Koesters, Thomas; Friedman, Kent P.; Fenchel, Matthias; Zhan, Yiqiang; Hermosillo, Gerardo; Babb, James; Jelescu, Ileana O.; Faul, David; Boada, Fernando E.; Shepherd, Timothy M.
2016-01-01
Simultaneous PET/MR of the brain is a promising new technology for characterizing patients with suspected cognitive impairment or epilepsy. Unlike CT though, MR signal intensities do not provide a direct correlate to PET photon attenuation correction (AC) and inaccurate radiotracer standard uptake value (SUV) estimation could limit future PET/MR clinical applications. We tested a novel AC method that supplements standard Dixon-based tissue segmentation with a superimposed model-based bone compartment. Methods We directly compared SUV estimation for MR-based AC methods to reference CT AC in 16 patients undergoing same-day, single 18FDG dose PET/CT and PET/MR for suspected neurodegeneration. Three Dixon-based MR AC methods were compared to CT – standard Dixon 4-compartment segmentation alone, Dixon with a superimposed model-based bone compartment, and Dixon with a superimposed bone compartment and linear attenuation correction optimized specifically for brain tissue. The brain was segmented using a 3D T1-weighted volumetric MR sequence and SUV estimations compared to CT AC for whole-image, whole-brain and 91 FreeSurfer-based regions-of-interest. Results Modifying the linear AC value specifically for brain and superimposing a model-based bone compartment reduced whole-brain SUV estimation bias of Dixon-based PET/MR AC by 95% compared to reference CT AC (P < 0.05) – this resulted in a residual −0.3% whole-brain mean SUV bias. Further, brain regional analysis demonstrated only 3 frontal lobe regions with SUV estimation bias of 5% or greater (P < 0.05). These biases appeared to correlate with high individual variability in the frontal bone thickness and pneumatization. Conclusion Bone compartment and linear AC modifications result in a highly accurate MR AC method in subjects with suspected neurodegeneration. This prototype MR AC solution appears equivalent than other recently proposed solutions, and does not require additional MR sequences and scan time. These
Range Restriction and Attenuation Corrections.
ERIC Educational Resources Information Center
Mumford, Michael D.; Mendoza, Jorge L.
The present paper reviews the techniques commonly used to correct an observed correlation coefficient for the simultaneous influence of attenuation and range restriction effects. It is noted that the procedure which is currently in use may be somewhat biased because it treats range restriction and attenuation as independent restrictive influences.…
Accurate adiabatic correction in the hydrogen molecule
Pachucki, Krzysztof; Komasa, Jacek
2014-12-14
A new formalism for the accurate treatment of adiabatic effects in the hydrogen molecule is presented, in which the electronic wave function is expanded in the James-Coolidge basis functions. Systematic increase in the size of the basis set permits estimation of the accuracy. Numerical results for the adiabatic correction to the Born-Oppenheimer interaction energy reveal a relative precision of 10{sup −12} at an arbitrary internuclear distance. Such calculations have been performed for 88 internuclear distances in the range of 0 < R ⩽ 12 bohrs to construct the adiabatic correction potential and to solve the nuclear Schrödinger equation. Finally, the adiabatic correction to the dissociation energies of all rovibrational levels in H{sub 2}, HD, HT, D{sub 2}, DT, and T{sub 2} has been determined. For the ground state of H{sub 2} the estimated precision is 3 × 10{sup −7} cm{sup −1}, which is almost three orders of magnitude higher than that of the best previous result. The achieved accuracy removes the adiabatic contribution from the overall error budget of the present day theoretical predictions for the rovibrational levels.
Accurate adiabatic correction in the hydrogen molecule
NASA Astrophysics Data System (ADS)
Pachucki, Krzysztof; Komasa, Jacek
2014-12-01
A new formalism for the accurate treatment of adiabatic effects in the hydrogen molecule is presented, in which the electronic wave function is expanded in the James-Coolidge basis functions. Systematic increase in the size of the basis set permits estimation of the accuracy. Numerical results for the adiabatic correction to the Born-Oppenheimer interaction energy reveal a relative precision of 10-12 at an arbitrary internuclear distance. Such calculations have been performed for 88 internuclear distances in the range of 0 < R ⩽ 12 bohrs to construct the adiabatic correction potential and to solve the nuclear Schrödinger equation. Finally, the adiabatic correction to the dissociation energies of all rovibrational levels in H2, HD, HT, D2, DT, and T2 has been determined. For the ground state of H2 the estimated precision is 3 × 10-7 cm-1, which is almost three orders of magnitude higher than that of the best previous result. The achieved accuracy removes the adiabatic contribution from the overall error budget of the present day theoretical predictions for the rovibrational levels.
Monte Carlo-based down-scatter correction of SPECT attenuation maps.
Bokulić, Tomislav; Vastenhouw, Brendan; de Jong, Hugo W A M; van Dongen, Alice J; van Rijk, Peter P; Beekman, Freek J
2004-08-01
Combined acquisition of transmission and emission data in single-photon emission computed tomography (SPECT) can be used for correction of non-uniform photon attenuation. However, down-scatter from a higher energy isotope (e.g. 99mTc) contaminates lower energy transmission data (e.g. 153Gd, 100 keV), resulting in underestimation of reconstructed attenuation coefficients. Window-based corrections are often not very accurate and increase noise in attenuation maps. We have developed a new correction scheme. It uses accurate scatter modelling to avoid noise amplification and does not require additional energy windows. The correction works as follows: Initially, an approximate attenuation map is reconstructed using down-scatter contaminated transmission data (step 1). An emission map is reconstructed based on the contaminated attenuation map (step 2). Based on this approximate 99mTc reconstruction and attenuation map, down-scatter in the 153Gd window is simulated using accelerated Monte Carlo simulation (step 3). This down-scatter estimate is used during reconstruction of a corrected attenuation map (step 4). Based on the corrected attenuation map, an improved 99mTc image is reconstructed (step 5). Steps 3-5 are repeated to incrementally improve the down-scatter estimate. The Monte Carlo simulator provides accurate down-scatter estimation with significantly less noise than down-scatter estimates acquired in an additional window. Errors in the reconstructed attenuation coefficients are reduced from ca. 40% to less than 5%. Furthermore, artefacts in 99mTc emission reconstructions are almost completely removed. These results are better than for window-based correction, both in simulation experiments and in physical phantom experiments. Monte Carlo down-scatter simulation in concert with statistical reconstruction provides accurate down-scatter correction of attenuation maps. PMID:15034678
Onboard Autonomous Corrections for Accurate IRF Pointing.
NASA Astrophysics Data System (ADS)
Jorgensen, J. L.; Betto, M.; Denver, T.
2002-05-01
filtered GPS updates, a world time clock, astrometric correction tables, and a attitude output transform system, that allow the ASC to deliver the spacecraft attitude relative to the Inertial Reference Frame (IRF) in realtime. This paper describes the operations of the onboard autonomy of the ASC, which in realtime removes the residuals from the attitude measurements, whereby a timely IRF attitude at arcsecond level, is delivered to the AOCS (or sent to ground). A discussion about achievable robustness and accuracy is given, and compared to inflight results from the operations of the two Advanced Stellar Compass's (ASC), which are flying in LEO onboard the German geo-potential research satellite CHAMP. The ASC's onboard CHAMP are dual head versions, i.e. each processing unit is attached to two star camera heads. The dual head configuration is primarily employed to achieve a carefree AOCS control with respect to the Sun, Moon and Earth, and to increase the attitude accuracy, but it also enables onboard estimation and removal of thermal generated biases.
An MRI-based attenuation correction method for combined PET/MRI applications
NASA Astrophysics Data System (ADS)
Fei, Baowei; Yang, Xiaofeng; Wang, Hesheng
2009-02-01
We are developing MRI-based attenuation correction methods for PET images. PET has high sensitivity but relatively low resolution and little anatomic details. MRI can provide excellent anatomical structures with high resolution and high soft tissue contrast. MRI can be used to delineate tumor boundaries and to provide an anatomic reference for PET, thereby improving quantitation of PET data. Combined PET/MRI can offer metabolic, functional and anatomic information and thus can provide a powerful tool to study the mechanism of a variety of diseases. Accurate attenuation correction represents an essential component for the reconstruction of artifact-free, quantitative PET images. Unfortunately, the present design of hybrid PET/MRI does not offer measured attenuation correction using a transmission scan. This problem may be solved by deriving attenuation maps from corresponding anatomic MR images. Our approach combines image registration, classification, and attenuation correction in a single scheme. MR images and the preliminary reconstruction of PET data are first registered using our automatic registration method. MRI images are then classified into different tissue types using our multiscale fuzzy C-mean classification method. The voxels of classified tissue types are assigned theoretical tissue-dependent attenuation coefficients to generate attenuation correction factors. Corrected PET emission data are then reconstructed using a threedimensional filtered back projection method and an order subset expectation maximization method. Results from simulated images and phantom data demonstrated that our attenuation correction method can improve PET data quantitation and it can be particularly useful for combined PET/MRI applications.
Hybrid approach for attenuation correction in PET/MR scanners
NASA Astrophysics Data System (ADS)
Santos Ribeiro, A.; Rota Kops, E.; Herzog, H.; Almeida, P.
2014-01-01
Aim: Attenuation correction (AC) of PET images is still one of the major limitations of hybrid PET/MR scanners. Different methods have been proposed to obtain the AC map from morphological MR images. Although, segmentation methods normally fail to differentiate air and bone regions, while template or atlas methods usually cannot accurately represent regions anatomically different from the template image. In this study a feed forward neural network (FFNN) algorithm is presented which directly outputs the attenuation coefficients by non-linear regression of the images acquired with an ultrashort echo time (UTE) sequence guided by the template-based AC map (TAC-map). Materials and methods: MR as well as CT data were acquired in four subjects. The UTE images and the TAC-map were the inputs of the presented FFNN algorithm for training as well as classification. The resulting attenuation maps were compared with CT-based, PNN-based and TAC maps. All the AC maps were used to reconstruct the PET emission data which were then compared for the different methods. Results: For each subject dice coefficients D were calculated between each method and the respective CT-based AC maps. The resulting Ds show higher values for all FFNN-based tissues comparatively to both TAC-based and PNN-based methods, particularly for bone tissue (D=0.77, D=0.51 and D=0.71, respectively). The AC-corrected PET images with the FFNN-based map show an overall lower relative difference (RD=3.90%) than those AC-corrected with the PNN-based (RD=4.44%) or template-based (RD=4.43%) methods. Conclusion: Our results show that an enhancement of current methods can be performed by combining both information of new MR image sequence techniques and general information provided from template techniques. Nevertheless, the number of tested subjects is statistically low and current analysis for a larger dataset is being carried out.
Jeong, Hyunjo; Zhang, Shuzeng; Li, Xiongbing; Barnard, Dan
2015-09-15
The accurate measurement of acoustic nonlinearity parameter β for fluids or solids generally requires making corrections for diffraction effects due to finite size geometry of transmitter and receiver. These effects are well known in linear acoustics, while those for second harmonic waves have not been well addressed and therefore not properly considered in previous studies. In this work, we explicitly define the attenuation and diffraction corrections using the multi-Gaussian beam (MGB) equations which were developed from the quasilinear solutions of the KZK equation. The effects of making these corrections are examined through the simulation of β determination in water. Diffraction corrections are found to have more significant effects than attenuation corrections, and the β values of water can be estimated experimentally with less than 5% errors when the exact second harmonic diffraction corrections are used together with the negligible attenuation correction effects on the basis of linear frequency dependence between attenuation coefficients, α{sub 2} ≃ 2α{sub 1}.
Accurately Detecting Students' Lies regarding Relational Aggression by Correctional Instructions
ERIC Educational Resources Information Center
Dickhauser, Oliver; Reinhard, Marc-Andre; Marksteiner, Tamara
2012-01-01
This study investigates the effect of correctional instructions when detecting lies about relational aggression. Based on models from the field of social psychology, we predict that correctional instruction will lead to a less pronounced lie bias and to more accurate lie detection. Seventy-five teachers received videotapes of students' true denial…
NASA Astrophysics Data System (ADS)
Gillen, Rebecca; Firbank, Michael J.; Lloyd, Jim; O'Brien, John T.
2015-09-01
This study investigated if the appearance and diagnostic accuracy of HMPAO brain perfusion SPECT images could be improved by using CT-based attenuation and scatter correction compared with the uniform attenuation correction method. A cohort of subjects who were clinically categorized as Alzheimer’s Disease (n=38 ), Dementia with Lewy Bodies (n=29 ) or healthy normal controls (n=30 ), underwent SPECT imaging with Tc-99m HMPAO and a separate CT scan. The SPECT images were processed using: (a) correction map derived from the subject’s CT scan or (b) the Chang uniform approximation for correction or (c) no attenuation correction. Images were visually inspected. The ratios between key regions of interest known to be affected or spared in each condition were calculated for each correction method, and the differences between these ratios were evaluated. The images produced using the different corrections were noted to be visually different. However, ROI analysis found similar statistically significant differences between control and dementia groups and between AD and DLB groups regardless of the correction map used. We did not identify an improvement in diagnostic accuracy in images which were corrected using CT-based attenuation and scatter correction, compared with those corrected using a uniform correction map.
Markerless attenuation correction for carotid MRI surface receiver coils in combined PET/MR imaging.
Eldib, Mootaz; Bini, Jason; Robson, Philip M; Calcagno, Claudia; Faul, David D; Tsoumpas, Charalampos; Fayad, Zahi A
2015-06-21
The purpose of the study was to evaluate the effect of attenuation of MR coils on quantitative carotid PET/MR exams. Additionally, an automated attenuation correction method for flexible carotid MR coils was developed and evaluated. The attenuation of the carotid coil was measured by imaging a uniform water phantom injected with 37 MBq of 18F-FDG in a combined PET/MR scanner for 24 min with and without the coil. In the same session, an ultra-short echo time (UTE) image of the coil on top of the phantom was acquired. Using a combination of rigid and non-rigid registration, a CT-based attenuation map was registered to the UTE image of the coil for attenuation and scatter correction. After phantom validation, the effect of the carotid coil attenuation and the attenuation correction method were evaluated in five subjects. Phantom studies indicated that the overall loss of PET counts due to the coil was 6.3% with local region-of-interest (ROI) errors reaching up to 18.8%. Our registration method to correct for attenuation from the coil decreased the global error and local error (ROI) to 0.8% and 3.8%, respectively. The proposed registration method accurately captured the location and shape of the coil with a maximum spatial error of 2.6 mm. Quantitative analysis in human studies correlated with the phantom findings, but was dependent on the size of the ROI used in the analysis. MR coils result in significant error in PET quantification and thus attenuation correction is needed. The proposed strategy provides an operator-free method for attenuation and scatter correction for a flexible MRI carotid surface coil for routine clinical use. PMID:26020273
Markerless attenuation correction for carotid MRI surface receiver coils in combined PET/MR imaging
NASA Astrophysics Data System (ADS)
Eldib, Mootaz; Bini, Jason; Robson, Philip M.; Calcagno, Claudia; Faul, David D.; Tsoumpas, Charalampos; Fayad, Zahi A.
2015-06-01
The purpose of the study was to evaluate the effect of attenuation of MR coils on quantitative carotid PET/MR exams. Additionally, an automated attenuation correction method for flexible carotid MR coils was developed and evaluated. The attenuation of the carotid coil was measured by imaging a uniform water phantom injected with 37 MBq of 18F-FDG in a combined PET/MR scanner for 24 min with and without the coil. In the same session, an ultra-short echo time (UTE) image of the coil on top of the phantom was acquired. Using a combination of rigid and non-rigid registration, a CT-based attenuation map was registered to the UTE image of the coil for attenuation and scatter correction. After phantom validation, the effect of the carotid coil attenuation and the attenuation correction method were evaluated in five subjects. Phantom studies indicated that the overall loss of PET counts due to the coil was 6.3% with local region-of-interest (ROI) errors reaching up to 18.8%. Our registration method to correct for attenuation from the coil decreased the global error and local error (ROI) to 0.8% and 3.8%, respectively. The proposed registration method accurately captured the location and shape of the coil with a maximum spatial error of 2.6 mm. Quantitative analysis in human studies correlated with the phantom findings, but was dependent on the size of the ROI used in the analysis. MR coils result in significant error in PET quantification and thus attenuation correction is needed. The proposed strategy provides an operator-free method for attenuation and scatter correction for a flexible MRI carotid surface coil for routine clinical use.
Impact of MR based attenuation correction on neurological PET studies
Su, Yi; Rubin, Brian B.; McConathy, Jonathan; Laforest, Richard; Qi, Jing; Sharma, Akash; Priatna, Agus; Benzinger, Tammie L.S.
2016-01-01
Hybrid positron emission tomography (PET) and magnetic resonance (MR) scanners have become a reality in recent years with the benefits of reduced radiation exposure, reduction of imaging time, and potential advantages in quantification. Appropriate attenuation correction remains a challenge. Biases in PET activity measurements were demonstrated using the current MR based attenuation correction technique. We aim to investigate the impact of using standard MRAC technique on the clinical and research utility of PET/MR hybrid scanner for amyloid imaging. Methods Florbetapir scans were obtained on 40 participants on a Biograph mMR hybrid scanner with simultaneous MR acquisition. PET images were reconstructed using both MR and CT derived attenuation map. Quantitative analysis was performed for both datasets to assess the impact of MR based attenuation correction to absolute PET activity measurements as well as target to reference ratio (SUVR). Clinical assessment was also performed by a nuclear medicine physician to determine amyloid status based on the criteria in the FDA prescribing information for florbetapir. Results MR based attenuation correction led to underestimation of PET activity for most part of the brain with a small overestimation for deep brain regions. There is also an overestimation of SUVR values with cerebellar reference. SUVR measurements obtained from the two attenuation correction methods were strongly correlated. Clinical assessment of amyloid status resulted in identical classification as positive or negative regardless of the attenuation correction methods. Conclusions MR based attenuation correction cause biases in quantitative measurements. The biases may be accounted for by a linear model, although the spatial variation cannot be easily modelled. The quantitative differences however did not affect clinical assessment as positive or negative. PMID:26823562
NASA Astrophysics Data System (ADS)
Maintas, Dimitris; Houzard, Claire; Ksyar, Rachid; Mognetti, Thomas; Maintas, Catherine; Scheiber, Christian; Itti, Roland
2006-12-01
It is considered that one of the great strengths of PET imaging is the ability to correct for body attenuation. This enables better lesion uptake quantification and quality of PET images. The aim of this work is to compare the sensitivity of non-attenuation-corrected (NAC) PET images, the gamma photons (GPAC) and CT attenuation-corrected (CTAC) images in detecting and staging of lung cancer. We have studied 66 patients undergoing PET/CT examinations for detecting and staging NSC lung cancer. The patients were injected with 18-FDG; 5 MBq/kg under fasting conditions and examination was started 60 min later. Transmission data were acquired by a spiral CT X-ray tube and by gamma photons emitting Cs-137l source and were used for the patient body attenuation correction without correction for respiratory motion. In 55 of 66 patients we performed both attenuation correction procedures and in 11 patients only CT attenuation correction. In seven patients with solitary nodules PET was negative and in 59 patients with lung cancer PET/CT was positive for pulmonary or other localization. In the group of 55 patients we found 165 areas of focal increased 18-FDG uptake in NAC, 165 in CTAC and 164 in GPAC PET images.In the patients with only CTAC we found 58 areas of increased 18-FDG uptake on NAC and 58 areas lesions on CTAC. In the patients with positive PET we found 223 areas of focal increased uptake in NAC and 223 areas in CTAC images. The sensitivity of NAC was equal to the sensitivity of CTAC and GPAC images. The visualization of peripheral lesions was better in NAC images and the lesions were better localized in attenuation-corrected images. In three lesions of the thorax the localization was better in GPAC and fused images than in CTAC images.
An Accurate Temperature Correction Model for Thermocouple Hygrometers 1
Savage, Michael J.; Cass, Alfred; de Jager, James M.
1982-01-01
Numerous water relation studies have used thermocouple hygrometers routinely. However, the accurate temperature correction of hygrometer calibration curve slopes seems to have been largely neglected in both psychrometric and dewpoint techniques. In the case of thermocouple psychrometers, two temperature correction models are proposed, each based on measurement of the thermojunction radius and calculation of the theoretical voltage sensitivity to changes in water potential. The first model relies on calibration at a single temperature and the second at two temperatures. Both these models were more accurate than the temperature correction models currently in use for four psychrometers calibrated over a range of temperatures (15-38°C). The model based on calibration at two temperatures is superior to that based on only one calibration. The model proposed for dewpoint hygrometers is similar to that for psychrometers. It is based on the theoretical voltage sensitivity to changes in water potential. Comparison with empirical data from three dewpoint hygrometers calibrated at four different temperatures indicates that these instruments need only be calibrated at, e.g. 25°C, if the calibration slopes are corrected for temperature. PMID:16662241
An accurate temperature correction model for thermocouple hygrometers.
Savage, M J; Cass, A; de Jager, J M
1982-02-01
Numerous water relation studies have used thermocouple hygrometers routinely. However, the accurate temperature correction of hygrometer calibration curve slopes seems to have been largely neglected in both psychrometric and dewpoint techniques.In the case of thermocouple psychrometers, two temperature correction models are proposed, each based on measurement of the thermojunction radius and calculation of the theoretical voltage sensitivity to changes in water potential. The first model relies on calibration at a single temperature and the second at two temperatures. Both these models were more accurate than the temperature correction models currently in use for four psychrometers calibrated over a range of temperatures (15-38 degrees C). The model based on calibration at two temperatures is superior to that based on only one calibration.The model proposed for dewpoint hygrometers is similar to that for psychrometers. It is based on the theoretical voltage sensitivity to changes in water potential. Comparison with empirical data from three dewpoint hygrometers calibrated at four different temperatures indicates that these instruments need only be calibrated at, e.g. 25 degrees C, if the calibration slopes are corrected for temperature. PMID:16662241
Attenuation correction for the large non-human primate brain imaging using microPET
NASA Astrophysics Data System (ADS)
Naidoo-Variawa, S.; Lehnert, W.; Kassiou, M.; Banati, R.; Meikle, S. R.
2010-04-01
Assessment of the biodistribution and pharmacokinetics of radiopharmaceuticals in vivo is often performed on animal models of human disease prior to their use in humans. The baboon brain is physiologically and neuro-anatomically similar to the human brain and is therefore a suitable model for evaluating novel CNS radioligands. We previously demonstrated the feasibility of performing baboon brain imaging on a dedicated small animal PET scanner provided that the data are accurately corrected for degrading physical effects such as photon attenuation in the body. In this study, we investigated factors affecting the accuracy and reliability of alternative attenuation correction strategies when imaging the brain of a large non-human primate (papio hamadryas) using the microPET Focus 220 animal scanner. For measured attenuation correction, the best bias versus noise performance was achieved using a 57Co transmission point source with a 4% energy window. The optimal energy window for a 68Ge transmission source operating in singles acquisition mode was 20%, independent of the source strength, providing bias-noise performance almost as good as for 57Co. For both transmission sources, doubling the acquisition time had minimal impact on the bias-noise trade-off for corrected emission images, despite observable improvements in reconstructed attenuation values. In a [18F]FDG brain scan of a female baboon, both measured attenuation correction strategies achieved good results and similar SNR, while segmented attenuation correction (based on uncorrected emission images) resulted in appreciable regional bias in deep grey matter structures and the skull. We conclude that measured attenuation correction using a single pass 57Co (4% energy window) or 68Ge (20% window) transmission scan achieves an excellent trade-off between bias and propagation of noise when imaging the large non-human primate brain with a microPET scanner.
Variational attenuation correction in two-view confocal microscopy
2013-01-01
Background Absorption and refraction induced signal attenuation can seriously hinder the extraction of quantitative information from confocal microscopic data. This signal attenuation can be estimated and corrected by algorithms that use physical image formation models. Especially in thick heterogeneous samples, current single view based models are unable to solve the underdetermined problem of estimating the attenuation-free intensities. Results We present a variational approach to estimate both, the real intensities and the spatially variant attenuation from two views of the same sample from opposite sides. Assuming noise-free measurements throughout the whole volume and pure absorption, this would in theory allow a perfect reconstruction without further assumptions. To cope with real world data, our approach respects photon noise, estimates apparent bleaching between the two recordings, and constrains the attenuation field to be smooth and sparse to avoid spurious attenuation estimates in regions lacking valid measurements. Conclusions We quantify the reconstruction quality on simulated data and compare it to the state-of-the art two-view approach and commonly used one-factor-per-slice approaches like the exponential decay model. Additionally we show its real-world applicability on model organisms from zoology (zebrafish) and botany (Arabidopsis). The results from these experiments show that the proposed approach improves the quantification of confocal microscopic data of thick specimen. PMID:24350574
Mehranian, Abolfazl; Arabi, Hossein; Zaidi, Habib
2016-03-01
Attenuation correction is an essential component of the long chain of data correction techniques required to achieve the full potential of quantitative positron emission tomography (PET) imaging. The development of combined PET/magnetic resonance imaging (MRI) systems mandated the widespread interest in developing novel strategies for deriving accurate attenuation maps with the aim to improve the quantitative accuracy of these emerging hybrid imaging systems. The attenuation map in PET/MRI should ideally be derived from anatomical MR images; however, MRI intensities reflect proton density and relaxation time properties of biological tissues rather than their electron density and photon attenuation properties. Therefore, in contrast to PET/computed tomography, there is a lack of standardized global mapping between the intensities of MRI signal and linear attenuation coefficients at 511 keV. Moreover, in standard MRI sequences, bones and lung tissues do not produce measurable signals owing to their low proton density and short transverse relaxation times. MR images are also inevitably subject to artifacts that degrade their quality, thus compromising their applicability for the task of attenuation correction in PET/MRI. MRI-guided attenuation correction strategies can be classified in three broad categories: (i) segmentation-based approaches, (ii) atlas-registration and machine learning methods, and (iii) emission/transmission-based approaches. This paper summarizes past and current state-of-the-art developments and latest advances in PET/MRI attenuation correction. The advantages and drawbacks of each approach for addressing the challenges of MR-based attenuation correction are comprehensively described. The opportunities brought by both MRI and PET imaging modalities for deriving accurate attenuation maps and improving PET quantification will be elaborated. Future prospects and potential clinical applications of these techniques and their integration in commercial
Dual energy CT for attenuation correction with PET/CT
Xia, Ting; Alessio, Adam M.; Kinahan, Paul E.
2014-01-15
Purpose: The authors evaluate the energy dependent noise and bias properties of monoenergetic images synthesized from dual-energy CT (DECT) acquisitions. These monoenergetic images can be used to estimate attenuation coefficients at energies suitable for positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging. This is becoming more relevant with the increased use of quantitative imaging by PET/CT and SPECT/CT scanners. There are, however, potential variations in the noise and bias of synthesized monoenergetic images as a function of energy. Methods: The authors used analytic approximations and simulations to estimate the noise and bias of synthesized monoenergetic images of water-filled cylinders with different shapes and the NURBS-based cardiac-torso (NCAT) phantom from 40 to 520 keV, the range of SPECT and PET energies. The dual-kVp spectra were based on the GE Lightspeed VCT scanner at 80 and 140 kVp with added filtration of 0.5 mm Cu. The authors evaluated strategies of noise suppression with sinogram smoothing and dose minimization with reduction of tube currents at the two kVp settings. The authors compared the impact of DECT-based attenuation correction with single-kVp CT-based attenuation correction on PET quantitation for the NCAT phantom for soft tissue and high-Z materials of bone and iodine contrast enhancement. Results: Both analytic calculations and simulations displayed the expected minimum noise value for a synthesized monoenergetic image at an energy between the mean energies of the two spectra. In addition the authors found that the normalized coefficient of variation in the synthesized attenuation map increased with energy but reached a plateau near 160 keV, and then remained constant with increasing energy up to 511 keV and beyond. The bias was minimal, as the linear attenuation coefficients of the synthesized monoenergetic images were within 2.4% of the known true values across the entire energy range
NASA Astrophysics Data System (ADS)
Kim, E.; Bowsher, J.; Thomas, A. S.; Sakhalkar, H.; Dewhirst, M.; Oldham, M.
2008-10-01
revealed highly inhomogeneous vasculature perfusion within the tumour. Optical-ECT emission images yielded high-resolution 3D images of the fluorescent protein distribution in the tumour. Attenuation-uncorrected optical-ECT images showed clear loss of signal in regions of high attenuation, including regions of high perfusion, where attenuation is increased by increased vascular ink stain. Application of attenuation correction showed significant changes in an apparent expression of fluorescent proteins, confirming the importance of the attenuation correction. In conclusion, this work presents the first development and application of an attenuation correction for optical-ECT imaging. The results suggest that successful attenuation correction for optical-ECT is feasible and is essential for quantitatively accurate optical-ECT imaging.
Effects of attenuation map accuracy on attenuation-corrected micro-SPECT images
2013-01-01
Background In single-photon emission computed tomography (SPECT), attenuation of photon flux in tissue affects quantitative accuracy of reconstructed images. Attenuation maps derived from X-ray computed tomography (CT) can be employed for attenuation correction. The attenuation coefficients as well as registration accuracy between SPECT and CT can be influenced by several factors. Here we investigate how such inaccuracies influence micro-SPECT quantification. Methods Effects of (1) misalignments between micro-SPECT and micro-CT through shifts and rotation, (2) globally altered attenuation coefficients and (3) combinations of these were evaluated. Tests were performed with a NEMA NU 4–2008 phantom and with rat cadavers containing sources with known activity. Results Changes in measured activities within volumes of interest in phantom images ranged from <1.5% (125I) and <0.6% (201Tl, 99mTc and 111In) for 1-mm shifts to <4.5% (125I) and <1.7% (201Tl, 99mTc and 111In) with large misregistration (3 mm). Changes induced by 15° rotation were smaller than those by 3-mm shifts. By significantly altering attenuation coefficients (±10%), activity changes of <5.2% for 125I and <2.7% for 201Tl, 99mTc and 111In were induced. Similar trends were seen in rat studies. Conclusions While getting sufficient accuracy of attenuation maps in clinical imaging is highly challenging, our results indicate that micro-SPECT quantification is quite robust to various imperfections of attenuation maps. PMID:23369630
Cardiac function assessed by attenuation-corrected radionuclide pressure-volume indices
Maurer, A.H.; Siegel, J.A.; Blasius, K.M.; Deneberg, B.S.; Spann, J.F.; Malmud, L.S.
1985-07-01
Using attenuation-corrected radionuclide volumes and arm-cuff peak systolic pressures, the authors established the mean value for the ratio of left ventricular (LV) peak systolic pressure/end systolic volume at rest for 15 healthy persons. In 43 patients with coronary disease, this ratio was more sensitive as an indicator of abnormal LV function and for predicting coronary artery disease than the resting ejection fraction. The slope of an end systolic pressure-volume line was also calculated from data obtained under three loading conditions: at rest, during isometric handgrip testing, and after the sublingual administration of nitroglycerin. These results represent an improvement over previous radionuclide pressure-volume measurements that have not used attenuation correction and show the need for accurate, nongeometric measurements of the LV end systolic volume.
Uniform attenuation correction using the frequency-distance principle
Zeng, Gengsheng L.
2007-11-15
The frequency-distance principle (FDP) is a well-known relationship that relates the distance between the object and the detector to the slope in the two-dimensional Fourier transform of the projection sinogram. This relationship has been previously applied to compensation of the distance dependent collimator blurring in SPECT (single photon emission computed tomography) in the literature. This paper makes an attempt to use the FDP to correct for uniform attenuation in SPECT. Computer simulations reveal that this technique works well for objects consisting of point sources but does not work well for distributed objects.
2010-01-01
Background Calibrating mammograms to produce a standardized breast density measurement for breast cancer risk analysis requires an accurate spatial measure of the compressed breast thickness. Thickness inaccuracies due to the nominal system readout value and compression paddle orientation induce unacceptable errors in the calibration. Method A thickness correction was developed and evaluated using a fully specified two-component surrogate breast model. A previously developed calibration approach based on effective radiation attenuation coefficient measurements was used in the analysis. Water and oil were used to construct phantoms to replicate the deformable properties of the breast. Phantoms consisting of measured proportions of water and oil were used to estimate calibration errors without correction, evaluate the thickness correction, and investigate the reproducibility of the various calibration representations under compression thickness variations. Results The average thickness uncertainty due to compression paddle warp was characterized to within 0.5 mm. The relative calibration error was reduced to 7% from 48-68% with the correction. The normalized effective radiation attenuation coefficient (planar) representation was reproducible under intra-sample compression thickness variations compared with calibrated volume measures. Conclusion Incorporating this thickness correction into the rigid breast tissue equivalent calibration method should improve the calibration accuracy of mammograms for risk assessments using the reproducible planar calibration measure. PMID:21080916
MR Imaging-Guided Attenuation Correction of PET Data in PET/MR Imaging.
Izquierdo-Garcia, David; Catana, Ciprian
2016-04-01
Attenuation correction (AC) is one of the most important challenges in the recently introduced combined PET/magnetic resonance (MR) scanners. PET/MR AC (MR-AC) approaches aim to develop methods that allow accurate estimation of the linear attenuation coefficients of the tissues and other components located in the PET field of view. MR-AC methods can be divided into 3 categories: segmentation, atlas, and PET based. This review provides a comprehensive list of the state-of-the-art MR-AC approaches and their pros and cons. The main sources of artifacts are presented. Finally, this review discusses the current status of MR-AC approaches for clinical applications. PMID:26952727
Blumhagen, Jan O. Ladebeck, Ralf; Fenchel, Matthias; Braun, Harald; Quick, Harald H.; Faul, David; Scheffler, Klaus
2014-02-15
Purpose: In quantitative PET imaging, it is critical to accurately measure and compensate for the attenuation of the photons absorbed in the tissue. While in PET/CT the linear attenuation coefficients can be easily determined from a low-dose CT-based transmission scan, in whole-body MR/PET the computation of the linear attenuation coefficients is based on the MR data. However, a constraint of the MR-based attenuation correction (AC) is the MR-inherent field-of-view (FoV) limitation due to static magnetic field (B{sub 0}) inhomogeneities and gradient nonlinearities. Therefore, the MR-based human AC map may be truncated or geometrically distorted toward the edges of the FoV and, consequently, the PET reconstruction with MR-based AC may be biased. This is especially of impact laterally where the patient arms rest beside the body and are not fully considered. Methods: A method is proposed to extend the MR FoV by determining an optimal readout gradient field which locally compensates B{sub 0} inhomogeneities and gradient nonlinearities. This technique was used to reduce truncation in AC maps of 12 patients, and the impact on the PET quantification was analyzed and compared to truncated data without applying the FoV extension and additionally to an established approach of PET-based FoV extension. Results: The truncation artifacts in the MR-based AC maps were successfully reduced in all patients, and the mean body volume was thereby increased by 5.4%. In some cases large patient-dependent changes in SUV of up to 30% were observed in individual lesions when compared to the standard truncated attenuation map. Conclusions: The proposed technique successfully extends the MR FoV in MR-based attenuation correction and shows an improvement of PET quantification in whole-body MR/PET hybrid imaging. In comparison to the PET-based completion of the truncated body contour, the proposed method is also applicable to specialized PET tracers with little uptake in the arms and might
Proximity corrected accurate in-die registration metrology
NASA Astrophysics Data System (ADS)
Daneshpanah, M.; Laske, F.; Wagner, M.; Roeth, K.-D.; Czerkas, S.; Yamaguchi, H.; Fujii, N.; Yoshikawa, S.; Kanno, K.; Takamizawa, H.
2014-07-01
193nm immersion lithography is the mainstream production technology for the 20nm and 14nm logic nodes. Multi-patterning of an increasing number of critical layers puts extreme pressure on wafer intra-field overlay, to which mask registration error is a major contributor [1]. The International Technology Roadmap for Semiconductors (ITRS [2]) requests a registration error below 4 nm for each mask of a multi-patterning set forming one layer on the wafer. For mask metrology at the 20nm and 14nm logic nodes, maintaining a precision-to-tolerance (P/T) ratio below 0.25 will be very challenging. Full characterization of mask registration errors in the active area of the die will become mandatory. It is well-known that differences in pattern density and asymmetries in the immediate neighborhood of a feature give rise to apparent shifts in position when measured by optical metrology systems, so-called optical proximity effects. These effects can easily be similar in magnitude to real mask placement errors, and uncorrected can result in mis-qualification of the mask. Metrology results from KLA-Tencor's next generation mask metrology system are reported, applying a model-based algorithm [3] which includes corrections for proximity errors. The proximity corrected, model-based measurements are compared to standard measurements and a methodology presented that verifies the correction performance of the new algorithm.
NASA Astrophysics Data System (ADS)
Hu, Yongxiang; Behrenfeld, Mike; Hostetler, Chris; Pelon, Jacques; Trepte, Charles; Hair, John; Slade, Wayne; Cetinic, Ivona; Vaughan, Mark; Lu, Xiaomei; Zhai, Pengwang; Weimer, Carl; Winker, David; Verhappen, Carolus C.; Butler, Carolyn; Liu, Zhaoyan; Hunt, Bill; Omar, Ali; Rodier, Sharon; Lifermann, Anne; Josset, Damien; Hou, Weilin; MacDonnell, David; Rhew, Ray
2016-06-01
Beam attenuation coefficient, c, provides an important optical index of plankton standing stocks, such as phytoplankton biomass and total particulate carbon concentration. Unfortunately, c has proven difficult to quantify through remote sensing. Here, we introduce an innovative approach for estimating c using lidar depolarization measurements and diffuse attenuation coefficients from ocean color products or lidar measurements of Brillouin scattering. The new approach is based on a theoretical formula established from Monte Carlo simulations that links the depolarization ratio of sea water to the ratio of diffuse attenuation Kd and beam attenuation C (i.e., a multiple scattering factor). On July 17, 2014, the CALIPSO satellite was tilted 30° off-nadir for one nighttime orbit in order to minimize ocean surface backscatter and demonstrate the lidar ocean subsurface measurement concept from space. Depolarization ratios of ocean subsurface backscatter are measured accurately. Beam attenuation coefficients computed from the depolarization ratio measurements compare well with empirical estimates from ocean color measurements. We further verify the beam attenuation coefficient retrievals using aircraft-based high spectral resolution lidar (HSRL) data that are collocated with in-water optical measurements.
Huang, C; Ouyang, J; Reese, T G; Wu, Y; El Fakhri, G; Ackerman, J L
2015-10-21
Due to the lack of signal from solid bone in normal MR sequences for the purpose of MR-based attenuation correction, investigators have proposed using the ultrashort echo time (UTE) pulse sequence, which yields signal from bone. However, the UTE-based segmentation approach might not fully capture the intra- and inter-subject bone density variation, which will inevitably lead to bias in reconstructed PET images. In this work, we investigated using the water- and fat-suppressed proton projection imaging (WASPI) sequence to obtain accurate and continuous attenuation for bones. This approach is capable of accounting for intra- and inter-subject bone attenuation variations. Using data acquired from a phantom, we have found that that attenuation correction based on the WASPI sequence is more accurate and precise when compared to either conventional MR attenuation correction or UTE-based segmentation approaches. PMID:26405761
NASA Astrophysics Data System (ADS)
Jeong, Hyunjo; Zhang, Shuzeng; Cho, Sungjong; Li, Xiongbing
2016-04-01
In recent studies with nonlinear Rayleigh surface waves, harmonic generation measurements have been successfully employed to characterize material damage and microstructural changes, and found to be sensitive to early stages of damage process. A nonlinearity parameter of Rayleigh surface waves was derived and frequently measured to quantify the level of damage. The accurate measurement of the nonlinearity parameter generally requires making corrections for beam diffraction and medium attenuation. These effects are not generally known for nonlinear Rayleigh waves, and therefore not properly considered in most of previous studies. In this paper, the nonlinearity parameter for a Rayleigh surface wave is defined from the plane wave displacement solutions. We explicitly define the attenuation and diffraction corrections for fundamental and second harmonic Rayleigh wave beams radiated from a uniform line source. Attenuation corrections are obtained from the quasilinear theory of plane Rayleigh wave equations. To obtain closed-form expressions for diffraction corrections, multi-Gaussian beam (MGB) models are employed to represent the integral solutions derived from the quasilinear theory of the full two-dimensional wave equation without parabolic approximation. Diffraction corrections are presented for a couple of transmitter-receiver geometries, and the effects of making attenuation and diffraction corrections are examined through the simulation of nonlinearity parameter determination in a solid sample.
Using BRDFs for accurate albedo calculations and adjacency effect corrections
Borel, C.C.; Gerstl, S.A.W.
1996-09-01
In this paper the authors discuss two uses of BRDFs in remote sensing: (1) in determining the clear sky top of the atmosphere (TOA) albedo, (2) in quantifying the effect of the BRDF on the adjacency point-spread function and on atmospheric corrections. The TOA spectral albedo is an important parameter retrieved by the Multi-angle Imaging Spectro-Radiometer (MISR). Its accuracy depends mainly on how well one can model the surface BRDF for many different situations. The authors present results from an algorithm which matches several semi-empirical functions to the nine MISR measured BRFs that are then numerically integrated to yield the clear sky TOA spectral albedo in four spectral channels. They show that absolute accuracies in the albedo of better than 1% are possible for the visible and better than 2% in the near infrared channels. Using a simplified extensive radiosity model, the authors show that the shape of the adjacency point-spread function (PSF) depends on the underlying surface BRDFs. The adjacency point-spread function at a given offset (x,y) from the center pixel is given by the integral of transmission-weighted products of BRDF and scattering phase function along the line of sight.
NASA Astrophysics Data System (ADS)
Gu, Ji-Young; Suk, Mi-Kyung; Nam, Kyung-Yeub; Ko, Jeong-Seok; Ryzhkov, Alexander
2016-04-01
To obtain high-quality radar quantitative precipitation estimation data, reliable radar calibration and efficient attenuation correction are very important. Because microwave radiation at shorter wavelength experiences strong attenuation in precipitation, accounting for this attenuation is the essential work at shorter wavelength radar. In this study, the performance of different attenuation/differential attenuation correction schemes at C band is tested for two strong rain events which occurred in central Oklahoma. And also, a new attenuation correction scheme (combination of self-consistency and hot-spot concept methodology) that separates relative contributions of strong convective cells and the rest of the storm to the path-integrated total and differential attenuation is among the algorithms explored. A quantitative use of weather radar measurement such as rainfall estimation relies on the reliable attenuation correction. We examined the impact of attenuation correction on estimates of rainfall in heavy rain events by using cross-checking with S-band radar measurements which are much less affected by attenuation and compared the storm rain totals obtained from the corrected Z and KDP and rain gages in these cases. This new approach can be utilized at shorter wavelength radars efficiently. Therefore, it is very useful to Weather Radar Center of Korea Meteorological Administration preparing X-band research dual Pol radar network.
Attenuation Correction for Magnetic Resonance Coils in Combined PET/MR Imaging: A Review.
Eldib, Mootaz; Bini, Jason; Faul, David D; Oesingmann, Niels; Tsoumpas, Charalampos; Fayad, Zahi A
2016-04-01
With the introduction of clinical PET/magnetic resonance (MR) systems, novel attenuation correction methods are needed, as there are no direct MR methods to measure the attenuation of the objects in the field of view (FOV). A unique challenge for PET/MR attenuation correction is that coils for MR data acquisition are located in the FOV of the PET camera and could induce significant quantitative errors. In this review, current methods and techniques to correct for the attenuation of a variety of coils are summarized and evaluated. PMID:26952728
Ultra low-dose CT attenuation correction in PET SPM
NASA Astrophysics Data System (ADS)
Wang, Shyh-Jen; Yang, Bang-Hung; Tsai, Chia-Jung; Yang, Ching-Ching; Lee, Jason J. S.; Wu, Tung-Hsin
2010-07-01
The use of CT images for attenuation correction (CTAC) allows significantly shorter scanning time and a high quality noise-free attenuation map compared with conventional germanium-68 transmission scan because at least 10 4 times greater of photon flux would be generated from a CT scan under standard operating condition. However, this CTAC technique would potentially introduce more radiation risk to the patients owing to the higher radiation exposure from CT scan. Statistic parameters mapping (SPM) is a prominent technique in nuclear medicine community for the analysis of brain imaging data. The purpose of this study is to assess the feasibility of low-dose CT (LDCT) and ultra low-dose CT (UDCT) in PET SPM applications. The study was divided into two parts. The first part was to evaluate of tracer uptake distribution pattern and quantity analysis by using the striatal phantom to initially assess the feasibility of AC for clinical purpose. The second part was to examine the group SPM analysis using the Hoffman brain phantom. The phantom study is to simulate the human brain and to reduce the experimental uncertainty of real subjects. The initial studies show that the results of PET SPM analysis have no significant differences between LDCT and UDCT comparing to the current used default CTAC. Moreover, the dose of the LDCT is lower than that of the default CT by a factor of 9, and UDCT can even yield a 42 times dose reduction. We have demonstrated the SPM results while using LDCT and UDCT for PET AC is comparable to those using default CT setting, suggesting their feasibility in PET SPM applications. In addition, the necessity of UDCT in PET SPM studies to avoid excess radiation dose is also evident since most of the subjects involved are non-cancer patients or children and some normal subjects are even served as a comparison group in the experiment. It is our belief that additional attempts to decrease the radiation dose would be valuable, especially for children and
Metal artifact reduction strategies for improved attenuation correction in hybrid PET/CT imaging
Abdoli, Mehrsima; Dierckx, Rudi A. J. O.; Zaidi, Habib
2012-06-15
Metallic implants are known to generate bright and dark streaking artifacts in x-ray computed tomography (CT) images, which in turn propagate to corresponding functional positron emission tomography (PET) images during the CT-based attenuation correction procedure commonly used on hybrid clinical PET/CT scanners. Therefore, visual artifacts and overestimation and/or underestimation of the tracer uptake in regions adjacent to metallic implants are likely to occur and as such, inaccurate quantification of the tracer uptake and potential erroneous clinical interpretation of PET images is expected. Accurate quantification of PET data requires metal artifact reduction (MAR) of the CT images prior to the application of the CT-based attenuation correction procedure. In this review, the origins of metallic artifacts and their impact on clinical PET/CT imaging are discussed. Moreover, a brief overview of proposed MAR methods and their advantages and drawbacks is presented. Although most of the presented MAR methods are mainly developed for diagnostic CT imaging, their potential application in PET/CT imaging is highlighted. The challenges associated with comparative evaluation of these methods in a clinical environment in the absence of a gold standard are also discussed.
NASA Astrophysics Data System (ADS)
Liu, Yuxiang
Attenuation of electromagnetic radiation due to rain or other wet hydrometeors along the propagation path has been studied extensively in the radar meteorology community. Recently, use of short range dual-polarization X-band radar systems has gained momentum due to lower system cost compared with the much more expensive S-band systems. Advances in dual-polarization radar research have shown that the specific attenuation and differential attenuation between horizontal and vertical polarized waves caused by oblate, highly oriented raindrops can be estimated using the specific differential phase. This advance leads to correction of the measured reflectivity (Zh) and the differential reflectivity (Zdr) due to path attenuation. This thesis addresses via theory, simulations and data analyses the accuracy and optimal estimation of attenuation-correction procedures at X-band frequency. Real-time implementation of the correction algorithm was developed for the first generation of X-band dual-polarized Doppler radar network (Integration Project 1, IP1) operated by the NSF Center for Collaborate Adaptive Sensing of the Atmosphere (CASA). We evaluate the algorithm for correcting the Zh, and the Zdr for rain attenuation using simulations and X-band radar data under ideal and noisy situations. Our algorithm is able to adjust the parameters according to the changes in temperature, drop shapes, and a certain class of drop size distributions (DSD) with very fast convergence. The X-band radar data were obtained from the National Institute of Earth Science and Disaster Prevention (NIED), Japan, and from CASA IP1. The algorithm accurately corrects NIED's data when compared with ground truth calculated from in situ disdrometer-based DSD measurements for a Typhoon event. We have implemented, in real-time, the algorithm in all the CASA IP1 radar nodes. We also evaluate our preliminary method that separately estimates rain and wet ice attenuation using microphysical outputs from a
Bias atlases for segmentation-based PET attenuation correction using PET-CT and MR
Ouyang, Jinsong; Chun, Se Young; Petibon, Yoann; Bonab, Ali A.; Alpert, Nathaniel; Fakhri, Georges El
2014-01-01
This study was to obtain voxel-wise PET accuracy and precision using tissue-segmentation for attenuation correction. We applied multiple thresholds to the CTs of 23 patients to classify tissues. For six of the 23 patients, MR images were also acquired. The MR fat/in-phase ratio images were used for fat segmentation. Segmented tissue classes were used to create attenuation maps, which were used for attenuation correction in PET reconstruction. PET bias images were then computed using the PET reconstructed with the original CT as the reference. We registered the CTs for all the patients and transformed the corresponding bias images accordingly. We then obtained the mean and standard deviation bias atlas using all the registered bias images. Our CT-based study shows that four-class segmentation (air, lungs, fat, other tissues), which is available on most PET-MR scanners, yields 15.1%, 4.1%, 6.6%, and 12.9% RMSE bias in lungs, fat, non-fat soft-tissues, and bones, respectively. An accurate fat identification is achievable using fat/in-phase MR images. Furthermore, we have found that three-class segmentation (air, lungs, other tissues) yields less than 5% standard deviation of bias within the heart, liver, and kidneys. This implies that three-class segmentation can be sufficient to achieve small variation of bias for imaging these three organs. Finally, we have found that inter- and intra-patient lung density variations contribute almost equally to the overall standard deviation of bias within the lungs. PMID:24966415
Attenuation correction for small animal SPECT imaging using x-ray CT data
Hwang, Andrew B.; Hasegawa, Bruce H.
2005-09-15
Photon attenuation in small animal nuclear medicine scans can be significant when using isotopes that emit lower energy photons such as iodine-125. We have developed a method to use microCT data to perform attenuation corrected small animal single-photon emission computed tomography (SPECT). A microCT calibration phantom was first imaged, and the resulting calibration curve was used to convert microCT image values to linear attenuation coefficient values that were then used in an iterative SPECT reconstruction algorithm. This method was applied to reconstruct a SPECT image of a uniform phantom filled with {sup 125}I-NaI. Without attenuation correction, the image suffered a 30% decrease in intensity in the center of the image, which was removed with the addition of attenuation correction. This reduced the relative standard deviation in the region of interest from 10% to 6%.
Arabi, Hossein; Zaidi, Habib
2016-07-01
Quantitative whole-body PET/MR imaging is challenged by the lack of accurate and robust strategies for attenuation correction. In this work, a new pseudo-CT generation approach, referred to as sorted atlas pseudo-CT (SAP), is proposed for accurate extraction of bones and estimation of lung attenuation properties. This approach improves the Gaussian process regression (GPR) kernel proposed by Hofmann et al. which relies on the information provided by a co-registered atlas (CT and MRI) using a GPR kernel to predict the distribution of attenuation coefficients. Our approach uses two separate GPR kernels for lung and non-lung tissues. For non-lung tissues, the co-registered atlas dataset was sorted on the basis of local normalized cross-correlation similarity to the target MR image to select the most similar image in the atlas for each voxel. For lung tissue, the lung volume was incorporated in the GPR kernel taking advantage of the correlation between lung volume and corresponding attenuation properties to predict the attenuation coefficients of the lung. In the presence of pathological tissues in the lungs, the lesions are segmented on PET images corrected for attenuation using MRI-derived three-class attenuation map followed by assignment of soft-tissue attenuation coefficient. The proposed algorithm was compared to other techniques reported in the literature including Hofmann's approach and the three-class attenuation correction technique implemented on the Philips Ingenuity TF PET/MR where CT-based attenuation correction served as reference. Fourteen patients with head and neck cancer undergoing PET/CT and PET/MR examinations were used for quantitative analysis. SUV measurements were performed on 12 normal uptake regions as well as high uptake malignant regions. Moreover, a number of similarity measures were used to evaluate the accuracy of extracted bones. The Dice similarity metric revealed that the extracted bone improved from 0.58 ± 0.09 to 0.65 ± 0.07 when
Investigation of Attenuation Correction for Small-Animal Single Photon Emission Computed Tomography
Lee, Hsin-Hui; Chen, Jyh-Cheng
2013-01-01
The quantitative accuracy of SPECT is limited by photon attenuation and scatter effect when photons interact with atoms. In this study, we developed a new attenuation correction (AC) method, CT-based mean attenuation correction (CTMAC) method, and compared it with various methods that were often used currently to assess the AC phenomenon by using the small-animal SPECT/CT data that were acquired from various physical phantoms and a rat. The physical phantoms and an SD rat, which were injected with 99mTc, were scanned by a parallel-hole small-animal SPECT, and then they were imaged by the 80 kVp micro-CT. Scatter was estimated and corrected by the triple-energy window (TEW) method. Absolute quantification was derived from a known activity point source scan. In the physical-phantom studies, we compared the images with original, scatter correction (SC) only, and the scatter-corrected images with AC performed by using Chang's method, CT-based attenuation correction (CTAC), CT-based iterative attenuation compensation during reconstruction (CTIACR), and the CTMAC. From the correction results, we find out that the errors of the previous six configurations are mostly quite similar. The CTMAC needs the shortest correction time while obtaining good AC results. PMID:23840278
Mollet, Pieter; Keereman, Vincent; Bini, Jason; Izquierdo-Garcia, David; Fayad, Zahi A.; Vandenberghe, Stefaan
2014-01-01
Quantitative PET imaging relies on accurate attenuation correction. Recently, there has been growing interest in combining state-of-the-art PET systems with MR imaging in a sequential or fully integrated setup. As CT becomes unavailable for these systems, an alternative approach to the CT-based reconstruction of attenuation coefficients (μ values) at 511 keV must be found. Deriving μ values directly from MR images is difficult because MR signals are related to the proton density and relaxation properties of tissue. Therefore, most research groups focus on segmentation or atlas registration techniques. Although studies have shown that these methods provide viable solutions in particular applications, some major drawbacks limit their use in whole-body PET/MR. Previously, we used an annulus-shaped PET transmission source inside the field of view of a PET scanner to measure attenuation coefficients at 511 keV. In this work, we describe the use of this method in studies of patients with the sequential time-of-flight (TOF) PET/MR scanner installed at the Icahn School of Medicine at Mount Sinai, New York, NY. Methods Five human PET/MR and CT datasets were acquired. The transmission-based attenuation correction method was compared with conventional CT-based attenuation correction and the 3-segment, MR-based attenuation correction available on the TOF PET/MR imaging scanner. Results The transmission-based method overcame most problems related to the MR-based technique, such as truncation artifacts of the arms, segmentation artifacts in the lungs, and imaging of cortical bone. Additionally, the TOF capabilities of the PET detectors allowed the simultaneous acquisition of transmission and emission data. Compared with the MR-based approach, the transmission-based method provided average improvements in PET quantification of 6.4%, 2.4%, and 18.7% in volumes of interest inside the lung, soft tissue, and bone tissue, respectively. Conclusion In conclusion, a transmission
Scatter correction of vessel dropout behind highly attenuating structures in 4D-DSA
NASA Astrophysics Data System (ADS)
Hermus, James; Mistretta, Charles; Szczykutowicz, Timothy P.
2015-03-01
In Computed Tomographic (CT) image reconstruction for 4 dimensional digital subtraction angiography (4D-DSA), loss of vessel contrast has been observed behind highly attenuating anatomy, such as large contrast filled aneurysms. Although this typically occurs only in a limited range of projection angles, the observed contrast time course can be altered. In this work we propose an algorithm to correct for highly attenuating anatomy within the fill projection data, i.e. aneurysms. The algorithm uses a 3D-SA volume to create a correction volume that is multiplied by the 4D-DSA volume in order to correct for signal dropout within the 4D-DSA volume. The algorithm was designed to correct for highly attenuating material in the fill volume only, however with alterations to a single step of the algorithm, artifacts due to highly attenuating materials in the mask volume (i.e. dental implants) can be mitigated as well. We successfully applied our algorithm to a case of vessel dropout due to the presence of a large attenuating aneurysm. The performance was qualified visually as the affected vessel no longer dropped out on corrected 4D-DSA time frames. The correction was quantified by plotting the signal intensity along the vessel. Our analysis demonstrated our correction does not alter vessel signal values outside of the vessel dropout region but does increase the vessel values within the dropout region as expected. We have demonstrated that this correction algorithm acts to correct vessel dropout in areas with highly attenuating materials.
Cortés-Blanco, A; Fujii, C; Goris, M L
1999-12-01
We propose a method to assess an attenuation correction method in myocardial perfusion SPECT. Three types of images are obtained: one resulting from a classic acquisition and filtered back-projection (classic), and those resulting from acquisition with a transmission source and an iterative reconstruction, with (music) or without (hybrid) the attenuation correction factored in to compare the three types of images and classify them as normal or abnormal, a three dimensional inter-patient quantitative comparison method was used. Differences were computed as fractions of the myocardial volume in which density differences are significant by population standards. In 7 cases the cumulative difference between prone and supine in hybrid images was 124 and 45 in music images. In 10 cases the cumulative difference between classic vs music images was 279, and between classic and hybrid 86. The AC changed 4/12 cases from abnormal to normal. The attenuation correction effect was concentrated on the septal and inferior walls, but neither exclusively nor evenly among patients. The attenuation correction effectively minimizes attenuation effects by a factor of 2.7, due to a correction of at least 69%. The correction has a small but substantial effect on the results. PMID:10611567
NASA Astrophysics Data System (ADS)
Toyokuni, Genti; Takenaka, Hiroshi
2012-06-01
We propose a method for modeling global seismic wave propagation through an attenuative Earth model including the center. This method enables accurate and efficient computations since it is based on the 2.5-D approach, which solves wave equations only on a 2-D cross section of the whole Earth and can correctly model 3-D geometrical spreading. We extend a numerical scheme for the elastic waves in spherical coordinates using the finite-difference method (FDM), to solve the viscoelastodynamic equation. For computation of realistic seismic wave propagation, incorporation of anelastic attenuation is crucial. Since the nature of Earth material is both elastic solid and viscous fluid, we should solve stress-strain relations of viscoelastic material, including attenuative structures. These relations represent the stress as a convolution integral in time, which has had difficulty treating viscoelasticity in time-domain computation such as the FDM. However, we now have a method using so-called memory variables, invented in the 1980s, followed by improvements in Cartesian coordinates. Arbitrary values of the quality factor (Q) can be incorporated into the wave equation via an array of Zener bodies. We also introduce the multi-domain, an FD grid of several layers with different grid spacings, into our FDM scheme. This allows wider lateral grid spacings with depth, so as not to perturb the FD stability criterion around the Earth center. In addition, we propose a technique to avoid the singularity problem of the wave equation in spherical coordinates at the Earth center. We develop a scheme to calculate wavefield variables on this point, based on linear interpolation for the velocity-stress, staggered-grid FDM. This scheme is validated through a comparison of synthetic seismograms with those obtained by the Direct Solution Method for a spherically symmetric Earth model, showing excellent accuracy for our FDM scheme. As a numerical example, we apply the method to simulate seismic
Mehranian, Abolfazl; Zaidi, Habib
2015-06-21
In standard segmentation-based MRI-guided attenuation correction (MRAC) of PET data on hybrid PET/MRI systems, the inter/intra-patient variability of linear attenuation coefficients (LACs) is ignored owing to the assignment of a constant LAC to each tissue class. This can lead to PET quantification errors, especially in the lung regions. In this work, we aim to derive continuous and patient-specific lung LACs from time-of-flight (TOF) PET emission data using the maximum likelihood reconstruction of activity and attenuation (MLAA) algorithm. The MLAA algorithm was constrained for estimation of lung LACs only in the standard 4-class MR attenuation map using Gaussian lung tissue preference and Markov random field smoothness priors. MRAC maps were derived from segmentation of CT images of 19 TOF-PET/CT clinical studies into background air, lung, soft tissue and fat tissue classes, followed by assignment of predefined LACs of 0, 0.0224, 0.0864 and 0.0975 cm(-1), respectively. The lung LACs of the resulting attenuation maps were then estimated from emission data using the proposed MLAA algorithm. PET quantification accuracy of MRAC and MLAA methods was evaluated against the reference CT-based AC method in the lungs, lesions located in/near the lungs and neighbouring tissues. The results show that the proposed MLAA algorithm is capable of retrieving lung density gradients and compensate fairly for respiratory-phase mismatch between PET and corresponding attenuation maps. It was found that the mean of the estimated lung LACs generally follow the trend of the reference CT-based attenuation correction (CTAC) method. Quantitative analysis revealed that the MRAC method resulted in average relative errors of -5.2 ± 7.1% and -6.1 ± 6.7% in the lungs and lesions, respectively. These were reduced by the MLAA algorithm to -0.8 ± 6.3% and -3.3 ± 4.7%, respectively. In conclusion, we demonstrated the potential and capability of emission-based methods in deriving patient
NASA Astrophysics Data System (ADS)
Schneebeli, M.; Sakuragi, J.; Biscaro, T.; Angelis, C. F.; Carvalho da Costa, I.; Morales, C.; Baldini, L.; Machado, L. A. T.
2012-09-01
A polarimetric X-band radar has been deployed during one month (April 2011) for a field campaign in Fortaleza, Brazil, together with three additional laser disdrometers. The disdrometers are capable of measuring the raindrop size distributions (DSDs), hence making it possible to forward-model theoretical polarimetric X-band radar observables at the point where the instruments are located. This set-up allows to thoroughly test the accuracy of the X-band radar measurements as well as the algorithms that are used to correct the radar data for radome and rain attenuation. For the campaign in Fortaleza it was found that radome attenuation dominantly affects the measurements. With an algorithm that is based on the self-consistency of the polarimetric observables, the radome induced reflectivity offset was estimated. Offset corrected measurements were then further corrected for rain attenuation with two different schemes. The performance of the post-processing steps was analyzed by comparing the data with disdrometer-inferred polarimetric variables that were measured at a distance of 20 km from the radar. Radome attenuation reached values up to 14 dB which was found to be consistent with an empirical radome attenuation vs. rain intensity relation that was previously developed for the same radar type. In contrast to previous work, our results suggest that radome attenuation should be estimated individually for every view direction of the radar in order to obtain homogenous reflectivity fields.
NASA Technical Reports Server (NTRS)
Cao, Fang; Fichot, Cedric G.; Hooker, Stanford B.; Miller, William L.
2014-01-01
Photochemical processes driven by high-energy ultraviolet radiation (UVR) in inshore, estuarine, and coastal waters play an important role in global bio geochemical cycles and biological systems. A key to modeling photochemical processes in these optically complex waters is an accurate description of the vertical distribution of UVR in the water column which can be obtained using the diffuse attenuation coefficients of down welling irradiance (Kd()). The Sea UV Sea UVc algorithms (Fichot et al., 2008) can accurately retrieve Kd ( 320, 340, 380,412, 443 and 490 nm) in oceanic and coastal waters using multispectral remote sensing reflectances (Rrs(), Sea WiFS bands). However, SeaUVSeaUVc algorithms are currently not optimized for use in optically complex, inshore waters, where they tend to severely underestimate Kd(). Here, a new training data set of optical properties collected in optically complex, inshore waters was used to re-parameterize the published SeaUVSeaUVc algorithms, resulting in improved Kd() retrievals for turbid, estuarine waters. Although the updated SeaUVSeaUVc algorithms perform best in optically complex waters, the published SeaUVSeaUVc models still perform well in most coastal and oceanic waters. Therefore, we propose a composite set of SeaUVSeaUVc algorithms, optimized for Kd() retrieval in almost all marine systems, ranging from oceanic to inshore waters. The composite algorithm set can retrieve Kd from ocean color with good accuracy across this wide range of water types (e.g., within 13 mean relative error for Kd(340)). A validation step using three independent, in situ data sets indicates that the composite SeaUVSeaUVc can generate accurate Kd values from 320 490 nm using satellite imagery on a global scale. Taking advantage of the inherent benefits of our statistical methods, we pooled the validation data with the training set, obtaining an optimized composite model for estimating Kd() in UV wavelengths for almost all marine waters. This
Accuracy of CT-based attenuation correction in PET/CT bone imaging
NASA Astrophysics Data System (ADS)
Abella, Monica; Alessio, Adam M.; Mankoff, David A.; MacDonald, Lawrence R.; Vaquero, Juan Jose; Desco, Manuel; Kinahan, Paul E.
2012-05-01
We evaluate the accuracy of scaling CT images for attenuation correction of PET data measured for bone. While the standard tri-linear approach has been well tested for soft tissues, the impact of CT-based attenuation correction on the accuracy of tracer uptake in bone has not been reported in detail. We measured the accuracy of attenuation coefficients of bovine femur segments and patient data using a tri-linear method applied to CT images obtained at different kVp settings. Attenuation values at 511 keV obtained with a 68Ga/68Ge transmission scan were used as a reference standard. The impact of inaccurate attenuation images on PET standardized uptake values (SUVs) was then evaluated using simulated emission images and emission images from five patients with elevated levels of FDG uptake in bone at disease sites. The CT-based linear attenuation images of the bovine femur segments underestimated the true values by 2.9 ± 0.3% for cancellous bone regardless of kVp. For compact bone the underestimation ranged from 1.3% at 140 kVp to 14.1% at 80 kVp. In the patient scans at 140 kVp the underestimation was approximately 2% averaged over all bony regions. The sensitivity analysis indicated that errors in PET SUVs in bone are approximately proportional to errors in the estimated attenuation coefficients for the same regions. The variability in SUV bias also increased approximately linearly with the error in linear attenuation coefficients. These results suggest that bias in bone uptake SUVs of PET tracers ranges from 2.4% to 5.9% when using CT scans at 140 and 120 kVp for attenuation correction. Lower kVp scans have the potential for considerably more error in dense bone. This bias is present in any PET tracer with bone uptake but may be clinically insignificant for many imaging tasks. However, errors from CT-based attenuation correction methods should be carefully evaluated if quantitation of tracer uptake in bone is important.
NASA Astrophysics Data System (ADS)
Habte, Frezghi; Natarajan, Arutselvan; Paik, David S.; Gambhir, Sanjiv S.
2014-03-01
Cerenkov luminescence imaging (CLI) is an emerging cost effective modality that uses conventional small animal optical imaging systems and clinically available radionuclide probes for light emission. CLI has shown good correlation with PET for organs of high uptake such as kidney, spleen, thymus and subcutaneous tumors in mouse models. However, CLI has limitations for deep tissue quantitative imaging since the blue-weighted spectral characteristics of Cerenkov radiation attenuates highly by mammalian tissue. Large organs such as the liver have also shown higher signal due to the contribution of emission of light from a greater thickness of tissue. In this study, we developed a simple model that estimates the effective tissue attenuation coefficient in order to correct the CLI signal intensity with a priori estimated depth and thickness of specific organs. We used several thin slices of ham to build a phantom with realistic attenuation. We placed radionuclide sources inside the phantom at different tissue depths and imaged it using an IVIS Spectrum (Perkin-Elmer, Waltham, MA, USA) and Inveon microPET (Preclinical Solutions Siemens, Knoxville, TN). We also performed CLI and PET of mouse models and applied the proposed attenuation model to correct CLI measurements. Using calibration factors obtained from phantom study that converts the corrected CLI measurements to %ID/g, we obtained an average difference of less that 10% for spleen and less than 35% for liver compared to conventional PET measurements. Hence, the proposed model has a capability of correcting the CLI signal to provide comparable measurements with PET data.
An improved MR sequence for attenuation correction in PET/MR hybrid imaging.
Sagiyama, Koji; Watanabe, Yuji; Kamei, Ryotaro; Shinyama, Daiki; Baba, Shingo; Honda, Hiroshi
2016-04-01
The aim of this study was to investigate the effects of MR parameters on tissue segmentation and determine the optimal MR sequence for attenuation correction in PET/MR hybrid imaging. Eight healthy volunteers were examined using a PET/MR hybrid scanner with six three-dimensional turbo-field-echo sequences for attenuation correction by modifying the echo time, k-space trajectory in the phase-encoding direction, and image contrast. MR images for attenuation correction were obtained from six MR sequences in each session; each volunteer underwent four sessions. Two radiologists assessed the attenuation correction maps generated from the MR images with respect to segmentation errors and ghost artifacts on a five-point scale, and the scores were decided by consensus. Segmentation accuracy and reproducibility were compared. Multiple regression analysis was performed to determine the effects of each MR parameter. The two three-dimensional turbo-field-echo sequences with an in-phase echo time and radial k-space sampling showed the highest total scores for segmentation accuracy, with a high reproducibility. In multiple regression analysis, the score with the shortest echo time (-3.44, P<0.0001) and Cartesian sampling in the anterior/posterior phase-encoding direction (-2.72, P=0.002) was significantly lower than that with in-phase echo time and Cartesian sampling in the right/left phase-encoding direction. Radial k-space sampling provided a significantly higher score (+5.08, P<0.0001) compared with Cartesian sampling. Furthermore, radial sampling improved intrasubject variations in the segmentation score (-8.28%, P=0.002). Image contrast had no significant effect on the total score or reproducibility. These results suggest that three-dimensional turbo-field-echo MR sequences with an in-phase echo time and radial k-space sampling provide improved MR-based attenuation correction maps. PMID:26656909
Attenuation correction of emission PET images with average CT: Interpolation from breath-hold CT
NASA Astrophysics Data System (ADS)
Huang, Tzung-Chi; Zhang, Geoffrey; Chen, Chih-Hao; Yang, Bang-Hung; Wu, Nien-Yun; Wang, Shyh-Jen; Wu, Tung-Hsin
2011-05-01
Misregistration resulting from the difference of temporal resolution in PET and CT scans occur frequently in PET/CT imaging, which causes distortion in tumor quantification in PET. Respiration cine average CT (CACT) for PET attenuation correction has been reported to improve the misalignment effectively by several papers. However, the radiation dose to the patient from a four-dimensional CT scan is relatively high. In this study, we propose a method to interpolate respiratory CT images over a respiratory cycle from inhalation and exhalation breath-hold CT images, and use the average CT from the generated CT set for PET attenuation correction. The radiation dose to the patient is reduced using this method. Six cancer patients of various lesion sites underwent routine free-breath helical CT (HCT), respiration CACT, interpolated average CT (IACT), and 18F-FDG PET. Deformable image registration was used to interpolate the middle phases of a respiratory cycle based on the end-inspiration and end-expiration breath-hold CT scans. The average CT image was calculated from the eight interpolated CT image sets of middle respiratory phases and the two original inspiration and expiration CT images. Then the PET images were reconstructed by these three methods for attenuation correction using HCT, CACT, and IACT. Misalignment of PET image using either CACT or IACT for attenuation correction in PET/CT was improved. The difference in standard uptake value (SUV) from tumor in PET images was most significant between the use of HCT and CACT, while the least significant between the use of CACT and IACT. Besides the similar improvement in tumor quantification compared to the use of CACT, using IACT for PET attenuation correction reduces the radiation dose to the patient.
Hossack, John A.
2013-01-01
Abstract. Intravascular near-infrared fluorescence (NIRF) imaging offers a new approach for characterizing atherosclerotic plaque, but random catheter positioning within the vessel lumen results in variable light attenuation and can yield inaccurate measurements. We hypothesized that NIRF measurements could be corrected for variable light attenuation through blood by tracking the location of the NIRF catheter with intravascular ultrasound (IVUS). In this study, a combined NIRF-IVUS catheter was designed to acquire coregistered NIRF and IVUS data, an automated image processing algorithm was developed to measure catheter-to-vessel wall distances, and depth-dependent attenuation of the fluorescent signal was corrected by an analytical light propagation model. Performance of the catheter sensing distance correction method was evaluated in coronary artery phantoms and ex vivo arteries. The correction method produced NIRF estimates of fluorophore concentrations, in coronary artery phantoms, with an average root mean square error of 17.5%. In addition, the correction method resulted in a statistically significant improvement in correlation between spatially resolved NIRF measurements and known fluorophore spatial distributions in ex vivo arteries (from r=0.24 to 0.69, p<0.01, n=6). This work demonstrates that catheter-to-vessel wall distances, measured from IVUS images, can be employed to compensate for inaccuracies caused by variable intravascular NIRF sensing distances. PMID:23698320
A Cavity Corrected 3D-RISM Functional for Accurate Solvation Free Energies
2014-01-01
We show that an Ng bridge function modified version of the three-dimensional reference interaction site model (3D-RISM-NgB) solvation free energy method can accurately predict the hydration free energy (HFE) of a set of 504 organic molecules. To achieve this, a single unique constant parameter was adjusted to the computed HFE of single atom Lennard-Jones solutes. It is shown that 3D-RISM is relatively accurate at predicting the electrostatic component of the HFE without correction but requires a modification of the nonpolar contribution that originates in the formation of the cavity created by the solute in water. We use a free energy functional with the Ng scaling of the direct correlation function [Ng, K. C. J. Chem. Phys.1974, 61, 2680]. This produces a rapid, reliable small molecule HFE calculation for applications in drug design. PMID:24634616
A Cavity Corrected 3D-RISM Functional for Accurate Solvation Free Energies.
Truchon, Jean-François; Pettitt, B Montgomery; Labute, Paul
2014-03-11
We show that an Ng bridge function modified version of the three-dimensional reference interaction site model (3D-RISM-NgB) solvation free energy method can accurately predict the hydration free energy (HFE) of a set of 504 organic molecules. To achieve this, a single unique constant parameter was adjusted to the computed HFE of single atom Lennard-Jones solutes. It is shown that 3D-RISM is relatively accurate at predicting the electrostatic component of the HFE without correction but requires a modification of the nonpolar contribution that originates in the formation of the cavity created by the solute in water. We use a free energy functional with the Ng scaling of the direct correlation function [Ng, K. C. J. Chem. Phys. 1974, 61, 2680]. This produces a rapid, reliable small molecule HFE calculation for applications in drug design. PMID:24634616
Errors in MR-based attenuation correction for brain imaging with PET/MR scanners
NASA Astrophysics Data System (ADS)
Rota Kops, Elena; Herzog, Hans
2013-02-01
AimAttenuation correction of PET data acquired by hybrid MR/PET scanners remains a challenge, even if several methods for brain and whole-body measurements have been developed recently. A template-based attenuation correction for brain imaging proposed by our group is easy to handle and delivers reliable attenuation maps in a short time. However, some potential error sources are analyzed in this study. We investigated the choice of template reference head among all the available data (error A), and possible skull anomalies of the specific patient, such as discontinuities due to surgery (error B). Materials and methodsAn anatomical MR measurement and a 2-bed-position transmission scan covering the whole head and neck region were performed in eight normal subjects (4 females, 4 males). Error A: Taking alternatively one of the eight heads as reference, eight different templates were created by nonlinearly registering the images to the reference and calculating the average. Eight patients (4 females, 4 males; 4 with brain lesions, 4 w/o brain lesions) were measured in the Siemens BrainPET/MR scanner. The eight templates were used to generate the patients' attenuation maps required for reconstruction. ROI and VOI atlas-based comparisons were performed employing all the reconstructed images. Error B: CT-based attenuation maps of two volunteers were manipulated by manually inserting several skull lesions and filling a nasal cavity. The corresponding attenuation coefficients were substituted with the water's coefficient (0.096/cm). ResultsError A: The mean SUVs over the eight templates pairs for all eight patients and all VOIs did not differ significantly one from each other. Standard deviations up to 1.24% were found. Error B: After reconstruction of the volunteers' BrainPET data with the CT-based attenuation maps without and with skull anomalies, a VOI-atlas analysis was performed revealing very little influence of the skull lesions (less than 3%), while the filled nasal
Fullerton, G D; Keener, C R; Cameron, I L
1994-12-01
The authors describe empirical corrections to ideally dilute expressions for freezing point depression of aqueous solutions to arrive at new expressions accurate up to three molal concentration. The method assumes non-ideality is due primarily to solute/solvent interactions such that the correct free water mass Mwc is the mass of water in solution Mw minus I.M(s) where M(s) is the mass of solute and I an empirical solute/solvent interaction coefficient. The interaction coefficient is easily derived from the constant in the linear regression fit to the experimental plot of Mw/M(s) as a function of 1/delta T (inverse freezing point depression). The I-value, when substituted into the new thermodynamic expressions derived from the assumption of equivalent activity of water in solution and ice, provides accurate predictions of freezing point depression (+/- 0.05 degrees C) up to 2.5 molal concentration for all the test molecules evaluated; glucose, sucrose, glycerol and ethylene glycol. The concentration limit is the approximate monolayer water coverage limit for the solutes which suggests that direct solute/solute interactions are negligible below this limit. This is contrary to the view of many authors due to the common practice of including hydration forces (a soft potential added to the hard core atomic potential) in the interaction potential between solute particles. When this is recognized the two viewpoints are in fundamental agreement. PMID:7699200
Methods of Attenuation Correction for Dual-Wavelength and Dual-Polarization Weather Radar Data
NASA Technical Reports Server (NTRS)
Meneghini, R.; Liao, L.
2007-01-01
In writing the integral equations for the median mass diameter and number concentration, or comparable parameters of the raindrop size distribution, it is apparent that the forms of the equations for dual-polarization and dual-wavelength radar data are identical when attenuation effects are included. The differential backscattering and extinction coefficients appear in both sets of equations: for the dual-polarization equations, the differences are taken with respect to polarization at a fixed frequency while for the dual-wavelength equations, the differences are taken with respect to frequency at a fixed polarization. An alternative to the integral equation formulation is that based on the k-Z (attenuation coefficient-radar reflectivity factor) parameterization. This-technique was originally developed for attenuating single-wavelength radars, a variation of which has been applied to the TRMM Precipitation Radar data (PR). Extensions of this method have also been applied to dual-polarization data. In fact, it is not difficult to show that nearly identical equations are applicable as well to dualwavelength radar data. In this case, the equations for median mass diameter and number concentration take the form of coupled, but non-integral equations. Differences between this and the integral equation formulation are a consequence of the different ways in which attenuation correction is performed under the two formulations. For both techniques, the equations can be solved either forward from the radar outward or backward from the final range gate toward the radar. Although the forward-going solutions tend to be unstable as the attenuation out to the range of interest becomes large in some sense, an independent estimate of path attenuation is not required. This is analogous to the case of an attenuating single-wavelength radar where the forward solution to the Hitschfeld-Bordan equation becomes unstable as the attenuation increases. To circumvent this problem, the
NASA Astrophysics Data System (ADS)
Singh, Malkiat; Bettenhausen, Michael H.
2011-08-01
Faraday rotation changes the polarization plane of linearly polarized microwaves which propagate through the ionosphere. To correct for ionospheric polarization error, it is necessary to have electron density profiles on a global scale that represent the ionosphere in real time. We use raytrace through the combined models of ionospheric conductivity and electron density (ICED), Bent, and Gallagher models (RIBG model) to specify the ionospheric conditions by ingesting the GPS data from observing stations that are as close as possible to the observation time and location of the space system for which the corrections are required. To accurately calculate Faraday rotation corrections, we also utilize the raytrace utility of the RIBG model instead of the normal shell model assumption for the ionosphere. We use WindSat data, which exhibits a wide range of orientations of the raypath and a high data rate of observations, to provide a realistic data set for analysis. The standard single-shell models at 350 and 400 km are studied along with a new three-shell model and compared with the raytrace method for computation time and accuracy. We have compared the Faraday results obtained with climatological (International Reference Ionosphere and RIBG) and physics-based (Global Assimilation of Ionospheric Measurements) ionospheric models. We also study the impact of limitations in the availability of GPS data on the accuracy of the Faraday rotation calculations.
NASA Astrophysics Data System (ADS)
Wu, C.; de Jong, J. R.; Gratama van Andel, H. A.; van der Have, F.; Vastenhouw, B.; Laverman, P.; Boerman, O. C.; Dierckx, R. A. J. O.; Beekman, F. J.
2011-09-01
Attenuation of photon flux on trajectories between the source and pinhole apertures affects the quantitative accuracy of reconstructed single-photon emission computed tomography (SPECT) images. We propose a Chang-based non-uniform attenuation correction (NUA-CT) for small-animal SPECT/CT with focusing pinhole collimation, and compare the quantitative accuracy with uniform Chang correction based on (i) body outlines extracted from x-ray CT (UA-CT) and (ii) on hand drawn body contours on the images obtained with three integrated optical cameras (UA-BC). Measurements in phantoms and rats containing known activities of isotopes were conducted for evaluation. In 125I, 201Tl, 99mTc and 111In phantom experiments, average relative errors comparing to the gold standards measured in a dose calibrator were reduced to 5.5%, 6.8%, 4.9% and 2.8%, respectively, with NUA-CT. In animal studies, these errors were 2.1%, 3.3%, 2.0% and 2.0%, respectively. Differences in accuracy on average between results of NUA-CT, UA-CT and UA-BC were less than 2.3% in phantom studies and 3.1% in animal studies except for 125I (3.6% and 5.1%, respectively). All methods tested provide reasonable attenuation correction and result in high quantitative accuracy. NUA-CT shows superior accuracy except for 125I, where other factors may have more impact on the quantitative accuracy than the selected attenuation correction.
Low-dose interpolated average CT for attenuation correction in cardiac PET/CT
NASA Astrophysics Data System (ADS)
Wu, Tung-Hsin; Zhang, Geoffrey; Wang, Shyh-Jen; Chen, Chih-Hao; Yang, Bang-Hung; Wu, Nien-Yun; Huang, Tzung-Chi
2010-07-01
Because of the advantages in the use of high photon flux and thus the short scan times of CT imaging, the traditional 68Ge scans for positron emission tomography (PET) image attenuation correction have been replaced by CT scans in the modern PET/CT technology. The combination of fast CT scan and slow PET scan often causes image misalignment between the PET and CT images due to respiration motion. Use of the average CT derived from cine CT images is reported to reduce such misalignment. However, the radiation dose to patients is higher with cine CT scans. This study introduces a method that uses breath-hold CT images and their interpolations to generate the average CT for PET image attenuation correction. Breath-hold CT sets are taken at end-inspiration and end-expiration. Deformable image registration is applied to generate a voxel-to-voxel motion matrix between the two CT sets. The motion is equally divided into 5 steps from inspiration to expiration and 5 steps from expiration to inspiration, generating a total of 8 phases of interpolated CT sets. An average CT image is generated from all the 10 phase CT images, including original inhale/exhale CT and 8 interpolated CT sets. Quantitative comparison shows that the reduction of image misalignment artifacts using the average CT from the interpolation technique for PET attenuation correction is at a similar level as that using cine average CT, while the dose to the patient from the CT scans is reduced significantly. The interpolated average CT method hence provides a low dose alternative to cine CT scans for PET attenuation correction.
NASA Astrophysics Data System (ADS)
Gu, Xiao-Yue; Li, Lin; Yin, Peng-Fei; Yun, Ming-Kai; Chai, Pei; Huang, Xian-Chao; Sun, Xiao-Li; Wei, Long
2015-10-01
The Positron Emission Mammography imaging system (PEMi) provides a novel nuclear diagnosis method dedicated for breast imaging. With a better resolution than whole body PET, PEMi can detect millimeter-sized breast tumors. To address the requirement of semi-quantitative analysis with a radiotracer concentration map of the breast, a new attenuation correction method based on a three-dimensional seeded region growing image segmentation (3DSRG-AC) method has been developed. The method gives a 3D connected region as the segmentation result instead of image slices. The continuity property of the segmentation result makes this new method free of activity variation of breast tissues. The threshold value chosen is the key process for the segmentation method. The first valley in the grey level histogram of the reconstruction image is set as the lower threshold, which works well in clinical application. Results show that attenuation correction for PEMi improves the image quality and the quantitative accuracy of radioactivity distribution determination. Attenuation correction also improves the probability of detecting small and early breast tumors. Supported by Knowledge Innovation Project of The Chinese Academy of Sciences (KJCX2-EW-N06)
Yada, Nobuhiro; Onishi, Hideo
2016-01-01
Objective(s): In this study, we aimed to validate the accuracy of computed tomography-based attenuation correction (CTAC), using the bilinear scaling method. Methods: The measured attenuation coefficient (μm) was compared to the theoretical attenuation coefficient (μt), using four different CT scanners and an RMI 467 phantom. The effective energy of CT beam X-rays was calculated, using the aluminum half-value layer method and was used in conjunction with an attenuation coefficient map to convert the CT numbers to μm values for the photon energy of 140 keV. We measured the CT number of RMI 467 phantom for each of the four scanners and compared the μm and μt values for the effective energies of CT beam X-rays, effective atomic numbers, and physical densities. Results: The μm values for CT beam X-rays with low effective energies decreased in high construction elements, compared with CT beam X-rays of high effective energies. As the physical density increased, the μm values elevated linearly. Compared with other scanners, the μm values obtained from the scanner with CT beam X-rays of maximal effective energy increased once the effective atomic number exceeded 10.00. The μm value of soft tissue was equivalent to the μt value. However, the ratios of maximal difference between μm and μt values were 25.4% (lung tissue) and 21.5% (bone tissue), respectively. Additionally, the maximal difference in μm values was 6.0% in the bone tissue for each scanner. Conclusion: The bilinear scaling method could accurately convert CT numbers to μ values in soft tissues. PMID:27408896
NASA Astrophysics Data System (ADS)
Park, Seyoun; Robinson, Adam; Quon, Harry; Kiess, Ana P.; Shen, Colette; Wong, John; Plishker, William; Shekhar, Raj; Lee, Junghoon
2016-03-01
In this paper, we propose a CT-CBCT registration method to accurately predict the tumor volume change based on daily cone-beam CTs (CBCTs) during radiotherapy. CBCT is commonly used to reduce patient setup error during radiotherapy, but its poor image quality impedes accurate monitoring of anatomical changes. Although physician's contours drawn on the planning CT can be automatically propagated to daily CBCTs by deformable image registration (DIR), artifacts in CBCT often cause undesirable errors. To improve the accuracy of the registration-based segmentation, we developed a DIR method that iteratively corrects CBCT intensities by local histogram matching. Three popular DIR algorithms (B-spline, demons, and optical flow) with the intensity correction were implemented on a graphics processing unit for efficient computation. We evaluated their performances on six head and neck (HN) cancer cases. For each case, four trained scientists manually contoured the nodal gross tumor volume (GTV) on the planning CT and every other fraction CBCTs to which the propagated GTV contours by DIR were compared. The performance was also compared with commercial image registration software based on conventional mutual information (MI), VelocityAI (Varian Medical Systems Inc.). The volume differences (mean±std in cc) between the average of the manual segmentations and automatic segmentations are 3.70+/-2.30 (B-spline), 1.25+/-1.78 (demons), 0.93+/-1.14 (optical flow), and 4.39+/-3.86 (VelocityAI). The proposed method significantly reduced the estimation error by 9% (B-spline), 38% (demons), and 51% (optical flow) over the results using VelocityAI. Although demonstrated only on HN nodal GTVs, the results imply that the proposed method can produce improved segmentation of other critical structures over conventional methods.
Ai, Hua; Pan, Tinsu
2015-01-01
Cardiac imaging is a promising application for combined PET/MR imaging. However, current MR imaging protocols for whole-body attenuation correction can produce spatial mismatch between PET and MR-derived attenuation data owing to a disparity between the two modalities' imaging speeds. We assessed the feasibility of using a respiration-averaged MR (AMR) method for attenuation correction of cardiac PET data in PET/MR images. First, to demonstrate the feasibility of motion imaging with MR, we used a 3T MR system and a two-dimensional fast spoiled gradient-recalled echo (SPGR) sequence to obtain AMR images ofa moving phantom. Then, we used the same sequence to obtain AMR images of a patient's thorax under free-breathing conditions. MR images were converted into PET attenuation maps using a three-class tissue segmentation method with two sets of predetermined CT numbers, one calculated from the patient-specific (PS) CT images and the other from a reference group (RG) containing 54 patient CT datasets. The MR-derived attenuation images were then used for attenuation correction of the cardiac PET data, which were compared to the PET data corrected with average CT (ACT) images. In the myocardium, the voxel-by-voxel differences and the differences in mean slice activity between the AMR-corrected PET data and the ACT-corrected PET data were found to be small (less than 7%). The use of AMR-derived attenuation images in place of ACT images for attenuation correction did not affect the summed stress score. These results demonstrate the feasibility of using the proposed SPGR-based MR imaging protocol to obtain patient AMR images and using those images for cardiac PET attenuation correction. Additional studies with more clinical data are warranted to further evaluate the method. PMID:26218995
Correction for multiple scattering of unpolarized photons in attenuation coefficient measurements
Fernandez, J.E.; Sumini, M.; Satori, R.
1995-01-01
Calculations of the diffusion of unpolarized photons in thin thickness targets have been performed with recourse to a vector transport model taking rigorously into account the polarization introduced by the scattering interactions. An order-of-interactions solution of the Boltzmann transport equation for photons was used to describe the multiple scattering terms due to the prevailing effects in the X-ray regime. An analytical expression for the correction factor to the attenuation coefficient is given in term of the solid angle subtended by the detector and the energy interval characterizing the detection response. Although the main corrections are due to the influence of the pure Rayleigh effect, first- and second-order chains involving the Rayleigh and Compton effects have been considered as possible sources of overlapping contributions to the transmitted intensity. The extent of the corrections is estimated and some examples are given for pure element targets.
Attenuated MP2 with a Long-Range Dispersion Correction for Treating Nonbonded Interactions.
Goldey, Matthew B; Belzunces, Bastien; Head-Gordon, Martin
2015-09-01
Attenuated second order Møller-Plesset theory (MP2) captures intermolecular binding energies at equilibrium geometries with high fidelity with respect to reference methods, yet must fail to reproduce dispersion energies at stretched geometries due to the removal of fully long-range dispersion. For this problem to be ameliorated, long-range correction using the VV10 van der Waals density functional is added to attenuated MP2, capturing short-range correlation with attenuated MP2 and long-range dispersion with VV10. Attenuated MP2 with long-range VV10 dispersion in the aug-cc-pVTZ (aTZ) basis set, MP2-V(terfc, aTZ), is parametrized for noncovalent interactions using the S66 database and tested on a variety of noncovalent databases, describing potential energy surfaces and equilibrium binding energies equally well. Further, a spin-component scaled (SCS) version, SCS-MP2-V(2terfc, aTZ), is produced using the W4-11 database as a supplemental thermochemistry training set, and the resulting method reproduces the quality of MP2-V(terfc, aTZ) for noncovalent interactions and exceeds the performance of SCS-MP2/aTZ for thermochemistry. PMID:26575911
What is the benefit of CT-based attenuation correction in myocardial perfusion SPET?
Apostolopoulos, Dimitrios J; Savvopoulos, Christos
2016-01-01
In multimodality imaging, CT-derived transmission maps are used for attenuation correction (AC) of SPET or PET data. Regarding SPET myocardial perfusion imaging (MPI), however, the bene����t of CT-based AC (CT-AC) has been questioned. Although most attenuation-related artifacts are removed by this technique, new false defects may appear while some true perfusion abnormalities may be masked. The merits and the drawbacks of CT-AC in MPI SPET are reviewed and discussed in this editorial. In conclusion, CT-AC is most helpful in men, overweight in particular, and in those with low or low to intermediate pre-test probability of coronary artery disease (CAD). It is also useful for the evaluation of myocardial viability. In high-risk patients though, CT-AC may underestimate the presence or the extent of CAD. In any case, corrected and non-corrected images should be viewed side-by-side and both considered in the interpretation of the study. PMID:27331200
Filter Paper: Solution to High Self-Attenuation Corrections in HEPA Filter Measurements
Oberer, R.B.; Harold, N.B.; Gunn, C.A.; Brummett, M.; Chaing, L.G.
2005-10-01
An 8 by 8 by 6 inch High Efficiency Particulate Air (HEPA) filter was measured as part of a uranium holdup survey in June of 2005 as it has been routinely measured every two months since 1998. Although the survey relies on gross gamma count measurements, this was one of a few measurements that had been converted to a quantitative measurement in 1998. The measurement was analyzed using the traditional Generalized Geometry Holdup (GGH) approach, using HMS3 software, with an area calibration and self-attenuation corrected with an empirical correction factor of 1.06. A result of 172 grams of {sup 235}U was reported. The actual quantity of {sup 235}U in the filter was approximately 1700g. Because of this unusually large discrepancy, the measurement of HEPA filters will be discussed. Various techniques for measuring HEPA filters will be described using the measurement of a 24 by 24 by 12 inch HEPA filter as an example. A new method to correct for self attenuation will be proposed for this measurement Following the discussion of the 24 by 24 by 12 inch HEPA filter, the measurement of the 8 by 8 by 6 inch will be discussed in detail.
Attenuation correction in emission tomography using the emission data—A review
Li, Yusheng
2016-01-01
The problem of attenuation correction (AC) for quantitative positron emission tomography (PET) had been considered solved to a large extent after the commercial availability of devices combining PET with computed tomography (CT) in 2001; single photon emission computed tomography (SPECT) has seen a similar development. However, stimulated in particular by technical advances toward clinical systems combining PET and magnetic resonance imaging (MRI), research interest in alternative approaches for PET AC has grown substantially in the last years. In this comprehensive literature review, the authors first present theoretical results with relevance to simultaneous reconstruction of attenuation and activity. The authors then look back at the early history of this research area especially in PET; since this history is closely interwoven with that of similar approaches in SPECT, these will also be covered. We then review algorithmic advances in PET, including analytic and iterative algorithms. The analytic approaches are either based on the Helgason–Ludwig data consistency conditions of the Radon transform, or generalizations of John’s partial differential equation; with respect to iterative methods, we discuss maximum likelihood reconstruction of attenuation and activity (MLAA), the maximum likelihood attenuation correction factors (MLACF) algorithm, and their offspring. The description of methods is followed by a structured account of applications for simultaneous reconstruction techniques: this discussion covers organ-specific applications, applications specific to PET/MRI, applications using supplemental transmission information, and motion-aware applications. After briefly summarizing SPECT applications, we consider recent developments using emission data other than unscattered photons. In summary, developments using time-of-flight (TOF) PET emission data for AC have shown promising advances and open a wide range of applications. These techniques may both remedy
NASA Technical Reports Server (NTRS)
Emery, William J.; Yu, Yunyue; Wick, Gary A.; Schluessel, Peter; Reynolds, Richard W.
1994-01-01
A new satellite sea surface temperature (SST) algorithm is developed that uses nearly coincident measurements from the microwave special sensor microwave imager (SSM/I) to correct for atmospheric moisture attenuation of the infrared signal from the advanced very high resolution radiometer (AVHRR). This new SST algorithm is applied to AVHRR imagery from the South Pacific and Norwegian seas, which are then compared with simultaneous in situ (ship based) measurements of both skin and bulk SST. In addition, an SST algorithm using a quadratic product of the difference between the two AVHRR thermal infrared channels is compared with the in situ measurements. While the quadratic formulation provides a considerable improvement over the older cross product (CPSST) and multichannel (MCSST) algorithms, the SSM/I corrected SST (called the water vapor or WVSST) shows overall smaller errors when compared to both the skin and bulk in situ SST observations. Applied to individual AVHRR images, the WVSST reveals an SST difference pattern (CPSST-WVSST) similar in shape to the water vapor structure while the CPSST-quadratic SST difference appears unrelated in pattern to the nearly coincident water vapor pattern. An application of the WVSST to week-long composites of global area coverage (GAC) AVHRR data demonstrates again the manner in which the WVSST corrects the AVHRR for atmospheric moisture attenuation. By comparison the quadratic SST method underestimates the SST corrections in the lower latitudes and overestimates the SST in th e higher latitudes. Correlations between the AVHRR thermal channel differences and the SSM/I water vapor demonstrate the inability of the channel difference to represent water vapor in the midlatitude and high latitudes during summer. Compared against drifting buoy data the WVSST and the quadratic SST both exhibit the same general behavior with the relatively small differences with the buoy temperatures.
NASA Astrophysics Data System (ADS)
Richter, K.; Blaskow, R.; Stelling, N.; Maas, H.-G.
2015-08-01
The characterization of the vertical forest structure is highly relevant for ecological research and for better understanding forest ecosystems. Full-waveform airborne laser scanner systems providing a complete time-resolved digitization of every laser pulse echo may deliver very valuable information on the biophysical structure in forest stands. To exploit the great potential offered by full-waveform airborne laser scanning data, the development of suitable voxel based data analysis methods is straightforward. Beyond extracting additional 3D points, it is very promising to derive voxel attributes from the digitized waveform directly. However, the 'history' of each laser pulse echo is characterized by attenuation effects caused by reflections in higher regions of the crown. As a result, the received waveform signals within the canopy have a lower amplitude than it would be observed for an identical structure without the previous canopy structure interactions (Romanczyk et al., 2012). To achieve a radiometrically correct voxel space representation, the loss of signal strength caused by partial reflections on the path of a laser pulse through the canopy has to be compensated by applying suitable attenuation correction models. The basic idea of the correction procedure is to enhance the waveform intensity values in lower parts of the canopy for portions of the pulse intensity, which have been reflected in higher parts of the canopy. To estimate the enhancement factor an appropriate reference value has to be derived from the data itself. Based on pulse history correction schemes presented in previous publications, the paper will discuss several approaches for reference value estimation. Furthermore, the results of experiments with two different data sets (leaf-on/leaf-off) are presented.
Evaluation of the Effect of Attenuation Correction by External CT in a Semiconductor SPECT.
Uchibe, Taku; Miyai, Masahiro; Yata, Nobuhiro; Haramoto, Masuo; Yamamoto, Yasushi; Nakamura, Megumi; Kitagaki, Hajime; Takahashi, Yasuyuki
2016-07-01
The discovery of NM530c with a cadmium-zinc-telluride detector (CdZnTe-SPECT) is superior to the conventional Anger-type SPECT with a sodium-iodide detector (NaI-SPECT) in terms of sensitivity and spatial resolution. However, in the clinical example, even in CdZnTe-SPECT, a count decrease in myocardium due to the attenuation of the gamma ray is an issue. This study was conducted to evaluate the effect of computed tomography attenuation correction (CTAC) in CdZnTe-SPECT with the help of external CT. We evaluated the revision effect of uniformity, influence by the difference in attenuation distance, contrast ratio, an uptake rate using the heart phantom. As a result of the phantom studies, a good revision effect was obtained. In the clinical study, there was a statistical significant difference between the contrast ratio before and after CTAC in the inferior wall. In addition, the contrast ratio before and after CTAC in CdZnTe-SPECT image was equal to those of NaI-SPECT image. It was suggested that CTAC using external CT in CdZnTe-SPECT was clinically useful for inferior wall. PMID:27440705
Towards improved hardware component attenuation correction in PET/MR hybrid imaging
NASA Astrophysics Data System (ADS)
Paulus, D. H.; Tellmann, L.; Quick, H. H.
2013-11-01
In positron emission tomography/computed tomography (PET/CT) hybrid imaging attenuation correction (AC) of the patient tissue and patient table is performed by converting the CT-based Hounsfield units (HU) to linear attenuation coefficients (LAC) of PET. When applied to the new field of hardware component AC in PET/magnetic resonance (MR) hybrid imaging, this conversion method may result in local overcorrection of PET activity values. The aim of this study thus was to optimize the conversion parameters for CT-based AC of hardware components in PET/MR. Systematic evaluation and optimization of the HU to LAC conversion parameters has been performed for the hardware component attenuation map (µ-map) of a flexible radiofrequency (RF) coil used in PET/MR imaging. Furthermore, spatial misregistration of this RF coil to its µ-map was simulated by shifting the µ-map in different directions and the effect on PET quantification was evaluated. Measurements of a PET NEMA standard emission phantom were performed on an integrated hybrid PET/MR system. Various CT parameters were used to calculate different µ-maps for the flexible RF coil and to evaluate the impact on the PET activity concentration. A 511 keV transmission scan of the local RF coil was used as standard of reference to adapt the slope of the conversion from HUs to LACs at 511 keV. The average underestimation of the PET activity concentration due to the non-attenuation corrected RF coil in place was calculated to be 5.0% in the overall phantom. When considering attenuation only in the upper volume of the phantom, the average difference to the reference scan without RF coil is 11.0%. When the PET/CT conversion is applied, an average overestimation of 3.1% (without extended CT scale) and 4.2% (with extended CT scale) is observed in the top volume of the NEMA phantom. Using the adapted conversion resulting from this study, the deviation in the top volume of the phantom is reduced to -0.5% and shows the lowest
Jodłowski, Paweł
2016-03-01
The Cutshall transmission method of determination of self-attenuation correction in (210)Pb measurements by gamma-spectrometry gives the results burdened with errors of up to 10%. The author proposes introducing into the Cutshall correction Cs,Cuts an additional revision factor CCs,Cuts to eliminate errors. The proposed formula of the revision factor describes the CCs,Cuts value depending on the experimentally obtained Cs,Cuts correction. Formula holds true in wide ranges of the measurement geometries and linear attenuation coefficients of both the standard and the sample. PMID:26702546
Téllez, Helena; Druce, John; Hong, Jong-Eun; Ishihara, Tatsumi; Kilner, John A
2015-03-01
The accuracy and precision of isotopic analysis in Time-of-Flight secondary ion mass spectrometry (ToF-SIMS) relies on the appropriate reduction of the dead-time and detector saturation effects, especially when analyzing species with high ion yields or present in high concentrations. Conventional approaches to avoid these problems are based on Poisson dead-time correction and/or an overall decrease of the total secondary ion intensity by reducing the target current. This ultimately leads to poor detection limits for the minor isotopes and high uncertainties of the measured isotopic ratios. An alternative strategy consists of the attenuation of those specific secondary ions that saturate the detector, providing an effective extension of the linear dynamic range. In this work, the selective attenuation of secondary ion signals (SASI) approach is applied to the study of oxygen transport properties in electroceramic materials by isotopic labeling with stable (18)O tracer and ToF-SIMS depth profiling. The better analytical performance in terms of accuracy and precision allowed a more reliable determination of the oxygen surface exchange and diffusion coefficients while maintaining good mass resolution and limits of detection for other minor secondary ion species. This improvement is especially relevant to understand the ionic transport mechanisms and properties of solid materials, such as the parallel diffusion pathways (e.g., oxygen diffusion through bulk, grain boundary, or dislocations) in electroceramic materials with relevant applications in energy storage and conversion devices. PMID:25647357
Dey, Joyoni; Segars, W. Paul; Pretorius, P. Hendrik; King, Michael A.
2015-01-01
Purpose We investigate the differences without/with respiratory motion correction in apparent imaging agent localization induced in reconstructed emission images when the attenuation maps used for attenuation correction (from CT) are misaligned with the patient anatomy during emission imaging due to differences in respiratory state. Methods We investigated use of attenuation maps acquired at different states of a 2 cm amplitude respiratory cycle (at end-expiration, at end-inspiration, the center map, the average transmission map, and a large breath-hold beyond range of respiration during emission imaging) to correct for attenuation in MLEM reconstruction for several anatomical variants of the NCAT phantom which included both with and without non-rigid motion between heart and sub-diaphragmatic regions (such as liver, kidneys etc). We tested these cases with and without emission motion correction and attenuation map alignment/non-alignment. Results For the NCAT default male anatomy the false count-reduction due to breathing was largely removed upon emission motion correction for the large majority of the cases. Exceptions (for the default male) were for the cases when using the large-breathhold end-inspiration map (TI_EXT), when we used the end-expiration (TE) map, and to a smaller extent, the end-inspiration map (TI). However moving the attenuation maps rigidly to align the heart region, reduced the remaining count-reduction artifacts. For the female patient count-reduction remained post motion correction using rigid map-alignment due to the breast soft-tissue misalignment. Quantitatively, after the transmission (rigid) alignment correction, the polar-map 17-segment RMS error with respect to the reference (motion-less case) reduced by 46.5% on average for the extreme breathhold case. The reductions were 40.8% for end-expiration map and 31.9% for end-inspiration cases on the average, comparable to the semi-ideal case where each state uses its own attenuation map for
ERIC Educational Resources Information Center
Sheen, Younghee; Wright, David; Moldawa, Anna
2009-01-01
Building on Sheen's (2007) study of the effects of written corrective feedback (CF) on the acquisition of English articles, this article investigated whether direct focused CF, direct unfocused CF and writing practice alone produced differential effects on the accurate use of grammatical forms by adult ESL learners. Using six intact adult ESL…
Attenuation correction in SPECT using consistency conditions for the exponential ray transform.
Mennessier, C; Noo, F; Clackdoyle, R; Bal, G; Desbat, L
1999-10-01
Using data consistency conditions for the exponential ray transform, a method is derived to correct SPECT data for attenuation effects. No transmission measurements are required, and no operator-defined contours are needed. Furthermore, any 3D parallel-ray geometry can be considered for SPECT data acquisition, even unconventional geometries which do not lead to a set of 2D parallel-beam sinograms. The method is presented for both the 2D parallel-beam geometry and a particular 3D case, called the rotating slant hole geometry. Full details of the algorithms are given. Implementation has been carried out and results are presented in 2D and in 3D using simulated data. PMID:10533924
Minato, K; Tang, Y N; Bennett, G W; Brill, A
1987-01-01
Attenuation correction for single-photon emission computed tomography (SPECT) usually assumes a uniform attenuation distribution within the body surface contour. Previous methods to estimate this contour have used thresholding of a reconstructed section image. This method is often very sensitive to the selection of a threshold value, especially for nonuniform activity distributions within the body. We have proposed the "fixed-point Hachimura-Kuwahara filter" to extract contour primitives from SPECT images. The Hachimura-Kuwahara filter, which preserves edges but smoothes nonedge regions, is applied repeatedly to identify the invariant set-the fixed-point image-which is unchanged by this nonlinear, two-dimensional filtering operation. This image usually becomes a piecewise constant array. In order to detect the contour, the tracing algorithm based on the minimum distance connection criterion is applied to the extracted contour primitives. This procedure does not require choice of a threshold value in determining the contour. SPECT data from a water-filled elliptical phantom containing three sources was obtained and scattered projections were reconstructed. The automatic edge detection procedure was applied to the scattered window reconstruction, resulting in a reasonable outline of the phantom. PMID:18230438
An analytical algorithm for skew-slit imaging geometry with nonuniform attenuation correction
Huang Qiu; Zeng, Gengsheng L.
2006-04-15
The pinhole collimator is currently the collimator of choice in small animal single photon emission computed tomography (SPECT) imaging because it can provide high spatial resolution and reasonable sensitivity when the animal is placed very close to the pinhole. It is well known that if the collimator rotates around the object (e.g., a small animal) in a circular orbit to form a cone-beam imaging geometry with a planar trajectory, the acquired data are not sufficient for an exact artifact-free image reconstruction. In this paper a novel skew-slit collimator is mounted instead of the pinhole collimator in order to significantly reduce the image artifacts caused by the geometry. The skew-slit imaging geometry is a more generalized version of the pinhole imaging geometry. The multiple pinhole geometry can also be extended to the multiple-skew-slit geometry. An analytical algorithm for image reconstruction based on the tilted fan-beam inversion is developed with nonuniform attenuation compensation. Numerical simulation shows that the axial artifacts are evidently suppressed in the skew-slit images compared to the pinhole images and the attenuation correction is effective.
NASA Astrophysics Data System (ADS)
Schramm, G.; Maus, J.; Hofheinz, F.; Petr, J.; Lougovski, A.; Beuthien-Baumann, B.; Platzek, I.; van den Hoff, J.
2014-06-01
The aim of this paper is to describe a new automatic method for compensation of metal-implant-induced segmentation errors in MR-based attenuation maps (MRMaps) and to evaluate the quantitative influence of those artifacts on the reconstructed PET activity concentration. The developed method uses a PET-based delineation of the patient contour to compensate metal-implant-caused signal voids in the MR scan that is segmented for PET attenuation correction. PET emission data of 13 patients with metal implants examined in a Philips Ingenuity PET/MR were reconstructed with the vendor-provided method for attenuation correction (MRMaporig, PETorig) and additionally with a method for attenuation correction (MRMapcor, PETcor) developed by our group. MRMaps produced by both methods were visually inspected for segmentation errors. The segmentation errors in MRMaporig were classified into four classes (L1 and L2 artifacts inside the lung and B1 and B2 artifacts inside the remaining body depending on the assigned attenuation coefficients). The average relative SUV differences (\\varepsilon _{rel}^{av}) between PETorig and PETcor of all regions showing wrong attenuation coefficients in MRMaporig were calculated. Additionally, relative SUVmean differences (ɛrel) of tracer accumulations in hot focal structures inside or in the vicinity of these regions were evaluated. MRMaporig showed erroneous attenuation coefficients inside the regions affected by metal artifacts and inside the patients' lung in all 13 cases. In MRMapcor, all regions with metal artifacts, except for the sternum, were filled with the soft-tissue attenuation coefficient and the lung was correctly segmented in all patients. MRMapcor only showed small residual segmentation errors in eight patients. \\varepsilon _{rel}^{av} (mean ± standard deviation) were: ( - 56 ± 3)% for B1, ( - 43 ± 4)% for B2, (21 ± 18)% for L1, (120 ± 47)% for L2 regions. ɛrel (mean ± standard deviation) of hot focal structures were
Šoštarić, Marko; Babić, Dinko; Petrinec, Branko; Zgorelec, Željka
2016-07-01
We develop a simple and widely applicable method for determining the self-attenuation correction in gamma-ray spectrometry on environmental samples. The method relies on measurements of the transmission of photons over the matrices of a calibration standard and an analysed sample. Results of this experiment are used in subsequent Monte Carlo simulations in which we first determine the linear attenuation coefficients (μ) of the two matrices and then the self-attenuation correction for the analysed sample. The method is validated by reproducing, over a wide energy range, the literature data for the μ of water. We demonstrate the use of the method on a sample of sand, for which we find that the correction is considerable below ~400keV, where many naturally occurring radionuclides emit gamma rays. At the lowest inspected energy (~60keV), one measures an activity that is by a factor of ~1.8 smaller than its true value. PMID:27157125
Method for transforming CT images for attenuation correction in PET/CT imaging
Carney, Jonathan P.J.; Townsend, David W.; Rappoport, Vitaliy; Bendriem, Bernard
2006-04-15
A tube-voltage-dependent scheme is presented for transforming Hounsfield units (HU) measured by different computed tomography (CT) scanners at different x-ray tube voltages (kVp) to 511 keV linear attenuation values for attenuation correction in positron emission tomography (PET) data reconstruction. A Gammex 467 electron density CT phantom was imaged using a Siemens Sensation 16-slice CT, a Siemens Emotion 6-slice CT, a GE Lightspeed 16-slice CT, a Hitachi CXR 4-slice CT, and a Toshiba Aquilion 16-slice CT at kVp ranging from 80 to 140 kVp. All of these CT scanners are also available in combination with a PET scanner as a PET/CT tomograph. HU obtained for various reference tissue substitutes in the phantom were compared with the known linear attenuation values at 511 keV. The transformation, appropriate for lung, soft tissue, and bone, yields the function 9.6x10{sup -5}{center_dot}(HU+1000) below a threshold of {approx}50 HU and a{center_dot}(HU+1000)+b above the threshold, where a and b are fixed parameters that depend on the kVp setting. The use of the kVp-dependent scaling procedure leads to a significant improvement in reconstructed PET activity levels in phantom measurements, resolving errors of almost 40% otherwise seen for the case of dense bone phantoms at 80 kVp. Results are also presented for patient studies involving multiple CT scans at different kVp settings, which should all lead to the same 511 keV linear attenuation values. A linear fit to values obtained from 140 kVp CT images using the kVp-dependent scaling plotted as a function of the corresponding values obtained from 80 kVp CT images yielded y=1.003x-0.001 with an R{sup 2} value of 0.999, indicating that the same values are obtained to a high degree of accuracy.
Accurate elevation and normal moveout corrections of seismic reflection data on rugged topography
Liu, J.; Xia, J.; Chen, C.; Zhang, G.
2005-01-01
The application of the seismic reflection method is often limited in areas of complex terrain. The problem is the incorrect correction of time shifts caused by topography. To apply normal moveout (NMO) correction to reflection data correctly, static corrections are necessary to be applied in advance for the compensation of the time distortions of topography and the time delays from near-surface weathered layers. For environment and engineering investigation, weathered layers are our targets, so that the static correction mainly serves the adjustment of time shifts due to an undulating surface. In practice, seismic reflected raypaths are assumed to be almost vertical through the near-surface layers because they have much lower velocities than layers below. This assumption is acceptable in most cases since it results in little residual error for small elevation changes and small offsets in reflection events. Although static algorithms based on choosing a floating datum related to common midpoint gathers or residual surface-consistent functions are available and effective, errors caused by the assumption of vertical raypaths often generate pseudo-indications of structures. This paper presents the comparison of applying corrections based on the vertical raypaths and bias (non-vertical) raypaths. It also provides an approach of combining elevation and NMO corrections. The advantages of the approach are demonstrated by synthetic and real-world examples of multi-coverage seismic reflection surveys on rough topography. ?? The Royal Society of New Zealand 2005.
Towards improved hardware component attenuation correction in PET/MR hybrid imaging.
Paulus, D H; Tellmann, L; Quick, H H
2013-11-21
In positron emission tomography/computed tomography (PET/CT) hybrid imaging attenuation correction (AC) of the patient tissue and patient table is performed by converting the CT-based Hounsfield units (HU) to linear attenuation coefficients (LAC) of PET. When applied to the new field of hardware component AC in PET/magnetic resonance (MR) hybrid imaging, this conversion method may result in local overcorrection of PET activity values. The aim of this study thus was to optimize the conversion parameters for CT-based AC of hardware components in PET/MR. Systematic evaluation and optimization of the HU to LAC conversion parameters has been performed for the hardware component attenuation map (µ-map) of a flexible radiofrequency (RF) coil used in PET/MR imaging. Furthermore, spatial misregistration of this RF coil to its µ-map was simulated by shifting the µ-map in different directions and the effect on PET quantification was evaluated. Measurements of a PET NEMA standard emission phantom were performed on an integrated hybrid PET/MR system. Various CT parameters were used to calculate different µ-maps for the flexible RF coil and to evaluate the impact on the PET activity concentration. A 511 keV transmission scan of the local RF coil was used as standard of reference to adapt the slope of the conversion from HUs to LACs at 511 keV. The average underestimation of the PET activity concentration due to the non-attenuation corrected RF coil in place was calculated to be 5.0% in the overall phantom. When considering attenuation only in the upper volume of the phantom, the average difference to the reference scan without RF coil is 11.0%. When the PET/CT conversion is applied, an average overestimation of 3.1% (without extended CT scale) and 4.2% (with extended CT scale) is observed in the top volume of the NEMA phantom. Using the adapted conversion resulting from this study, the deviation in the top volume of the phantom is reduced to -0.5% and shows the lowest
Attenuation of near-surface diffracted energy in deep seismic data by DMO correction
NASA Astrophysics Data System (ADS)
Klinkby, Lone; Pedersen, Morten Wendell
1998-03-01
Seismic data are often contaminated by scattered waves from shallow diffractors such as offshore installations and structural irregularities. As the waves travel in the water layer and shallow sub-bottom they are damped considerably less than near-vertical reflected waves. Far from the diffractors the stacking velocity of the noise will be nearly identical to the stacking velocities of the primary reflections. This implies that CMP stacking of normal-moveout corrected data does not suppress the noise, and the necessary attenuation will typically be done separately by prestack 2D velocity-filtering or array simulation in the shot and receiver domains. However, by changing the moveout of the diffraction curves through DMO correction, the stacking velocity of the noise will be close to the true velocity of the diffracted waves, and suppression through CMP stacking is possible. As DMO is related to CDP smearing, which is a marginal problem for deep seismic data, it is normally not used as a part of the standard processing schemes. The noise suppression features of the DMO processor are demonstrated with a data example from the North Sea.
NASA Astrophysics Data System (ADS)
Nam, Woo Hyun; Ahn, Il Jun; Kim, Kyeong Min; Kim, Byung Il; Ra, Jong Beom
2013-10-01
Positron emission tomography (PET) is widely used for diagnosis and follow up assessment of radiotherapy. However, thoracic and abdominal PET suffers from false staging and incorrect quantification of the radioactive uptake of lesion(s) due to respiratory motion. Furthermore, respiratory motion-induced mismatch between a computed tomography (CT) attenuation map and PET data often leads to significant artifacts in the reconstructed PET image. To solve these problems, we propose a unified framework for respiratory-matched attenuation correction and motion compensation of respiratory-gated PET. For the attenuation correction, the proposed algorithm manipulates a 4D CT image virtually generated from two low-dose inhale and exhale CT images, rather than a real 4D CT image which significantly increases the radiation burden on a patient. It also utilizes CT-driven motion fields for motion compensation. To realize the proposed algorithm, we propose an improved region-based approach for non-rigid registration between body CT images, and we suggest a selection scheme of 3D CT images that are respiratory-matched to each respiratory-gated sinogram. In this work, the proposed algorithm was evaluated qualitatively and quantitatively by using patient datasets including lung and/or liver lesion(s). Experimental results show that the method can provide much clearer organ boundaries and more accurate lesion information than existing algorithms by utilizing two low-dose CT images.
Saha, Krishnendu; Hoyt, Sean C.; Murray, Bryon M.
2016-01-01
The acquisition and processing of the Jaszczak phantom is a recommended test by the American College of Radiology for evaluation of gamma camera system performance. To produce the reconstructed phantom image for quality evaluation, attenuation correction is applied. The attenuation of counts originating from the center of the phantom is greater than that originating from the periphery of the phantom causing an artifactual appearance of inhomogeneity in the reconstructed image and complicating phantom evaluation. Chang's mathematical formulation is a common method of attenuation correction applied on most gamma cameras that do not require an external transmission source such as computed tomography, radionuclide sources installed within the gantry of the camera or a flood source. Tomographic acquisition can be obtained in two different acquisition modes for dual-detector gamma camera; one where the two detectors are at 180° configuration and acquire projection images for a full 360°, and the other where the two detectors are positioned at a 90° configuration and acquire projections for only 180°. Though Chang's attenuation correction method has been used for 360° angle acquisition, its applicability for 180° angle acquisition remains a question with one vendor's camera software producing artifacts in the images. This work investigates whether Chang's attenuation correction technique can be applied to both acquisition modes by the development of a Chang's formulation-based algorithm that is applicable to both modes. Assessment of attenuation correction performance by phantom uniformity analysis illustrates improved uniformity with the proposed algorithm (22.6%) compared to the camera software (57.6%). PMID:27051167
Saha, Krishnendu; Hoyt, Sean C; Murray, Bryon M
2016-01-01
The acquisition and processing of the Jaszczak phantom is a recommended test by the American College of Radiology for evaluation of gamma camera system performance. To produce the reconstructed phantom image for quality evaluation, attenuation correction is applied. The attenuation of counts originating from the center of the phantom is greater than that originating from the periphery of the phantom causing an artifactual appearance of inhomogeneity in the reconstructed image and complicating phantom evaluation. Chang's mathematical formulation is a common method of attenuation correction applied on most gamma cameras that do not require an external transmission source such as computed tomography, radionuclide sources installed within the gantry of the camera or a flood source. Tomographic acquisition can be obtained in two different acquisition modes for dual-detector gamma camera; one where the two detectors are at 180° configuration and acquire projection images for a full 360°, and the other where the two detectors are positioned at a 90° configuration and acquire projections for only 180°. Though Chang's attenuation correction method has been used for 360° angle acquisition, its applicability for 180° angle acquisition remains a question with one vendor's camera software producing artifacts in the images. This work investigates whether Chang's attenuation correction technique can be applied to both acquisition modes by the development of a Chang's formulation-based algorithm that is applicable to both modes. Assessment of attenuation correction performance by phantom uniformity analysis illustrates improved uniformity with the proposed algorithm (22.6%) compared to the camera software (57.6%). PMID:27051167
NASA Astrophysics Data System (ADS)
Manduci, L.; Tenailleau, L.; Trolet, J. L.; De Vismes, A.; Lopez, G.; Piccione, M.
2010-01-01
The mass attenuation coefficients for a number of marine and terrestrial bioindicators were measured using γ spectrometry for energies between 22 and 80 keV. These values were then used to find the correction factor k for the apparent radioactivity. The experimental results were compared with a Monte Carlo simulation performed using PENELOPE in order to evaluate the reliability of the simplified calculation and to determine the correction factors.
Improved UTE-based attenuation correction for cranial PET-MR using dynamic magnetic field monitoring
Aitken, A. P.; Giese, D.; Tsoumpas, C.; Schleyer, P.; Kozerke, S.; Prieto, C.; Schaeffter, T.
2014-01-15
Purpose: Ultrashort echo time (UTE) MRI has been proposed as a way to produce segmented attenuation maps for PET, as it provides contrast between bone, air, and soft tissue. However, UTE sequences require samples to be acquired during rapidly changing gradient fields, which makes the resulting images prone to eddy current artifacts. In this work it is demonstrated that this can lead to misclassification of tissues in segmented attenuation maps (AC maps) and that these effects can be corrected for by measuring the true k-space trajectories using a magnetic field camera. Methods: The k-space trajectories during a dual echo UTE sequence were measured using a dynamic magnetic field camera. UTE images were reconstructed using nominal trajectories and again using the measured trajectories. A numerical phantom was used to demonstrate the effect of reconstructing with incorrect trajectories. Images of an ovine leg phantom were reconstructed and segmented and the resulting attenuation maps were compared to a segmented map derived from a CT scan of the same phantom, using the Dice similarity measure. The feasibility of the proposed method was demonstrated inin vivo cranial imaging in five healthy volunteers. Simulated PET data were generated for one volunteer to show the impact of misclassifications on the PET reconstruction. Results: Images of the numerical phantom exhibited blurring and edge artifacts on the bone–tissue and air–tissue interfaces when nominal k-space trajectories were used, leading to misclassification of soft tissue as bone and misclassification of bone as air. Images of the tissue phantom and thein vivo cranial images exhibited the same artifacts. The artifacts were greatly reduced when the measured trajectories were used. For the tissue phantom, the Dice coefficient for bone in MR relative to CT was 0.616 using the nominal trajectories and 0.814 using the measured trajectories. The Dice coefficients for soft tissue were 0.933 and 0.934 for the
Fick, Steven E; Ruggles, Dorea
2006-01-01
Radiation force balance (RFB) measurements of time-averaged, spatially-integrated ultrasound power transmitted into a reflectionless water load are based on measurements of the power received by the RFB target. When conical targets are used to intercept the output of collimated, circularly symmetric ultrasound sources operating at frequencies above a few megahertz, the correction for in-situ attenuation is significant, and differs significantly from predictions for idealized circumstances. Empirical attenuation correction factors for a 45° (half-angle) absorptive conical RFB target have been determined for 24 frequencies covering the 5 MHz to 30 MHz range. They agree well with previously unpublished attenuation calibration factors determined in 1994 for a similar target. PMID:27274946
Nyflot, Matthew J.; Lee, Tzu-Cheng; Alessio, Adam M.; Kinahan, Paul E.; Wollenweber, Scott D.; Stearns, Charles W.; Bowen, Stephen R.
2015-01-15
Purpose: Respiratory-correlated positron emission tomography (PET/CT) 4D PET/CT is used to mitigate errors from respiratory motion; however, the optimal CT attenuation correction (CTAC) method for 4D PET/CT is unknown. The authors performed a phantom study to evaluate the quantitative performance of CTAC methods for 4D PET/CT in the ground truth setting. Methods: A programmable respiratory motion phantom with a custom movable insert designed to emulate a lung lesion and lung tissue was used for this study. The insert was driven by one of five waveforms: two sinusoidal waveforms or three patient-specific respiratory waveforms. 3DPET and 4DPET images of the phantom under motion were acquired and reconstructed with six CTAC methods: helical breath-hold (3DHEL), helical free-breathing (3DMOT), 4D phase-averaged (4DAVG), 4D maximum intensity projection (4DMIP), 4D phase-matched (4DMATCH), and 4D end-exhale (4DEXH) CTAC. Recovery of SUV{sub max}, SUV{sub mean}, SUV{sub peak}, and segmented tumor volume was evaluated as RC{sub max}, RC{sub mean}, RC{sub peak}, and RC{sub vol}, representing percent difference relative to the static ground truth case. Paired Wilcoxon tests and Kruskal–Wallis ANOVA were used to test for significant differences. Results: For 4DPET imaging, the maximum intensity projection CTAC produced significantly more accurate recovery coefficients than all other CTAC methods (p < 0.0001 over all metrics). Over all motion waveforms, ratios of 4DMIP CTAC recovery were 0.2 ± 5.4, −1.8 ± 6.5, −3.2 ± 5.0, and 3.0 ± 5.9 for RC{sub max}, RC{sub peak}, RC{sub mean}, and RC{sub vol}. In comparison, recovery coefficients for phase-matched CTAC were −8.4 ± 5.3, −10.5 ± 6.2, −7.6 ± 5.0, and −13.0 ± 7.7 for RC{sub max}, RC{sub peak}, RC{sub mean}, and RC{sub vol}. When testing differences between phases over all CTAC methods and waveforms, end-exhale phases were significantly more accurate (p = 0.005). However, these differences were driven by
Oyeyemi, Victor B.; Krisiloff, David B.; Keith, John A.; Libisch, Florian; Pavone, Michele; Carter, Emily A.
2014-01-28
Oxygenated hydrocarbons play important roles in combustion science as renewable fuels and additives, but many details about their combustion chemistry remain poorly understood. Although many methods exist for computing accurate electronic energies of molecules at equilibrium geometries, a consistent description of entire combustion reaction potential energy surfaces (PESs) requires multireference correlated wavefunction theories. Here we use bond dissociation energies (BDEs) as a foundational metric to benchmark methods based on multireference configuration interaction (MRCI) for several classes of oxygenated compounds (alcohols, aldehydes, carboxylic acids, and methyl esters). We compare results from multireference singles and doubles configuration interaction to those utilizing a posteriori and a priori size-extensivity corrections, benchmarked against experiment and coupled cluster theory. We demonstrate that size-extensivity corrections are necessary for chemically accurate BDE predictions even in relatively small molecules and furnish examples of unphysical BDE predictions resulting from using too-small orbital active spaces. We also outline the specific challenges in using MRCI methods for carbonyl-containing compounds. The resulting complete basis set extrapolated, size-extensivity-corrected MRCI scheme produces BDEs generally accurate to within 1 kcal/mol, laying the foundation for this scheme's use on larger molecules and for more complex regions of combustion PESs.
Oyeyemi, Victor B; Krisiloff, David B; Keith, John A; Libisch, Florian; Pavone, Michele; Carter, Emily A
2014-01-28
Oxygenated hydrocarbons play important roles in combustion science as renewable fuels and additives, but many details about their combustion chemistry remain poorly understood. Although many methods exist for computing accurate electronic energies of molecules at equilibrium geometries, a consistent description of entire combustion reaction potential energy surfaces (PESs) requires multireference correlated wavefunction theories. Here we use bond dissociation energies (BDEs) as a foundational metric to benchmark methods based on multireference configuration interaction (MRCI) for several classes of oxygenated compounds (alcohols, aldehydes, carboxylic acids, and methyl esters). We compare results from multireference singles and doubles configuration interaction to those utilizing a posteriori and a priori size-extensivity corrections, benchmarked against experiment and coupled cluster theory. We demonstrate that size-extensivity corrections are necessary for chemically accurate BDE predictions even in relatively small molecules and furnish examples of unphysical BDE predictions resulting from using too-small orbital active spaces. We also outline the specific challenges in using MRCI methods for carbonyl-containing compounds. The resulting complete basis set extrapolated, size-extensivity-corrected MRCI scheme produces BDEs generally accurate to within 1 kcal/mol, laying the foundation for this scheme's use on larger molecules and for more complex regions of combustion PESs. PMID:25669533
Calbo, Joaquín; Ortí, Enrique; Sancho-García, Juan C; Aragó, Juan
2015-03-10
In this work, we present a thorough assessment of the performance of some representative double-hybrid density functionals (revPBE0-DH-NL and B2PLYP-NL) as well as their parent hybrid and GGA counterparts, in combination with the most modern version of the nonlocal (NL) van der Waals correction to describe very large weakly interacting molecular systems dominated by noncovalent interactions. Prior to the assessment, an accurate and homogeneous set of reference interaction energies was computed for the supramolecular complexes constituting the L7 and S12L data sets by using the novel, precise, and efficient DLPNO-CCSD(T) method at the complete basis set limit (CBS). The correction of the basis set superposition error and the inclusion of the deformation energies (for the S12L set) have been crucial for obtaining precise DLPNO-CCSD(T)/CBS interaction energies. Among the density functionals evaluated, the double-hybrid revPBE0-DH-NL and B2PLYP-NL with the three-body dispersion correction provide remarkably accurate association energies very close to the chemical accuracy. Overall, the NL van der Waals approach combined with proper density functionals can be seen as an accurate and affordable computational tool for the modeling of large weakly bonded supramolecular systems. PMID:26579747
NASA Astrophysics Data System (ADS)
Oyeyemi, Victor B.; Krisiloff, David B.; Keith, John A.; Libisch, Florian; Pavone, Michele; Carter, Emily A.
2014-01-01
Oxygenated hydrocarbons play important roles in combustion science as renewable fuels and additives, but many details about their combustion chemistry remain poorly understood. Although many methods exist for computing accurate electronic energies of molecules at equilibrium geometries, a consistent description of entire combustion reaction potential energy surfaces (PESs) requires multireference correlated wavefunction theories. Here we use bond dissociation energies (BDEs) as a foundational metric to benchmark methods based on multireference configuration interaction (MRCI) for several classes of oxygenated compounds (alcohols, aldehydes, carboxylic acids, and methyl esters). We compare results from multireference singles and doubles configuration interaction to those utilizing a posteriori and a priori size-extensivity corrections, benchmarked against experiment and coupled cluster theory. We demonstrate that size-extensivity corrections are necessary for chemically accurate BDE predictions even in relatively small molecules and furnish examples of unphysical BDE predictions resulting from using too-small orbital active spaces. We also outline the specific challenges in using MRCI methods for carbonyl-containing compounds. The resulting complete basis set extrapolated, size-extensivity-corrected MRCI scheme produces BDEs generally accurate to within 1 kcal/mol, laying the foundation for this scheme's use on larger molecules and for more complex regions of combustion PESs.
Ultra-low dose CT attenuation correction for PET/CT
NASA Astrophysics Data System (ADS)
Xia, Ting; Alessio, Adam M.; De Man, Bruno; Manjeshwar, Ravindra; Asma, Evren; Kinahan, Paul E.
2012-01-01
A challenge for positron emission tomography/computed tomography (PET/CT) quantitation is patient respiratory motion, which can cause an underestimation of lesion activity uptake and an overestimation of lesion volume. Several respiratory motion correction methods benefit from longer duration CT scans that are phase matched with PET scans. However, even with the currently available, lowest dose CT techniques, extended duration cine CT scans impart a substantially high radiation dose. This study evaluates methods designed to reduce CT radiation dose in PET/CT scanning. We investigated selected combinations of dose reduced acquisition and noise suppression methods that take advantage of the reduced requirement of CT for PET attenuation correction (AC). These include reducing CT tube current, optimizing CT tube voltage, adding filtration, CT sinogram smoothing and clipping. We explored the impact of these methods on PET quantitation via simulations on different digital phantoms. CT tube current can be reduced much lower for AC than that in low dose CT protocols. Spectra that are higher energy and narrower are generally more dose efficient with respect to PET image quality. Sinogram smoothing could be used to compensate for the increased noise and artifacts at radiation dose reduced CT images, which allows for a further reduction of CT dose with no penalty for PET image quantitation. When CT is not used for diagnostic and anatomical localization purposes, we showed that ultra-low dose CT for PET/CT is feasible. The significant dose reduction strategies proposed here could enable respiratory motion compensation methods that require extended duration CT scans and reduce radiation exposure in general for all PET/CT imaging.
Automatic detection of cardiovascular risk in CT attenuation correction maps in Rb-82 PET/CTs
NASA Astrophysics Data System (ADS)
Išgum, Ivana; de Vos, Bob D.; Wolterink, Jelmer M.; Dey, Damini; Berman, Daniel S.; Rubeaux, Mathieu; Leiner, Tim; Slomka, Piotr J.
2016-03-01
CT attenuation correction (CTAC) images acquired with PET/CT visualize coronary artery calcium (CAC) and enable CAC quantification. CAC scores acquired with CTAC have been suggested as a marker of cardiovascular disease (CVD). In this work, an algorithm previously developed for automatic CAC scoring in dedicated cardiac CT was applied to automatic CAC detection in CTAC. The study included 134 consecutive patients undergoing 82-Rb PET/CT. Low-dose rest CTAC scans were acquired (100 kV, 11 mAs, 1.4mm×1.4mm×3mm voxel size). An experienced observer defined the reference standard with the clinically used intensity level threshold for calcium identification (130 HU). Five scans were removed from analysis due to artifacts. The algorithm extracted potential CAC by intensity-based thresholding and 3D connected component labeling. Each candidate was described by location, size, shape and intensity features. An ensemble of extremely randomized decision trees was used to identify CAC. The data set was randomly divided into training and test sets. Automatically identified CAC was quantified using volume and Agatston scores. In 33 test scans, the system detected on average 469mm3/730mm3 (64%) of CAC with 36mm3 false positive volume per scan. The intraclass correlation coefficient for volume scores was 0.84. Each patient was assigned to one of four CVD risk categories based on the Agatston score (0-10, 11-100, 101-400, <400). The correct CVD category was assigned to 85% of patients (Cohen's linearly weighted κ0.82). Automatic detection of CVD risk based on CAC scoring in rest CTAC images is feasible. This may enable large scale studies evaluating clinical value of CAC scoring in CTAC data.
Beare, Richard; Brown, Michael J. I.; Pimbblet, Kevin
2014-12-20
We describe an accurate new method for determining absolute magnitudes, and hence also K-corrections, that is simpler than most previous methods, being based on a quadratic function of just one suitably chosen observed color. The method relies on the extensive and accurate new set of 129 empirical galaxy template spectral energy distributions from Brown et al. A key advantage of our method is that we can reliably estimate random errors in computed absolute magnitudes due to galaxy diversity, photometric error and redshift error. We derive K-corrections for the five Sloan Digital Sky Survey filters and provide parameter tables for use by the astronomical community. Using the New York Value-Added Galaxy Catalog, we compare our K-corrections with those from kcorrect. Our K-corrections produce absolute magnitudes that are generally in good agreement with kcorrect. Absolute griz magnitudes differ by less than 0.02 mag and those in the u band by ∼0.04 mag. The evolution of rest-frame colors as a function of redshift is better behaved using our method, with relatively few galaxies being assigned anomalously red colors and a tight red sequence being observed across the whole 0.0 < z < 0.5 redshift range.
NASA Astrophysics Data System (ADS)
Zohoun, Sylvain; Agoua, Eusèbe; Degan, Gérard; Perre, Patrick
2002-08-01
This paper presents an experimental study of the mass diffusion in the hygroscopic region of four temperate species and three tropical ones. In order to simplify the interpretation of the phenomena, a dimensionless parameter called reduced diffusivity is defined. This parameter varies from 0 to 1. The method used is firstly based on the determination of that parameter from results of the measurement of the mass flux which takes into account the conditions of operating standard device (tightness, dimensional variations and easy installation of samples of wood, good stability of temperature and humidity). Secondly the reasons why that parameter has to be corrected are presented. An abacus for this correction of mass diffusivity of wood in steady regime has been plotted. This work constitutes an advanced deal nowadays for characterising forest species.
NASA Astrophysics Data System (ADS)
Rosenthal, Yair; Lohmann, George P.
2002-09-01
Paired δ18O and Mg/Ca measurements on the same foraminiferal shells offer the ability to independently estimate sea surface temperature (SST) changes and assess their temporal relationship to the growth and decay of continental ice sheets. The accuracy of this method is confounded, however, by the absence of a quantitative method to correct Mg/Ca records for alteration by dissolution. Here we describe dissolution-corrected calibrations for Mg/Ca-paleothermometry in which the preexponent constant is a function of size-normalized shell weight: (1) for G. ruber (212-300 μm) (Mg/Ca)ruber = (0.025 wt + 0.11) e0.095T and (b) for G. sacculifer (355-425 μm) (Mg/Ca)sacc = (0.0032 wt + 0.181) e0.095T. The new calibrations improve the accuracy of SST estimates and are globally applicable. With this correction, eastern equatorial Atlantic SST during the Last Glacial Maximum is estimated to be 2.9° ± 0.4°C colder than today.
Torello, David; Thiele, Sebastian; Matlack, Kathryn H; Kim, Jin-Yeon; Qu, Jianmin; Jacobs, Laurence J
2015-02-01
This research considers the effects of diffraction, attenuation, and the nonlinearity of generating sources on measurements of nonlinear ultrasonic Rayleigh wave propagation. A new theoretical framework for correcting measurements made with air-coupled and contact piezoelectric receivers for the aforementioned effects is provided based on analytical models and experimental considerations. A method for extracting the nonlinearity parameter β11 is proposed based on a nonlinear least squares curve-fitting algorithm that is tailored for Rayleigh wave measurements. Quantitative experiments are conducted to confirm the predictions for the nonlinearity of the piezoelectric source and to demonstrate the effectiveness of the curve-fitting procedure. These experiments are conducted on aluminum 2024 and 7075 specimens and a β11(7075)/β11(2024) measure of 1.363 agrees well with previous literature and earlier work. The proposed work is also applied to a set of 2205 duplex stainless steel specimens that underwent various degrees of heat-treatment over 24h, and the results improve upon conclusions drawn from previous analysis. PMID:25287976
SPECT attenuation correction: an essential tool to realize nuclear cardiology's manifest destiny.
Garcia, Ernest V
2007-01-01
Single photon emission computed tomography (SPECT) myocardial perfusion imaging has attained widespread clinical acceptance as a standard of care for cardiac patients. Yet, physical phenomena degrade the accuracy of how our cardiac images are visually interpreted or quantitatively analyzed. This degradation results in cardiac images in which brightness or counts are not necessarily linear with tracer uptake or myocardial perfusion. Attenuation correction (AC) is a methodology that has evolved over the last 30 years to compensate for this degradation. Numerous AC clinical trials over the last 10 years have shown increased diagnostic accuracy over non-AC SPECT for detecting and localizing coronary artery disease, particularly for significantly increasing specificity and normalcy rate. This overwhelming evidence has prompted our professional societies to issue a joint position statement in 2004 recommending the use of AC to maximize SPECT diagnostic accuracy and clinical usefulness. Phantom and animal studies have convincingly shown how SPECT AC recovers the true regional myocardial activity concentration, while non-AC SPECT does not. Thus, AC is also an essential tool for extracting quantitative parameters from all types of cardiac radionuclide distributions, and plays an important role in establishing cardiac SPECT for flow, metabolic, innervation, and molecular imaging, our manifest destiny. PMID:17276302
Attenuation correction of PET images with interpolated average CT for thoracic tumors
NASA Astrophysics Data System (ADS)
Huang, Tzung-Chi; Mok, Greta S. P.; Wang, Shyh-Jen; Wu, Tung-Hsin; Zhang, Geoffrey
2011-04-01
To reduce positron emission tomography (PET) and computed tomography (CT) misalignments and standardized uptake value (SUV) errors, cine average CT (CACT) has been proposed to replace helical CT (HCT) for attenuation correction (AC). A new method using interpolated average CT (IACT) for AC is introduced to further reduce radiation dose with similar image quality. Six patients were recruited in this study. The end-inspiration and -expiration phases from cine CT were used as the two original phases. Deformable image registration was used to generate the interpolated phases. The IACT was calculated by averaging the original and interpolated phases. The PET images were then reconstructed with AC using CACT, HCT and IACT, respectively. Their misalignments were compared by visual assessment, mutual information, correlation coefficient and SUV. The doses from different CT maps were analyzed. The misalignments were reduced for CACT and IACT as compared to HCT. The maximum SUV difference between the use of IACT and CACT was ~3%, and it was ~20% between the use of HCT and CACT. The estimated dose for IACT was 0.38 mSv. The radiation dose using IACT could be reduced by 85% compared to the use of CACT. IACT is a good low-dose approximation of CACT for AC.
Correcting errors in the optical path difference in Fourier spectroscopy: a new accurate method.
Kauppinen, J; Kärkköinen, T; Kyrö, E
1978-05-15
A new computational method for calculating and correcting the errors of the optical path difference in Fourier spectrometers is presented. This method only requires an one-sided interferogram and a single well-separated line in the spectrum. The method also cancels out the linear phase error. The practical theory of the method is included, and an example of the progress of the method is illustrated by simulations. The method is also verified by several simulations in order to estimate its usefulness and accuracy. An example of the use of this method in practice is also given. PMID:20198027
NASA Astrophysics Data System (ADS)
Saïdou; Bochud, François; Laedermann, Jean-Pascal; Buchillier, Thierry; Njock Moïse, Kwato; Froidevaux, Pascal
2007-08-01
In this work the calibration of an HPGe detector for 210Pb measurement is realised by a liquid standard source and the determination of this radionuclide in solid environmental samples by gamma spectrometry takes into account a correction factor for self-attenuation of its 46.5 keV line. Experimental, theoretical and Monte Carlo investigations are undertaken to evaluate self-attenuation for cylindrical sample geometry. To validate this correction factor, 210Po (at equilibrium with 210Pb) alpha spectrometry procedure using microwave acid digestion under pressure is developed and proposed. The different self-attenuation correction methods are in coherence, and corrected 210Pb activities are in good agreement with the results of 210Po. Finally, self-attenuation corrections are proposed for environmental solid samples whose density ranges between 0.8 and 1.4 g/cm 3 and whose mass attenuation coefficient is around 0.4 cm 2/g.
Asymmetrical-fan tranmission CT on SPECT to derive {mu}-maps for attenuation correction
Loncaric, S.; Huang, G.; Ni, B.
1994-05-01
For proper attenuation correction of SPECT images, an appropriate {mu}-map properly registered with each imaging slices is needed. Among the many techniques for {mu}-map derivation, simultaneous or sequential fan-beam transmission CT (TCT), on the same SPECT system with the same acquisition settings, have advantages of being practical while ensuring registration. However, the problems are: (1) limited FOV for thoracic imaging, projection would be truncated with a typical size detector, (2) lack of room for placing the transmission source in many SPECT systems. We have developed a new sampling scheme to solve the problems mentioned above. This scheme uses an asymmetrical-fan geometry (AFG), which samples only half of the field, the other half would be sampled after an 180{degrees} detector rotation. This technique completes the minimum sampling requirement in a 360{degrees} detector rotation and yields a relatively large FOV defined by the outside edge of the sampling fan. We have confirmed the feasibility of the AFG sampling on a 3-head SPECT system to provide a large FOV for TCT of most patient. The TCT sampling scheme is achieved with an asymmetrical-fan collimator. We have developed the required new reconstruction algorithms and derived excellent reconstructed images of phantoms and human subjects. We propose to have this technique implemented in a short and fast transmission scan in a multi-head SPECT system, after emission imaging, because the detectors have to be pulled out to make room for the transmission source. The imaging field can even exceed the full field size of the detector. MS would be possible when an obtuse sampling fan is formed by shifting the source outward further, provided the central FOV is properly covered with a supplementary sampling scheme, e.g., using another TCT with a fan-beam collimator on another one of the detectors.
Multi-centre analysis of incidental findings on low-resolution CT attenuation correction images
Lawson, R; Kane, T; Elias, M; Howes, A; Birchall, J; Hogg, P
2014-01-01
Objective: To review new incidental findings detected on low-resolution CT attenuation correction (CTAC) images acquired during single-photon emission CT (SPECT-CT) myocardial perfusion imaging (MPI) and to determine whether the CTAC images had diagnostic value and warrant reporting. Methods: A multicentre study was performed in four UK nuclear medicine departments. CTAC images acquired as part of MPI performed using SPECT were evaluated to identify incidental findings. New findings considered to be clinically significant were evaluated further. Positive predictive value (PPV) was determined at the time of definitive diagnosis. Results: Of 1819 patients studied, 497 (27.3%) had a positive CTAC finding. 51 (2.8%) patients had findings that were clinically significant at the time of the CTAC report and had not been previously diagnosed. Only four (0.2%) of these were potentially detrimental to patient outcome. Conclusion: One centre had a PPV of 0%, and the study suggests that these CTAC images should not be reported. Two centres with more modern equipment had low PPVs of 0% and 6%, respectively, and further research is suggested prior to drawing a conclusion. The centre with best quality CT had a PPV of 67%, and the study suggests that CTAC images from this equipment should be reported. Advances in knowledge: This study is unique compared with previous studies that have reported only the potential to identify incidental findings on low-resolution CT images. This study both identifies and evaluates new clinically significant incidental findings, and it demonstrates that the benefit of reporting the CTAC images depends on the type of equipment used. PMID:25135310
Brady, Samuel L.; Shulkin, Barry L.
2015-02-15
Purpose: To develop ultralow dose computed tomography (CT) attenuation correction (CTAC) acquisition protocols for pediatric positron emission tomography CT (PET CT). Methods: A GE Discovery 690 PET CT hybrid scanner was used to investigate the change to quantitative PET and CT measurements when operated at ultralow doses (10–35 mA s). CT quantitation: noise, low-contrast resolution, and CT numbers for 11 tissue substitutes were analyzed in-phantom. CT quantitation was analyzed to a reduction of 90% volume computed tomography dose index (0.39/3.64; mGy) from baseline. To minimize noise infiltration, 100% adaptive statistical iterative reconstruction (ASiR) was used for CT reconstruction. PET images were reconstructed with the lower-dose CTAC iterations and analyzed for: maximum body weight standardized uptake value (SUV{sub bw}) of various diameter targets (range 8–37 mm), background uniformity, and spatial resolution. Radiation dose and CTAC noise magnitude were compared for 140 patient examinations (76 post-ASiR implementation) to determine relative dose reduction and noise control. Results: CT numbers were constant to within 10% from the nondose reduced CTAC image for 90% dose reduction. No change in SUV{sub bw}, background percent uniformity, or spatial resolution for PET images reconstructed with CTAC protocols was found down to 90% dose reduction. Patient population effective dose analysis demonstrated relative CTAC dose reductions between 62% and 86% (3.2/8.3–0.9/6.2). Noise magnitude in dose-reduced patient images increased but was not statistically different from predose-reduced patient images. Conclusions: Using ASiR allowed for aggressive reduction in CT dose with no change in PET reconstructed images while maintaining sufficient image quality for colocalization of hybrid CT anatomy and PET radioisotope uptake.
Wang, Siwei; Sun, Dongning; Dong, Yi; Xie, Weilin; Shi, Hongxiao; Yi, Lilin; Hu, Weisheng
2014-02-15
We have developed a radio-frequency local oscillator remote distribution system, which transfers a phase-stabilized 10.03 GHz signal over 100 km optical fiber. The phase noise of the remote signal caused by temperature and mechanical stress variations on the fiber is compensated by a high-precision phase-correction system, which is achieved using a single sideband modulator to transfer the phase correction from intermediate frequency to radio frequency, thus enabling accurate phase control of the 10 GHz signal. The residual phase noise of the remote 10.03 GHz signal is measured to be -70 dBc/Hz at 1 Hz offset, and long-term stability of less than 1×10⁻¹⁶ at 10,000 s averaging time is achieved. Phase error is less than ±0.03π. PMID:24562233
Patra, Sabyasachi; Agarwal, Chhavi; Gathibandhe, M; Goswami, A
2013-07-01
The Hybrid Monte Carlo method developed for attenuation correction has been extended for 500 ml cylindrical geometry. The method has been experimentally validated. Absolute efficiency studies for 500 ml aqueous, air and point source has been carried out using Monte Carlo simulation. It has been observed that point source efficiency is a good estimate of 500 ml source beyond sample-to-detector distance of 15 cm. It has been found that while HMC method for attenuation correction is valid at all sample-to-detector distances and over all transmittance range, the far-field and near-field formulae available in literature are valid only over a very narrow range of sample-to-detector distance. PMID:23523508
NASA Astrophysics Data System (ADS)
Kasaragod, Deepa; Sugiyama, Satoshi; Ikuno, Yasushi; Alonso-Caneiro, David; Yamanari, Masahiro; Fukuda, Shinichi; Oshika, Tetsuro; Hong, Young-Joo; Li, En; Makita, Shuichi; Miura, Masahiro; Yasuno, Yoshiaki
2016-03-01
Polarization sensitive optical coherence tomography (PS-OCT) is a functional extension of OCT that contrasts the polarization properties of tissues. It has been applied to ophthalmology, cardiology, etc. Proper quantitative imaging is required for a widespread clinical utility. However, the conventional method of averaging to improve the signal to noise ratio (SNR) and the contrast of the phase retardation (or birefringence) images introduce a noise bias offset from the true value. This bias reduces the effectiveness of birefringence contrast for a quantitative study. Although coherent averaging of Jones matrix tomography has been widely utilized and has improved the image quality, the fundamental limitation of nonlinear dependency of phase retardation and birefringence to the SNR was not overcome. So the birefringence obtained by PS-OCT was still not accurate for a quantitative imaging. The nonlinear effect of SNR to phase retardation and birefringence measurement was previously formulated in detail for a Jones matrix OCT (JM-OCT) [1]. Based on this, we had developed a maximum a-posteriori (MAP) estimator and quantitative birefringence imaging was demonstrated [2]. However, this first version of estimator had a theoretical shortcoming. It did not take into account the stochastic nature of SNR of OCT signal. In this paper, we present an improved version of the MAP estimator which takes into account the stochastic property of SNR. This estimator uses a probability distribution function (PDF) of true local retardation, which is proportional to birefringence, under a specific set of measurements of the birefringence and SNR. The PDF was pre-computed by a Monte-Carlo (MC) simulation based on the mathematical model of JM-OCT before the measurement. A comparison between this new MAP estimator, our previous MAP estimator [2], and the standard mean estimator is presented. The comparisons are performed both by numerical simulation and in vivo measurements of anterior and
Harnish, Roy; Prevrhal, Sven; Alavi, Abass; Zaidi, Habib; Lang, Thomas
2014-01-01
Background To determine if metal artefact reduction (MAR) combined with a priori knowledge of prosthesis material composition can be applied to obtain CT-based attenuation maps with sufficient accuracy for quantitative assessment of 18F-fluorodeoxyglucose uptake in lesions near metallic prostheses. Methods A custom hip prosthesis phantom with a lesion-sized cavity filled with 0.2 ml 18F-FDG solution having an activity of 3.367 MBq adjacent to a prosthesis bore was imaged twice with a chrome-cobalt steel hip prosthesis and a plastic replica, respectively. Scanning was performed on a clinical hybrid PET/CT system equipped with an additional external 137Cs transmission source. PET emission images were reconstructed from both phantom configurations with CT-based attenuation correction (CTAC) and with CT-based attenuation correction using MAR (MARCTAC). To compare results with the attenuation-correction method extant prior to the advent of PET/CT, we also carried out attenuation correction with 137Cs transmission-based attenuation correction (TXAC). CTAC and MARCTAC images were scaled to attenuation coefficients at 511 keV using a tri-linear function that mapped the highest CT values to the prosthesis alloy attenuation coefficient. Accuracy and spatial distribution of the lesion activity was compared between the three reconstruction schemes. Results Compared to the reference activity of 3.37 MBq, the estimated activity quantified from the PET image corrected by TXAC was 3.41 MBq. The activity estimated from PET images corrected by MARCTAC was similar in accuracy at 3.32 MBq. CTAC corrected PET images resulted in nearly 40% overestimation of lesion activity at 4.70 MBq. Comparison of PET images obtained with the plastic and metal prostheses in place showed that CTAC resulted in a marked distortion of the 18F-FDG distribution within the lesion, whereas application of MARCTAC and TXAC resulted in lesion distributions similar to those observed with the plastic replica
Attenuation correction of PET cardiac data with low-dose average CT in PET/CT
Pan Tinsu; Mawlawi, Osama; Luo, Dershan; Liu, Hui H.; Chi Paichun, M.; Mar, Martha V.; Gladish, Gregory; Truong, Mylene; Erasmus, Jeremy Jr.; Liao Zhongxing; Macapinlac, H. A.
2006-10-15
We proposed a low-dose average computer tomography (ACT) for attenuation correction (AC) of the PET cardiac data in PET/CT. The ACT was obtained from a cine CT scan of over one breath cycle per couch position while the patient was free breathing. We applied this technique on four patients who underwent tumor imaging with {sup 18}F-FDG in PET/CT, whose PET data showed high uptake of {sup 18}F-FDG in the heart and whose CT and PET data had misregistration. All four patients did not have known myocardiac infarction or ischemia. The patients were injected with 555-740 MBq of {sup 18}F-FDG and scanned 1 h after injection. The helical CT (HCT) data were acquired in 16 s for the coverage of 100 cm. The PET acquisition was 3 min per bed of 15 cm. The duration of cine CT acquisition per 2 cm was 5.9 s. We used a fast gantry rotation cycle time of 0.5 s to minimize motion induced reconstruction artifacts in the cine CT images, which were averaged to become the ACT images for AC of the PET data. The radiation dose was about 5 mGy for 5.9 s cine duration. The selection of 5.9 s was based on our analysis of the respiratory signals of 600 patients; 87% of the patients had average breath cycles of less than 6 s and 90% had standard deviations of less than 1 s in the period of breath cycle. In all four patient studies, registrations between the CT and the PET data were improved. An increase of average uptake in the anterior and the lateral walls up to 48% and a decrease of average uptake in the septal and the inferior walls up to 16% with ACT were observed. We also compared ACT and conventional slow scan CT (SSCT) of 4 s duration in one patient study and found ACT was better than SSCT in depicting average respiratory motion and the SSCT images showed motion-induced reconstruction artifacts. In conclusion, low-dose ACT improved registration of the CT and the PET data in the heart region in our study of four patients. ACT was superior than SSCT for depicting average respiration
NASA Astrophysics Data System (ADS)
Rui, Xue; Cheng, Lishui; Long, Yong; Fu, Lin; Alessio, Adam M.; Asma, Evren; Kinahan, Paul E.; De Man, Bruno
2015-09-01
For PET/CT systems, PET image reconstruction requires corresponding CT images for anatomical localization and attenuation correction. In the case of PET respiratory gating, multiple gated CT scans can offer phase-matched attenuation and motion correction, at the expense of increased radiation dose. We aim to minimize the dose of the CT scan, while preserving adequate image quality for the purpose of PET attenuation correction by introducing sparse view CT data acquisition. We investigated sparse view CT acquisition protocols resulting in ultra-low dose CT scans designed for PET attenuation correction. We analyzed the tradeoffs between the number of views and the integrated tube current per view for a given dose using CT and PET simulations of a 3D NCAT phantom with lesions inserted into liver and lung. We simulated seven CT acquisition protocols with {984, 328, 123, 41, 24, 12, 8} views per rotation at a gantry speed of 0.35 s. One standard dose and four ultra-low dose levels, namely, 0.35 mAs, 0.175 mAs, 0.0875 mAs, and 0.043 75 mAs, were investigated. Both the analytical Feldkamp, Davis and Kress (FDK) algorithm and the Model Based Iterative Reconstruction (MBIR) algorithm were used for CT image reconstruction. We also evaluated the impact of sinogram interpolation to estimate the missing projection measurements due to sparse view data acquisition. For MBIR, we used a penalized weighted least squares (PWLS) cost function with an approximate total-variation (TV) regularizing penalty function. We compared a tube pulsing mode and a continuous exposure mode for sparse view data acquisition. Global PET ensemble root-mean-squares-error (RMSE) and local ensemble lesion activity error were used as quantitative evaluation metrics for PET image quality. With sparse view sampling, it is possible to greatly reduce the CT scan dose when it is primarily used for PET attenuation correction with little or no measureable effect on the PET image. For the four ultra-low dose
Rui, Xue; Cheng, Lishui; Long, Yong; Fu, Lin; Alessio, Adam M; Asma, Evren; Kinahan, Paul E; De Man, Bruno
2015-10-01
For PET/CT systems, PET image reconstruction requires corresponding CT images for anatomical localization and attenuation correction. In the case of PET respiratory gating, multiple gated CT scans can offer phase-matched attenuation and motion correction, at the expense of increased radiation dose. We aim to minimize the dose of the CT scan, while preserving adequate image quality for the purpose of PET attenuation correction by introducing sparse view CT data acquisition.We investigated sparse view CT acquisition protocols resulting in ultra-low dose CT scans designed for PET attenuation correction. We analyzed the tradeoffs between the number of views and the integrated tube current per view for a given dose using CT and PET simulations of a 3D NCAT phantom with lesions inserted into liver and lung. We simulated seven CT acquisition protocols with {984, 328, 123, 41, 24, 12, 8} views per rotation at a gantry speed of 0.35 s. One standard dose and four ultra-low dose levels, namely, 0.35 mAs, 0.175 mAs, 0.0875 mAs, and 0.043 75 mAs, were investigated. Both the analytical Feldkamp, Davis and Kress (FDK) algorithm and the Model Based Iterative Reconstruction (MBIR) algorithm were used for CT image reconstruction. We also evaluated the impact of sinogram interpolation to estimate the missing projection measurements due to sparse view data acquisition. For MBIR, we used a penalized weighted least squares (PWLS) cost function with an approximate total-variation (TV) regularizing penalty function. We compared a tube pulsing mode and a continuous exposure mode for sparse view data acquisition. Global PET ensemble root-mean-squares-error (RMSE) and local ensemble lesion activity error were used as quantitative evaluation metrics for PET image quality.With sparse view sampling, it is possible to greatly reduce the CT scan dose when it is primarily used for PET attenuation correction with little or no measureable effect on the PET image. For the four ultra-low dose levels
NASA Astrophysics Data System (ADS)
Goldey, Matthew; Head-Gordon, Martin
2015-03-01
Second order Møller-Plesset perturbation theory (MP2) in finite basis sets describes several classes of noncovalent interactions poorly due to basis set superposition error (BSSE) and underlying inaccurate physics for dispersion interactions. Attenuation of the Coulomb operator provides a direct path toward improving MP2 for noncovalent interactions. In limited basis sets, we demonstrate improvements in accuracy for intermolecular interactions with a three to five-fold reduction in RMS errors. For a range of inter- and intermolecular test cases, attenuated MP2 even outperforms complete basis set estimates of MP2. Finite basis attenuated MP2 is useful for inter- and intramolecular interactions where higher cost approaches are intractable. Extending this approach, recent research pairs attenuated MP2 with long-range correction to describe potential energy landscapes, and further results for large systems with noncovalent interactions are shown. This work was supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We acknowledge computational resources obtained under NSF Award CHE-1048789.
Peng, Xiangda; Zhang, Yuebin; Chu, Huiying; Li, Yan; Zhang, Dinglin; Cao, Liaoran; Li, Guohui
2016-06-14
Classical molecular dynamic (MD) simulation of membrane proteins faces significant challenges in accurately reproducing and predicting experimental observables such as ion conductance and permeability due to its incapability of precisely describing the electronic interactions in heterogeneous systems. In this work, the free energy profiles of K(+) and Na(+) permeating through the gramicidin A channel are characterized by using the AMOEBA polarizable force field with a total sampling time of 1 μs. Our results indicated that by explicitly introducing the multipole terms and polarization into the electrostatic potentials, the permeation free energy barrier of K(+) through the gA channel is considerably reduced compared to the overestimated results obtained from the fixed-charge model. Moreover, the estimated maximum conductance, without any corrections, for both K(+) and Na(+) passing through the gA channel are much closer to the experimental results than any classical MD simulations, demonstrating the power of AMOEBA in investigating the membrane proteins. PMID:27171823
NASA Technical Reports Server (NTRS)
Hagstrom, Thomas; Hariharan, S. I.; Maccamy, R. C.
1993-01-01
We consider the solution of scattering problems for the wave equation using approximate boundary conditions at artificial boundaries. These conditions are explicitly viewed as approximations to an exact boundary condition satisfied by the solution on the unbounded domain. We study the short and long term behavior of the error. It is provided that, in two space dimensions, no local in time, constant coefficient boundary operator can lead to accurate results uniformly in time for the class of problems we consider. A variable coefficient operator is developed which attains better accuracy (uniformly in time) than is possible with constant coefficient approximations. The theory is illustrated by numerical examples. We also analyze the proposed boundary conditions using energy methods, leading to asymptotically correct error bounds.
Bubin, Sergiy; Stanke, Monika; Adamowicz, Ludwik
2011-08-21
In this work we report very accurate variational calculations of the complete pure vibrational spectrum of the D(2) molecule performed within the framework where the Born-Oppenheimer (BO) approximation is not assumed. After the elimination of the center-of-mass motion, D(2) becomes a three-particle problem in this framework. As the considered states correspond to the zero total angular momentum, their wave functions are expanded in terms of all-particle, one-center, spherically symmetric explicitly correlated Gaussian functions multiplied by even non-negative powers of the internuclear distance. The nonrelativistic energies of the states obtained in the non-BO calculations are corrected for the relativistic effects of the order of α(2) (where α = 1/c is the fine structure constant) calculated as expectation values of the operators representing these effects. PMID:21861559
NASA Astrophysics Data System (ADS)
Tolstoy, Leonid
In recent years there has been a huge interest in the development and use of dual-polarized radar systems operating at X-band (˜10 GHz) region of the electromagnetic spectrum. This is due to the fact that these systems are smaller and cheaper allowing for a network to be built, for example, for short range (typically < 30--40 km) hydrological applications. Such networks allow for higher cross-beam spatial resolutions while cheaper pedestals supporting a smaller antenna also allows for higher temporal resolution as compared with large S-band (long range) systems used by the National Weather Service. Dual-polarization radar techniques allow for correction of the strong attenuation of the electromagnetic radar signal due to rain at X-band and higher frequencies. However, practical attempts to develop reliable correction algorithms have been cumbered by the need to deal with the rather large statistical fluctuations or "noise" in the measured polarization parameters. Recently, the variational method was proposed, which overcomes this problem by using the forward model for polarization variables, and uses iterative approach to minimize the difference between modeled and observed values, in a least squares sense. This approach also allows for detection of hail and determination of the fraction of reflectivity due to the hail when the precipitation shaft is composed of a mixture of rain and hail. It was shown that this approach works well with S-band radar data. The purpose of this research is to extend the application of the variational method to the X-band dual-polarization radar data. The main objective is to correct for attenuation caused by rain mixed with wet ice hydrometeors (e.g., hail) in deep convection. The standard dual-polarization method of attenuation-correction using the differential propagation phase between H and V polarized waves cannot account for wet ice hydrometeors along the propagation path. The ultimate goal is to develop a feasible and robust
Strydhorst, Jared H. Ruddy, Terrence D.; Wells, R. Glenn
2015-04-15
Purpose: Our goal in this work was to investigate the impact of CT-based attenuation correction on measurements of rat myocardial perfusion with {sup 99m}Tc and {sup 201}Tl single photon emission computed tomography (SPECT). Methods: Eight male Sprague-Dawley rats were injected with {sup 99m}Tc-tetrofosmin and scanned in a small animal pinhole SPECT/CT scanner. Scans were repeated weekly over a period of 5 weeks. Eight additional rats were injected with {sup 201}Tl and also scanned following a similar protocol. The images were reconstructed with and without attenuation correction, and the relative perfusion was analyzed with the commercial cardiac analysis software. The absolute uptake of {sup 99m}Tc in the heart was also quantified with and without attenuation correction. Results: For {sup 99m}Tc imaging, relative segmental perfusion changed by up to +2.1%/−1.8% as a result of attenuation correction. Relative changes of +3.6%/−1.0% were observed for the {sup 201}Tl images. Interscan and inter-rat reproducibilities of relative segmental perfusion were 2.7% and 3.9%, respectively, for the uncorrected {sup 99m}Tc scans, and 3.6% and 4.3%, respectively, for the {sup 201}Tl scans, and were not significantly affected by attenuation correction for either tracer. Attenuation correction also significantly increased the measured absolute uptake of tetrofosmin and significantly altered the relationship between the rat weight and tracer uptake. Conclusions: Our results show that attenuation correction has a small but statistically significant impact on the relative perfusion measurements in some segments of the heart and does not adversely affect reproducibility. Attenuation correction had a small but statistically significant impact on measured absolute tracer uptake.
Akamatsu, Mana; Yamashita, Yasuo; Akamatsu, Go; Tsutsui, Yuji; Ohya, Nobuyoshi; Nakamura, Yasuhiko; Sasaki, Masayuki
2014-01-01
Objective(s): The aim of this study was to evaluate the influences of reconstruction and attenuation correction on the differences in the radioactivity distributions in 123I brain SPECT obtained by the hybrid SPECT/CT device. Methods: We used the 3-dimensional (3D) brain phantom, which imitates the precise structure of gray matter, white matter and bone regions. It was filled with 123I solution (20.1 kBq/mL) in the gray matter region and with K2HPO4 in the bone region. The SPECT/CT data were acquired by the hybrid SPECT/CT device. SPECT images were reconstructed by using filtered back projection with uniform attenuation correction (FBP-uAC), 3D ordered-subsets expectation-maximization with uniform AC (3D-OSEM-uAC) and 3D OSEM with CT-based non-uniform AC (3D-OSEM-CTAC). We evaluated the differences in the radioactivity distributions among these reconstruction methods using a 3D digital phantom, which was developed from CT images of the 3D brain phantom, as a reference. The normalized mean square error (NMSE) and regional radioactivity were calculated to evaluate the similarity of SPECT images to the 3D digital phantom. Results: The NMSE values were 0.0811 in FBP-uAC, 0.0914 in 3D-OSEM-uAC and 0.0766 in 3D-OSEM-CTAC. The regional radioactivity of FBP-uAC was 11.5% lower in the middle cerebral artery territory, and that of 3D-OSEM-uAC was 5.8% higher in the anterior cerebral artery territory, compared with the digital phantom. On the other hand, that of 3D-OSEM-CTAC was 1.8% lower in all brain areas. Conclusion: By using the hybrid SPECT/CT device, the brain SPECT reconstructed by 3D-OSEM with CT attenuation correction can provide an accurate assessment of the distribution of brain radioactivity.
2013-01-01
Background Population stratification is a systematic difference in allele frequencies between subpopulations. This can lead to spurious association findings in the case–control genome wide association studies (GWASs) used to identify single nucleotide polymorphisms (SNPs) associated with disease-linked phenotypes. Methods such as self-declared ancestry, ancestry informative markers, genomic control, structured association, and principal component analysis are used to assess and correct population stratification but each has limitations. We provide an alternative technique to address population stratification. Results We propose a novel machine learning method, ETHNOPRED, which uses the genotype and ethnicity data from the HapMap project to learn ensembles of disjoint decision trees, capable of accurately predicting an individual’s continental and sub-continental ancestry. To predict an individual’s continental ancestry, ETHNOPRED produced an ensemble of 3 decision trees involving a total of 10 SNPs, with 10-fold cross validation accuracy of 100% using HapMap II dataset. We extended this model to involve 29 disjoint decision trees over 149 SNPs, and showed that this ensemble has an accuracy of ≥ 99.9%, even if some of those 149 SNP values were missing. On an independent dataset, predominantly of Caucasian origin, our continental classifier showed 96.8% accuracy and improved genomic control’s λ from 1.22 to 1.11. We next used the HapMap III dataset to learn classifiers to distinguish European subpopulations (North-Western vs. Southern), East Asian subpopulations (Chinese vs. Japanese), African subpopulations (Eastern vs. Western), North American subpopulations (European vs. Chinese vs. African vs. Mexican vs. Indian), and Kenyan subpopulations (Luhya vs. Maasai). In these cases, ETHNOPRED produced ensembles of 3, 39, 21, 11, and 25 disjoint decision trees, respectively involving 31, 502, 526, 242 and 271 SNPs, with 10-fold cross validation accuracy of
NASA Astrophysics Data System (ADS)
Tang, Qiulin; Zeng, Gengsheng L.; Gullberg, Grant T.
2005-07-01
In this paper, we developed an analytical fan-beam reconstruction algorithm that compensates for uniform attenuation in SPECT. The new fan-beam algorithm is in the form of backprojection first, then filtering, and is mathematically exact. The algorithm is based on three components. The first one is the established generalized central-slice theorem, which relates the 1D Fourier transform of a set of arbitrary data and the 2D Fourier transform of the backprojected image. The second one is the fact that the backprojection of the fan-beam measurements is identical to the backprojection of the parallel measurements of the same object with the same attenuator. The third one is the stable analytical reconstruction algorithm for uniformly attenuated Radon data, developed by Metz and Pan. The fan-beam algorithm is then extended into a cone-beam reconstruction algorithm, where the orbit of the focal point of the cone-beam imaging geometry is a circle. This orbit geometry does not satisfy Tuy's condition and the obtained cone-beam algorithm is an approximation. In the cone-beam algorithm, the cone-beam data are first backprojected into the 3D image volume; then a slice-by-slice filtering is performed. This slice-by-slice filtering procedure is identical to that of the fan-beam algorithm. Both the fan-beam and cone-beam algorithms are efficient, and computer simulations are presented. The new cone-beam algorithm is compared with Bronnikov's cone-beam algorithm, and it is shown to have better performance with noisy projections.
NASA Astrophysics Data System (ADS)
Blanc, Émilie; Komatitsch, Dimitri; Chaljub, Emmanuel; Lombard, Bruno; Xie, Zhinan
2016-04-01
This paper concerns the numerical modelling of time-domain mechanical waves in viscoelastic media based on a generalized Zener model. To do so, classically in the literature relaxation mechanisms are introduced, resulting in a set of the so-called memory variables and thus in large computational arrays that need to be stored. A challenge is thus to accurately mimic a given attenuation law using a minimal set of relaxation mechanisms. For this purpose, we replace the classical linear approach of Emmerich & Korn with a nonlinear optimization approach with constraints of positivity. We show that this technique is more accurate than the linear approach. Moreover, it ensures that physically meaningful relaxation times that always honour the constraint of decay of total energy with time are obtained. As a result, these relaxation times can always be used in a stable way in a modelling algorithm, even in the case of very strong attenuation for which the classical linear approach may provide some negative and thus unusable coefficients.
Joshi, Urvi; Riphagen, Ingrid I.; Teule, Gerrit J. J.; van Lingen, Arthur; Hoekstra, Otto S.
2007-01-01
Purpose To perform a systematic review and meta-analysis to determine the diagnostic accuracy of attenuation-corrected (AC) vs. nonattenuation-corrected (NAC) 2-deoxy-2-[F-18]fluoro-d-glucose-positron emission tomography (FDG-PET) in oncological patients. Procedures Following a comprehensive search of the literature, two reviewers independently assessed the methodological quality of eligible studies. The diagnostic value of AC was studied through its sensitivity/specificity compared to histology, and by comparing the relative lesion detection rate reported with NAC-PET vs. AC, for full-ring and dual-head coincidence PET (FR- and DH-PET, respectively). Results Twelve studies were included. For FR-PET, the pooled sensitivity/specificity on a patient basis was 64/97% for AC and 62/99% for NAC, respectively. Pooled lesion detection with NAC vs. AC was 98% [95% confidence interval (95% CI): 96–99%, n = 1,012 lesions] for FR-PET, and 88% (95% CI:81–94%, n = 288 lesions) for DH-PET. Conclusions Findings suggest similar sensitivity/specificity and lesion detection for NAC vs. AC FR-PET and significantly higher lesion detection for NAC vs. AC DH-PET. PMID:17318671
Comparison of attenuation correction methods for TGS and SGS: Do we really need selenium-75?
Estep, R.J.; Prettyman, T.H.; Sheppard, G.A.
1996-09-01
We compared attenuation-coefficient mapping techniques for use in tomographic gamma scanner (TGS) image reconstructions to determine whether there is a significant improvement when using fully coupled methods. For the constrained least-squares image reconstruction method tested here, we found no significant improvement. We also compared the effectiveness of different transmission source combinations for 129- and 414-keV {sup 239}Pu TGS assays. We concluded that the best source combination for TGS assays of {sup 239}Pu and other isotopes is a mixture of {sup 133}Ba, {sup 54}Mn, and {sup 60}Co. Three other source combinations were found to be at least as effective as {sup 75}Se.
Martinez, L C; Calzado, A
2016-01-01
A parametric model is used for the calculation of the CT number of some selected human tissues of known compositions (Hi) in two hybrid systems, one SPECT-CT and one PET-CT. Only one well characterized substance, not necessarily tissue-like, needs to be scanned with the protocol of interest. The linear attenuation coefficients of these tissues for some energies of interest (μ(i)) have been calculated from their tabulated compositions and the NIST databases. These coefficients have been compared with those calculated with the bilinear model from the CT number (μ(B)i). No relevant differences have been found for bones and lung. In the soft tissue region, the differences can be up to 5%. These discrepancies are attributed to the different chemical composition for the tissues assumed by both methods. PMID:26454019
Chun, Se Young
2016-03-01
PET and SPECT are important tools for providing valuable molecular information about patients to clinicians. Advances in nuclear medicine hardware technologies and statistical image reconstruction algorithms enabled significantly improved image quality. Sequentially or simultaneously acquired anatomical images such as CT and MRI from hybrid scanners are also important ingredients for improving the image quality of PET or SPECT further. High-quality anatomical information has been used and investigated for attenuation and scatter corrections, motion compensation, and noise reduction via post-reconstruction filtering and regularization in inverse problems. In this article, we will review works using anatomical information for molecular image reconstruction algorithms for better image quality by describing mathematical models, discussing sources of anatomical information for different cases, and showing some examples. PMID:26941855
NASA Astrophysics Data System (ADS)
Richter, K.; Stelling, N.; Maas, H.-G.
2014-08-01
Full-waveform airborne laser scanning offers a great potential for various forestry applications. Especially applications requiring information on the vertical structure of the lower canopy parts benefit from the great amount of information contained in waveform data. To enable the derivation of vertical forest canopy structure, the development of suitable voxel based data analysis methods is straightforward. Beyond extracting additional 3D points, it is very promising to derive the voxel attributes from the digitized waveform directly. For this purpose, the differential backscatter cross sections have to be projected into a Cartesian voxel structure. Thereby the voxel entries represent amplitudes of the cross section and can be interpreted as a local measure for the amount of pulse reflecting matter. However, the "history" of each laser echo pulse is characterized by attenuation effects caused by reflections in higher regions of the crown. As a result, the received waveform signals within the canopy have a lower amplitude than it would be observed for an identical structure without the previous canopy structure interactions (Romanczyk et al., 2012). If the biophysical structure is determined from the raw waveform data, material in the lower parts of the canopy is thus under-represented. To achieve a radiometrically correct voxel space representation the loss of signal strength caused by partial reflections on the path of a laser pulse through the canopy has to be compensated. In this paper, we present an integral approach correcting the waveform at each recorded sample. The basic idea of the procedure is to enhance the waveform intensity values in lower parts of the canopy for portions of the pulse intensity, which have been reflected (and thus blocked) in higher parts of the canopy. The paper will discuss the developed correction method and show results from a validation both with synthetic and real world data.
NASA Astrophysics Data System (ADS)
Bahn, Y. K.; Park, H. H.; Lee, C. H.; Kim, H. S.; Lyu, K. Y.; Dong, K. R.; Chung, W. K.; Cho, J. H.
2014-04-01
In this study, phantom was used to evaluate attenuation correction computed tomography (CT) dose and image in case of pediatric positron emission tomography (PET)/CT scan. Three PET/CT scanners were used along with acryl phantom in the size for infant and ion-chamber dosimeter. The CT image acquisition conditions were changed from 10 to 20, 40, 80, 100 and 160 mA and from 80 to 100, 120 and 140 kVp, which aimed at evaluating penetrate dose and computed tomography dose indexvolume (CTDIvol) value. And NEMA PET Phantom™ was used to obtain PET image under the same CT conditions in order to evaluate each attenuation-corrected PET image based on standard uptake value (SUV) value and signal-to-noise ratio (SNR). In general, the penetrate dose was reduced by around 92% under the minimum CT conditions (80 kVp and 10 mA) with the decrease in CTDIvol value by around 88%, compared with the pediatric abdomen CT conditions (100 kVp and 100 mA). The PET image with its attenuation corrected according to each CT condition showed no change in SUV value and no influence on the SNR. In conclusion, if the minimum dose CT that is properly applied to body of pediatric patient is corrected for attenuation to ensure that the effective dose is reduced by around 90% or more compared with that for adult patient, this will be useful to reduce radiation exposure level.
SU-C-9A-06: The Impact of CT Image Used for Attenuation Correction in 4D-PET
Cui, Y; Bowsher, J; Yan, S; Cai, J; Das, S; Yin, F
2014-06-01
Purpose: To evaluate the appropriateness of using 3D non-gated CT image for attenuation correction (AC) in a 4D-PET (gated PET) imaging protocol used in radiotherapy treatment planning simulation. Methods: The 4D-PET imaging protocol in a Siemens PET/CT simulator (Biograph mCT, Siemens Medical Solutions, Hoffman Estates, IL) was evaluated. CIRS Dynamic Thorax Phantom (CIRS Inc., Norfolk, VA) with a moving glass sphere (8 mL) in the middle of its thorax portion was used in the experiments. The glass was filled with {sup 18}F-FDG and was in a longitudinal motion derived from a real patient breathing pattern. Varian RPM system (Varian Medical Systems, Palo Alto, CA) was used for respiratory gating. Both phase-gating and amplitude-gating methods were tested. The clinical imaging protocol was modified to use three different CT images for AC in 4D-PET reconstruction: first is to use a single-phase CT image to mimic actual clinical protocol (single-CT-PET); second is to use the average intensity projection CT (AveIP-CT) derived from 4D-CT scanning (AveIP-CT-PET); third is to use 4D-CT image to do the phase-matched AC (phase-matching- PET). Maximum SUV (SUVmax) and volume of the moving target (glass sphere) with threshold of 40% SUVmax were calculated for comparison between 4D-PET images derived with different AC methods. Results: The SUVmax varied 7.3%±6.9% over the breathing cycle in single-CT-PET, compared to 2.5%±2.8% in AveIP-CT-PET and 1.3%±1.2% in phasematching PET. The SUVmax in single-CT-PET differed by up to 15% from those in phase-matching-PET. The target volumes measured from single- CT-PET images also presented variations up to 10% among different phases of 4D PET in both phase-gating and amplitude-gating experiments. Conclusion: Attenuation correction using non-gated CT in 4D-PET imaging is not optimal process for quantitative analysis. Clinical 4D-PET imaging protocols should consider phase-matched 4D-CT image if available to achieve better accuracy.
Gallandi, Lukas; Marom, Noa; Rinke, Patrick; Körzdörfer, Thomas
2016-02-01
The performance of non-empirically tuned long-range corrected hybrid functionals for the prediction of vertical ionization potentials (IPs) and electron affinities (EAs) is assessed for a set of 24 organic acceptor molecules. Basis set-extrapolated coupled cluster singles, doubles, and perturbative triples [CCSD(T)] calculations serve as a reference for this study. Compared to standard exchange-correlation functionals, tuned long-range corrected hybrid functionals produce highly reliable results for vertical IPs and EAs, yielding mean absolute errors on par with computationally more demanding GW calculations. In particular, it is demonstrated that long-range corrected hybrid functionals serve as ideal starting points for non-self-consistent GW calculations. PMID:26731340
2015-11-01
In the article by Heuslein et al, which published online ahead of print on September 3, 2015 (DOI: 10.1161/ATVBAHA.115.305775), a correction was needed. Brett R. Blackman was added as the penultimate author of the article. The article has been corrected for publication in the November 2015 issue. PMID:26490278
Karton, A.; Martin, J. M. L.; Ruscic, B.; Chemistry; Weizmann Institute of Science
2007-06-01
A benchmark calculation of the atomization energy of the 'simple' organic molecule C2H6 (ethane) has been carried out by means of W4 theory. While the molecule is straightforward in terms of one-particle and n-particle basis set convergence, its large zero-point vibrational energy (and anharmonic correction thereto) and nontrivial diagonal Born-Oppenheimer correction (DBOC) represent interesting challenges. For the W4 set of molecules and C2H6, we show that DBOCs to the total atomization energy are systematically overestimated at the SCF level, and that the correlation correction converges very rapidly with the basis set. Thus, even at the CISD/cc-pVDZ level, useful correlation corrections to the DBOC are obtained. When applying such a correction, overall agreement with experiment was only marginally improved, but a more significant improvement is seen when hydrogen-containing systems are considered in isolation. We conclude that for closed-shell organic molecules, the greatest obstacles to highly accurate computational thermochemistry may not lie in the solution of the clamped-nuclei Schroedinger equation, but rather in the zero-point vibrational energy and the diagonal Born-Oppenheimer correction.
2015-12-01
In the article by Narayan et al (Narayan O, Davies JE, Hughes AD, Dart AM, Parker KH, Reid C, Cameron JD. Central aortic reservoir-wave analysis improves prediction of cardiovascular events in elderly hypertensives. Hypertension. 2015;65:629–635. doi: 10.1161/HYPERTENSIONAHA.114.04824), which published online ahead of print December 22, 2014, and appeared in the March 2015 issue of the journal, some corrections were needed.On page 632, Figure, panel A, the label PRI has been corrected to read RPI. In panel B, the text by the upward arrow, "10% increase in kd,” has been corrected to read, "10% decrease in kd." The corrected figure is shown below.The authors apologize for these errors. PMID:26558821
Cohen, Shlomi; Kozlovsky, Nitsan; Matar, Michael A; Kaplan, Zeev; Zohar, Joseph; Cohen, Hagit
2012-10-01
compared with exposed-SD animals. Intentional prevention of sleep in the early aftermath of stress exposure may well be beneficial in attenuating traumatic stress-related sequelae. Post-exposure SD may disrupt the consolidation of aversive or fearful memories by facilitating correctly timed interactions between glucocorticoid and adrenergic systems. PMID:22713910
NASA Astrophysics Data System (ADS)
Sun, Yuansheng; Periasamy, Ammasi
2010-03-01
Förster resonance energy transfer (FRET) microscopy is commonly used to monitor protein interactions with filter-based imaging systems, which require spectral bleedthrough (or cross talk) correction to accurately measure energy transfer efficiency (E). The double-label (donor+acceptor) specimen is excited with the donor wavelength, the acceptor emission provided the uncorrected FRET signal and the donor emission (the donor channel) represents the quenched donor (qD), the basis for the E calculation. Our results indicate this is not the most accurate determination of the quenched donor signal as it fails to consider the donor spectral bleedthrough (DSBT) signals in the qD for the E calculation, which our new model addresses, leading to a more accurate E result. This refinement improves E comparisons made with lifetime and spectral FRET imaging microscopy as shown here using several genetic (FRET standard) constructs, where cerulean and venus fluorescent proteins are tethered by different amino acid linkers.
NASA Astrophysics Data System (ADS)
Tranchida, Davide; Piccarolo, Stefano; Loos, Joachim; Alexeev, Alexander
2006-10-01
The Oliver and Pharr [J. Mater. Res. 7, 1564 (1992)] procedure is a widely used tool to analyze nanoindentation force curves obtained on metals or ceramics. Its application to polymers is, however, difficult, as Young's moduli are commonly overestimated mainly because of viscoelastic effects and pileup. However, polymers spanning a large range of morphologies have been used in this work to introduce a phenomenological correction factor. It depends on indenter geometry: sets of calibration indentations have to be performed on some polymers with known elastic moduli to characterize each indenter.
Bigdeli, T. Bernard; Lee, Donghyung; Webb, Bradley Todd; Riley, Brien P.; Vladimirov, Vladimir I.; Fanous, Ayman H.; Kendler, Kenneth S.; Bacanu, Silviu-Alin
2016-01-01
Motivation: For genetic studies, statistically significant variants explain far less trait variance than ‘sub-threshold’ association signals. To dimension follow-up studies, researchers need to accurately estimate ‘true’ effect sizes at each SNP, e.g. the true mean of odds ratios (ORs)/regression coefficients (RRs) or Z-score noncentralities. Naïve estimates of effect sizes incur winner’s curse biases, which are reduced only by laborious winner’s curse adjustments (WCAs). Given that Z-scores estimates can be theoretically translated on other scales, we propose a simple method to compute WCA for Z-scores, i.e. their true means/noncentralities. Results:WCA of Z-scores shrinks these towards zero while, on P-value scale, multiple testing adjustment (MTA) shrinks P-values toward one, which corresponds to the zero Z-score value. Thus, WCA on Z-scores scale is a proxy for MTA on P-value scale. Therefore, to estimate Z-score noncentralities for all SNPs in genome scans, we propose FDR Inverse Quantile Transformation (FIQT). It (i) performs the simpler MTA of P-values using FDR and (ii) obtains noncentralities by back-transforming MTA P-values on Z-score scale. When compared to competitors, realistic simulations suggest that FIQT is more (i) accurate and (ii) computationally efficient by orders of magnitude. Practical application of FIQT to Psychiatric Genetic Consortium schizophrenia cohort predicts a non-trivial fraction of sub-threshold signals which become significant in much larger supersamples. Conclusions: FIQT is a simple, yet accurate, WCA method for Z-scores (and ORs/RRs, via simple transformations). Availability and Implementation: A 10 lines R function implementation is available at https://github.com/bacanusa/FIQT. Contact: sabacanu@vcu.edu Supplementary information: Supplementary data are available at Bioinformatics online. PMID:27187203
Maebatake, Akira; Imamura, Ayaka; Kodera, Yui; Yamashita, Yasuo; Himuro, Kazuhiko; Baba, Shingo; Miwa, Kenta; Sasaki, Masayuki
2016-01-01
Objective(s): The aim of this study was to determine the optimal reconstruction parameters for iterative reconstruction in different devices and collimators for dopamine transporter (DaT) single-photon emission computed tomography (SPECT). The results were compared between filtered back projection (FBP) and different attenuation correction (AC) methods. Methods: An anthropomorphic striatal phantom was filled with 123I solutions at different striatum-to-background radioactivity ratios. Data were acquired using two SPECT/CT devices, equipped with a low-to-medium-energy general-purpose collimator (cameras A-1 and B-1) and a low-energy high-resolution (LEHR) collimator (cameras A-2 and B-2). The SPECT images were once reconstructed by FBP using Chang’s AC and once by ordered subset expectation maximization (OSEM) using both CTAC and Chang’s AC; moreover, scatter correction was performed. OSEM on cameras A-1 and A-2 included resolution recovery (RR). The images were analyzed, using the specific binding ratio (SBR). Regions of interest for the background were placed on both frontal and occipital regions. Results: The optimal number of iterations and subsets was 10i10s on camera A-1, 10i5s on camera A-2, and 7i6s on cameras B-1 and B-2. The optimal full width at half maximum of the Gaussian filter was 2.5 times the pixel size. In the comparison between FBP and OSEM, the quality was superior on OSEM-reconstructed images, although edge artifacts were observed in cameras A-1 and A-2. The SBR recovery of OSEM was higher than that of FBP on cameras A-1 and A-2, while no significant difference was detected on cameras B-1 and B-2. Good linearity of SBR was observed in all cameras. In the comparison between Chang’s AC and CTAC, a significant correlation was observed on all cameras. The difference in the background region influenced SBR differently in Chang’s AC and CTAC on cameras A-1 and B-1. Conclusion: Iterative reconstruction improved image quality on all cameras
NASA Astrophysics Data System (ADS)
1995-04-01
Seismic images of the Brooks Range, Arctic Alaska, reveal crustal-scale duplexing: Correction Geology, v. 23, p. 65 68 (January 1995) The correct Figure 4A, for the loose insert, is given here. See Figure 4A below. Corrected inserts will be available to those requesting copies of the article from the senior author, Gary S. Fuis, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025. Figure 4A. P-wave velocity model of Brooks Range region (thin gray contours) with migrated wide-angle reflections (heavy red lines) and migreated vertical-incidence reflections (short black lines) superimposed. Velocity contour interval is 0.25 km/s; 4,5, and 6 km/s contours are labeled. Estimated error in velocities is one contour interval. Symbols on faults shown at top are as in Figure 2 caption.
2016-02-01
Neogi T, Jansen TLTA, Dalbeth N, et al. 2015 Gout classification criteria: an American College of Rheumatology/European League Against Rheumatism collaborative initiative. Ann Rheum Dis 2015;74:1789–98. The name of the 20th author was misspelled. The correct spelling is Janitzia Vazquez-Mellado. We regret the error. PMID:26881284
2016-02-01
In the article by Guessous et al (Guessous I, Pruijm M, Ponte B, Ackermann D, Ehret G, Ansermot N, Vuistiner P, Staessen J, Gu Y, Paccaud F, Mohaupt M, Vogt B, Pechère-Bertschi A, Martin PY, Burnier M, Eap CB, Bochud M. Associations of ambulatory blood pressure with urinary caffeine and caffeine metabolite excretions. Hypertension. 2015;65:691–696. doi: 10.1161/HYPERTENSIONAHA.114.04512), which published online ahead of print December 8, 2014, and appeared in the March 2015 issue of the journal, a correction was needed.One of the author surnames was misspelled. Antoinette Pechère-Berstchi has been corrected to read Antoinette Pechère-Bertschi.The authors apologize for this error. PMID:26763012
Boellaard, R; van Lingen, A; van Balen, S C M; Lammertsma, A A
2004-02-21
The quality of thorax and pelvis transmission scans and therefore of attenuation correction in PET depends on patient thickness and transmission rod source strength. The purpose of the present study was to assess the feasibility of using count-based transmission scans, thereby guaranteeing more consistent image quality and more precise quantification than with fixed transmission scan duration. First, the relation between noise equivalent counts (NEC) of 10 min calibration transmission scans and rod source activity was determined over a period of 1.5 years. Second, the relation between transmission scan counts and uniform phantom diameter was studied numerically, determining the relative contribution of counts from lines of response passing through the phantom as compared with the total number of counts. Finally, the relation between patient weight and transmission scan duration was determined for 35 patients, who were scanned at the level of thorax or pelvis. After installation of new rod sources, the NEC of transmission scans first increased slightly (5%) with decreasing rod source activity and after 3 months decreased with a rate of 2-3% per month. The numerical simulation showed that the number of transmission scan counts from lines of response passing through the phantom increased with phantom diameter up to 7 cm. For phantoms larger than 7 cm, the number of these counts decreased at approximately the same rate as the total number of transmission scan counts. Patient data confirmed that the total number of transmission scan counts decreased with increasing patient weight with about 0.5% kg(-1). It can be concluded that count-based transmission scans compensate for radioactive decay of the rod sources. With count-based transmission scans, rod sources can be used for up to 1.5 years at the cost of a 50% increased transmission scan duration. For phantoms with diameters of more than 7 cm and for patients scanned at the level of thorax or pelvis, use of count
Park, C G; Ha, B
1995-09-01
Most of the attempts and efforts in cleft lip repair have been directed toward the skin incision. The importance of the orbicularis oris muscle repair has been emphasized in recent years. The well-designed skin incision with simple repair of the orbicularis oris muscle has produced a considerable improvement in the appearance of the upper lip; however, the repaired upper lip seems to change its shape abnormally in motion and has a tendency to be distorted with age if the orbicularis oris muscle is not repaired precisely and accurately. Following the dissection of the normal upper lip and unilateral cleft lip in cadavers, we could find two different components in the orbicularis oris muscle, a superficial and a deep component. One is a retractor and the other is a constrictor of the lip. They have antagonistic actions to each other during lip movement. We also can identify these two different components of the muscle in the cleft lip patient during operation. We thought inaccurate and mixed connection between these two different functional components could make the repaired lip distorted and unbalanced, which would get worse during growth. By identification and separate repair of the two different muscular components of the orbicularis oris muscle (i.e., repair of the superficial and deep components on the lateral side with the corresponding components on the medial side), better results in the dynamic and three-dimensional configuration of the upper lip can be achieved, and unfavorable distortion can be avoided as the patients grow.(ABSTRACT TRUNCATED AT 250 WORDS) PMID:7652051
Airborne laser fluorosensor measurements of fluorophore concentrations in surface waters are highly sensitive to interference from changes in optical attenuation. This interference can be eliminated by normalizing the fluorescence signal with the concurrent water Raman signal. In...
Silvani, M. I.; Almeida, G. L.; Lopes, R. T.
2014-11-11
Radiographic images acquired with point-like gamma-ray sources exhibit a desirable low penumbra effects specially when positioned far away from the set object-detector. Such an arrangement frequently is not affordable due to the limited flux provided by a distant source. A closer source, however, has two main drawbacks, namely the degradation of the spatial resolution - as actual sources are only approximately punctual - and the non-homogeneity of the beam hitting the detector, which creates a false attenuation map of the object being inspected. This non-homogeneity is caused by the beam divergence itself and by the different thicknesses traversed the beam even if the object were an homogeneous flat plate. In this work, radiographic images of objects with different geometries, such as flat plates and pipes have undergone a correction of beam divergence and attenuation addressing the experimental verification of the capability and soundness of an algorithm formerly developed to generate and process synthetic images. The impact of other parameters, including source-detector gap, attenuation coefficient, ratio defective-to-main hull thickness and counting statistics have been assessed for specifically tailored test-objects aiming at the evaluation of the ability of the proposed method to deal with different boundary conditions. All experiments have been carried out with an X-ray sensitive Imaging Plate and reactor-produced {sup 198}Au and {sup 165}Dy sources. The results have been compared with other technique showing a better capability to correct the attenuation map of inspected objects unveiling their inner structure otherwise concealed by the poor contrast caused by the beam divergence and attenuation, in particular for those regions far apart from the vertical of the source.
2015-05-22
The Circulation Research article by Keith and Bolli (“String Theory” of c-kitpos Cardiac Cells: A New Paradigm Regarding the Nature of These Cells That May Reconcile Apparently Discrepant Results. Circ Res. 2015:116:1216-1230. doi: 10.1161/CIRCRESAHA.116.305557) states that van Berlo et al (2014) observed that large numbers of fibroblasts and adventitial cells, some smooth muscle and endothelial cells, and rare cardiomyocytes originated from c-kit positive progenitors. However, van Berlo et al reported that only occasional fibroblasts and adventitial cells derived from c-kit positive progenitors in their studies. Accordingly, the review has been corrected to indicate that van Berlo et al (2014) observed that large numbers of endothelial cells, with some smooth muscle cells and fibroblasts, and more rarely cardiomyocytes, originated from c-kit positive progenitors in their murine model. The authors apologize for this error, and the error has been noted and corrected in the online version of the article, which is available at http://circres.ahajournals.org/content/116/7/1216.full ( PMID:25999426
NASA Astrophysics Data System (ADS)
1998-12-01
Alleged mosasaur bite marks on Late Cretaceous ammonites are limpet (patellogastropod) home scars Geology, v. 26, p. 947 950 (October 1998) This article had the following printing errors: p. 947, Abstract, line 11, “sepia” should be “septa” p. 947, 1st paragraph under Introduction, line 2, “creep” should be “deep” p. 948, column 1, 2nd paragraph, line 7, “creep” should be “deep” p. 949, column 1, 1st paragraph, line 1, “creep” should be “deep” p. 949, column 1, 1st paragraph, line 5, “19774” should be “1977)” p. 949, column 1, 4th paragraph, line 7, “in particular” should be “In particular” CORRECTION Mammalian community response to the latest Paleocene thermal maximum: An isotaphonomic study in the northern Bighorn Basin, Wyoming Geology, v. 26, p. 1011 1014 (November 1998) An error appeared in the References Cited. The correct reference appears below: Fricke, H. C., Clyde, W. C., O'Neil, J. R., and Gingerich, P. D., 1998, Evidence for rapid climate change in North America during the latest Paleocene thermal maximum: Oxygen isotope compositions of biogenic phosphate from the Bighorn Basin (Wyoming): Earth and Planetary Science Letters, v. 160, p. 193 208.
Shibutani, Takayuki; Onoguchi, Masahisa; Funayama, Risa; Nakajima, Kenichi; Matsuo, Shinro; Yoneyama, Hiroto; Konishi, Takahiro; Kinuya, Seigo
2015-11-01
The aim of this study was to reveal the optimal reconstruction parameters of ordered subset conjugates gradient minimizer (OSCGM) by no correction (NC), attenuation correction (AC), and AC+scatter correction (ACSC) using IQ-single photon emission computed tomography (SPECT) system in thallium-201 myocardial perfusion SPECT. Myocardial phantom acquired two patterns, with or without defect. Myocardial images were performed 5-point scale visual score and quantitative evaluations using contrast, uptake, and uniformity about the subset and update (subset×iteration) of OSCGM and the full width at half maximum (FWHM) of Gaussian filter by three corrections. We decided on optimal reconstruction parameters of OSCGM by three corrections. The number of subsets to create suitable images were 3 or 5 for NC and AC, 2 or 3 for ACSC. The updates to create suitable images were 30 or 40 for NC, 40 or 60 for AC, and 30 for ACSC. Furthermore, the FWHM of Gaussian filters were 9.6 mm or 12 mm for NC and ACSC, 7.2 mm or 9.6 mm for AC. In conclusion, the following optimal reconstruction parameters of OSCGM were decided; NC: subset 5, iteration 8 and FWHM 9.6 mm, AC: subset 5, iteration 8 and FWHM 7.2 mm, ACSC: subset 3, iteration 10 and FWHM 9.6 mm. PMID:26596202
Exact fan-beam and 4{pi}-acquisition cone-beam SPECT algorithms with uniform attenuation correction
Tang Qiulin; Zeng, Gengsheng L.; Wu Jiansheng; Gullberg, Grant T.
2005-11-15
This paper presents analytical fan-beam and cone-beam reconstruction algorithms that compensate for uniform attenuation in single photon emission computed tomography. First, a fan-beam algorithm is developed by obtaining a relationship between the two-dimensional (2D) Fourier transform of parallel-beam projections and fan-beam projections. Using this relationship, 2D Fourier transforms of equivalent parallel-beam projection data are obtained from the fan-beam projection data. Then a quasioptimal analytical reconstruction algorithm for uniformly attenuated Radon data, developed by Metz and Pan, is used to reconstruct the image. A cone-beam algorithm is developed by extending the fan-beam algorithm to 4{pi} solid angle geometry. The cone-beam algorithm is also an exact algorithm.
Schramm, Georg Maus, Jens; Hofheinz, Frank; Petr, Jan; Lougovski, Alexandr; Beuthien-Baumann, Bettina; Oehme, Liane; Platzek, Ivan; Hoff, Jörg van den
2015-11-15
Purpose: MR-based attenuation correction (MRAC) in routine clinical whole-body positron emission tomography and magnetic resonance imaging (PET/MRI) is based on tissue type segmentation. Due to lack of MR signal in cortical bone and the varying signal of spongeous bone, standard whole-body segmentation-based MRAC ignores the higher attenuation of bone compared to the one of soft tissue (MRAC{sub nobone}). The authors aim to quantify and reduce the bias introduced by MRAC{sub nobone} in the standard uptake value (SUV) of spinal and pelvic lesions in 20 PET/MRI examinations with [{sup 18}F]NaF. Methods: The authors reconstructed 20 PET/MR [{sup 18}F]NaF patient data sets acquired with a Philips Ingenuity TF PET/MRI. The PET raw data were reconstructed with two different attenuation images. First, the authors used the vendor-provided MRAC algorithm that ignores the higher attenuation of bone to reconstruct PET{sub nobone}. Second, the authors used a threshold-based algorithm developed in their group to automatically segment bone structures in the [{sup 18}F]NaF PET images. Subsequently, an attenuation coefficient of 0.11 cm{sup −1} was assigned to the segmented bone regions in the MRI-based attenuation image (MRAC{sub bone}) which was used to reconstruct PET{sub bone}. The automatic bone segmentation algorithm was validated in six PET/CT [{sup 18}F]NaF examinations. Relative SUV{sub mean} and SUV{sub max} differences between PET{sub bone} and PET{sub nobone} of 8 pelvic and 41 spinal lesions, and of other regions such as lung, liver, and bladder, were calculated. By varying the assigned bone attenuation coefficient from 0.11 to 0.13 cm{sup −1}, the authors investigated its influence on the reconstructed SUVs of the lesions. Results: The comparison of [{sup 18}F]NaF-based and CT-based bone segmentation in the six PET/CT patients showed a Dice similarity of 0.7 with a true positive rate of 0.72 and a false discovery rate of 0.33. The [{sup 18}F]NaF-based bone
Zaidi, Habib; Nkoulou, Rene; Bond, Sarah; Baskin, Aylin; Schindler, Thomas; Ratib, Osman; Declerck, Jerome
2013-08-01
The use of coronary calcium scoring (CaScCT) for attenuation correction (AC) of (13)N-ammonia PET/CT studies (NH3) is still being debated. We compare standard ACCT to CaScCT using various respiratory phases and co-registration methods for AC. Forty-one patients underwent a stress/rest NH3. Standard ACCT scans and CaScCT acquired during inspiration (CaScCTinsp, 26 patients) or expiration (CaScCTexp, 15 patients) were used to correct PET data for photon attenuation. Resulting images were compared using Pearson's correlation and Bland-Altman (BA) limits of agreement (LA) on segmental relative and absolute coronary blood flow (CBF) using both manual and automatic co-registration methods (rigid-body and deformable). For relative perfusion, CaScCTexp correlates better than CaScCTinsp with ACCT when using manual co-registration (r = 0.870; P < 0.001 and r = 0.732; P < 0.001, respectively). Automatic co-registration provides the best correlation between CaScCTexp and ACCT for relative perfusion (r = 0.956; P < 0.001). Both CaScCTinsp and CaScCTexp yielded excellent correlations with ACCT for CBF when using manual co-registration (r = 0.918; P < 0.001; BA mean bias 0.05 ml/min/g; LA: -0.42 to +0.3 ml/min/g and r = 0.97; P < 0.001; BA mean bias 0.1 ml/min/g; LA: -0.65 to +0.5 ml/min/g, respectively). The use of CaScCTexp and deformable co-registration is best suited for AC to quantify relative perfusion and CBF enabling substantial radiation dose reduction. PMID:23504215
Cheng, Nai-Ming; Fang, Yu-Hua Dean; Tsan, Din-Li
2016-01-01
Purpose We compared attenuation correction of PET images with helical CT (PET/HCT) and respiration-averaged CT (PET/ACT) in patients with non-small-cell lung cancer (NSCLC) with the goal of investigating the impact of respiration-averaged CT on 18F FDG PET texture parameters. Materials and Methods A total of 56 patients were enrolled. Tumors were segmented on pretreatment PET images using the adaptive threshold. Twelve different texture parameters were computed: standard uptake value (SUV) entropy, uniformity, entropy, dissimilarity, homogeneity, coarseness, busyness, contrast, complexity, grey-level nonuniformity, zone-size nonuniformity, and high grey-level large zone emphasis. Comparisons of PET/HCT and PET/ACT were performed using Wilcoxon signed-rank tests, intraclass correlation coefficients, and Bland-Altman analysis. Receiver operating characteristic (ROC) curves as well as univariate and multivariate Cox regression analyses were used to identify the parameters significantly associated with disease-specific survival (DSS). A fixed threshold at 45% of the maximum SUV (T45) was used for validation. Results SUV maximum and total lesion glycolysis (TLG) were significantly higher in PET/ACT. However, texture parameters obtained with PET/ACT and PET/HCT showed a high degree of agreement. The lowest levels of variation between the two modalities were observed for SUV entropy (9.7%) and entropy (9.8%). SUV entropy, entropy, and coarseness from both PET/ACT and PET/HCT were significantly associated with DSS. Validation analyses using T45 confirmed the usefulness of SUV entropy and entropy in both PET/HCT and PET/ACT for the prediction of DSS, but only coarseness from PET/ACT achieved the statistical significance threshold. Conclusions Our results indicate that 1) texture parameters from PET/ACT are clinically useful in the prediction of survival in NSCLC patients and 2) SUV entropy and entropy are robust to attenuation correction methods. PMID:26930211
NASA Astrophysics Data System (ADS)
Arockia Bazil Raj, A.; Padmavathi, S.
2016-07-01
Atmospheric parameters strongly affect the performance of Free Space Optical Communication (FSOC) system when the optical wave is propagating through the inhomogeneous turbulent medium. Developing a model to get an accurate prediction of optical attenuation according to meteorological parameters becomes significant to understand the behaviour of FSOC channel during different seasons. A dedicated free space optical link experimental set-up is developed for the range of 0.5 km at an altitude of 15.25 m. The diurnal profile of received power and corresponding meteorological parameters are continuously measured using the developed optoelectronic assembly and weather station, respectively, and stored in a data logging computer. Measured meteorological parameters (as input factors) and optical attenuation (as response factor) of size [177147 × 4] are used for linear regression analysis and to design the mathematical model that is more suitable to predict the atmospheric optical attenuation at our test field. A model that exhibits the R2 value of 98.76% and average percentage deviation of 1.59% is considered for practical implementation. The prediction accuracy of the proposed model is investigated along with the comparative results obtained from some of the existing models in terms of Root Mean Square Error (RMSE) during different local seasons in one-year period. The average RMSE value of 0.043-dB/km is obtained in the longer range dynamic of meteorological parameters variations.
Hu, Lingzhi E-mail: raymond.muzic@case.edu; Traughber, Melanie; Su, Kuan-Hao; Pereira, Gisele C.; Grover, Anu; Traughber, Bryan; Muzic, Raymond F. Jr. E-mail: raymond.muzic@case.edu
2014-10-15
Purpose: The ultrashort echo-time (UTE) sequence is a promising MR pulse sequence for imaging cortical bone which is otherwise difficult to image using conventional MR sequences and also poses strong attenuation for photons in radiation therapy and PET imaging. The authors report here a systematic characterization of cortical bone signal decay and a scanning time optimization strategy for the UTE sequence through k-space undersampling, which can result in up to a 75% reduction in acquisition time. Using the undersampled UTE imaging sequence, the authors also attempted to quantitatively investigate the MR properties of cortical bone in healthy volunteers, thus demonstrating the feasibility of using such a technique for generating bone-enhanced images which can be used for radiation therapy planning and attenuation correction with PET/MR. Methods: An angularly undersampled, radially encoded UTE sequence was used for scanning the brains of healthy volunteers. Quantitative MR characterization of tissue properties, including water fraction and R2{sup ∗} = 1/T2{sup ∗}, was performed by analyzing the UTE images acquired at multiple echo times. The impact of different sampling rates was evaluated through systematic comparison of the MR image quality, bone-enhanced image quality, image noise, water fraction, and R2{sup ∗} of cortical bone. Results: A reduced angular sampling rate of the UTE trajectory achieves acquisition durations in proportion to the sampling rate and in as short as 25% of the time required for full sampling using a standard Cartesian acquisition, while preserving unique MR contrast within the skull at the cost of a minimal increase in noise level. The R2{sup ∗} of human skull was measured as 0.2–0.3 ms{sup −1} depending on the specific region, which is more than ten times greater than the R2{sup ∗} of soft tissue. The water fraction in human skull was measured to be 60%–80%, which is significantly less than the >90% water fraction in
Visvikis, D; Costa, D C; Croasdale, I; Lonn, A H R; Bomanji, J; Gacinovic, S; Ell, P J
2003-03-01
The introduction of combined PET/CT systems has a number of advantages, including the utilisation of CT images for PET attenuation correction (AC). The potential advantage compared with existing methodology is less noisy transmission maps within shorter times of acquisition. The objective of our investigation was to assess the accuracy of CT attenuation correction (CTAC) and to study resulting bias and signal to noise ratio (SNR) in image-derived semi-quantitative uptake indices. A combined PET/CT system (GE Discovery LS) was used. Different size phantoms containing variable density components were used to assess the inherent accuracy of a bilinear transformation in the conversion of CT images to 511 keV attenuation maps. This was followed by a phantom study simulating tumour imaging conditions, with a tumour to background ratio of 5:1. An additional variable was the inclusion of contrast agent at different concentration levels. A CT scan was carried out followed by 5 min emission with 1-h and 3-min transmission frames. Clinical data were acquired in 50 patients, who had a CT scan under normal breathing conditions (CTAC(nb)) or under breath-hold with inspiration (CTAC(insp)) or expiration (CTAC(exp)), followed by a PET scan of 5 and 3 min per bed position for the emission and transmission scans respectively. Phantom and patient studies were reconstructed using segmented AC (SAC) and CTAC. In addition, measured AC (MAC) was performed for the phantom study using the 1-h transmission frame. Comparing the attenuation coefficients obtained using the CT- and the rod source-based attenuation maps, differences of 3% and <6% were recorded before and after segmentation of the measured transmission maps. Differences of up to 6% and 8% were found in the average count density (SUV(avg)) between the phantom images reconstructed with MAC and those reconstructed with CTAC and SAC respectively. In the case of CTAC, the difference increased up to 27% with the presence of contrast
Measurement of attenuation coefficients of the fundamental and second harmonic waves in water
NASA Astrophysics Data System (ADS)
Zhang, Shuzeng; Jeong, Hyunjo; Cho, Sungjong; Li, Xiongbing
2016-02-01
Attenuation corrections in nonlinear acoustics play an important role in the study of nonlinear fluids, biomedical imaging, or solid material characterization. The measurement of attenuation coefficients in a nonlinear regime is not easy because they depend on the source pressure and requires accurate diffraction corrections. In this work, the attenuation coefficients of the fundamental and second harmonic waves which come from the absorption of water are measured in nonlinear ultrasonic experiments. Based on the quasilinear theory of the KZK equation, the nonlinear sound field equations are derived and the diffraction correction terms are extracted. The measured sound pressure amplitudes are adjusted first for diffraction corrections in order to reduce the impact on the measurement of attenuation coefficients from diffractions. The attenuation coefficients of the fundamental and second harmonics are calculated precisely from a nonlinear least squares curve-fitting process of the experiment data. The results show that attenuation coefficients in a nonlinear condition depend on both frequency and source pressure, which are much different from a linear regime. In a relatively lower drive pressure, the attenuation coefficients increase linearly with frequency. However, they present the characteristic of nonlinear growth in a high drive pressure. As the diffraction corrections are obtained based on the quasilinear theory, it is important to use an appropriate source pressure for accurate attenuation measurements.
NASA Astrophysics Data System (ADS)
Heirwegh, C. M.; Chettle, D. R.; Pejović-Milić, A.
2010-02-01
The interpretation of measurements of bone strontium in vivo using energy dispersive x-ray fluorescence spectroscopy is presently hindered by overlying skin and soft-tissue absorption of the strontium x-rays. The use of imaging technologies to measure the overlying soft-tissue thickness at the index finger measuring site might allow correction of the strontium reading to estimate its concentration in bone. An examination of magnetic resonance (MR), computed tomography (CT) and high-frequency ultrasound (US) imaging technologies revealed that 55 MHz US had the smallest range of measurement uncertainty at 3.2% followed by 1 Tesla MR, 25 MHz US, 8 MHz US and CT at 4.3, 5.4, 6.6 and 7.1% uncertainty, respectively. Of these, only CT imaging appeared to underestimate total thickness (p < 0.05). Furthermore, an inter-study comparison on the accuracy of US measurements of the overlying tissue thickness at finger and ankle in nine subjects was investigated. The 8 MHz US system used in prior in vivo experiments was found to perform satisfactorily in a repeat study of ankle measurements, but indicated that finger thickness measurements may have been misread in previous studies by up to 17.7% (p < 0.025). Repeat ankle measurements were not significantly different from initial measurements at 2.2% difference.
Medich, David C.; Munro, John J. III
2010-05-15
depths of 1 and 10 cm and angles of 0 deg. and 180 deg. This was in contrast to that of the Model M-19 Ir-192 source which exhibited approximately 3.5%-4.4% variation in its energy correction factors from phantom depths of 0.5-10 cm. The absorbed dose energy correction factor for the Ir-192 source, on the other hand, was independent of angle to within 1%. Conclusions: The application of a single energy correction factor for Yb-169 TLD based dosimetry would introduce a high degree of measurement uncertainty that may not be reasonable for the clinical characterization of a brachytherapy source; rather, an absorbed dose energy correction function will need to be developed for these sources. This correction function should be specific to each source model, type of TLD used, and to the experimental setup to obtain accurate and precise dosimetric measurements.
ALPHA ATTENUATION DUE TO DUST LOADING
Dailey, A; Dennis Hadlock, D
2007-08-09
Previous studies had been done in order to show the attenuation of alpha particles in filter media. These studies provided an accurate correction for this attenuation, but there had not yet been a study with sufficient results to properly correct for attenuation due to dust loading on the filters. At the Savannah River Site, filter samples are corrected for attenuation due to dust loading at 20%. Depending on the facility the filter comes from and the duration of the sampling period, the proper correction factor may vary. The objective of this study was to determine self-absorption curves for each of three counting instruments. Prior work indicated significant decreases in alpha count rate (as much as 38%) due to dust loading, especially on filters from facilities where sampling takes place over long intervals. The alpha count rate decreased because of a decrease in the energy of the alpha. The study performed resulted in a set of alpha absorption curves for each of three detectors. This study also took into account the affects of the geometry differences in the different counting equipment used.
NASA Astrophysics Data System (ADS)
Wang, Lu; Wu, Li-Wei; Wei, Le; Gao, Juan; Sun, Cui-Li; Chai, Pei; Li, Dao-Wu
2014-02-01
The accuracy of attenuation correction in positron emission tomography scanners depends mainly on deriving the reliable 511-keV linear attenuation coefficient distribution in the scanned objects. In the PET/CT system, the linear attenuation distribution is usually obtained from the intensities of the CT image. However, the intensities of the CT image relate to the attenuation of photons in an energy range of 40 keV-140 keV. Before implementing PET attenuation correction, the intensities of CT images must be transformed into the PET 511-keV linear attenuation coefficients. However, the CT scan parameters can affect the effective energy of CT X-ray photons and thus affect the intensities of the CT image. Therefore, for PET/CT attenuation correction, it is crucial to determine the conversion curve with a given set of CT scan parameters and convert the CT image into a PET linear attenuation coefficient distribution. A generalized method is proposed for converting a CT image into a PET linear attenuation coefficient distribution. Instead of some parameter-dependent phantom calibration experiments, the conversion curve is calculated directly by employing the consistency conditions to yield the most consistent attenuation map with the measured PET data. The method is evaluated with phantom experiments and small animal experiments. In phantom studies, the estimated conversion curve fits the true attenuation coefficients accurately, and accurate PET attenuation maps are obtained by the estimated conversion curves and provide nearly the same correction results as the true attenuation map. In small animal studies, a more complicated attenuation distribution of the mouse is obtained successfully to remove the attenuation artifact and improve the PET image contrast efficiently.
Ai, H; Pan, T; Hwang, K
2014-06-15
Purpose: To determine the feasibility of identifying cortical bone on MR images with a short-TE 3D fast-GRE sequence for attenuation correction of PET data in PET/MR. Methods: A water-fat-bone phantom was constructed with two pieces of beef shank. MR scans were performed on a 3T MR scanner (GE Discovery™ MR750). A 3D GRE sequence was first employed to measure the level of residual signal in cortical bone (TE{sub 1}/TE{sub 2}/TE{sub 3}=2.2/4.4/6.6ms, TR=20ms, flip angle=25°). For cortical bone segmentation, a 3D fast-GRE sequence (TE/TR=0.7/1.9ms, acquisition voxel size=2.5×2.5×3mm{sup 3}) was implemented along with a 3D Dixon sequence (TE{sub 1}/TE{sub 2}/TR=1.2/2.3/4.0ms, acquisition voxel size=1.25×1.25×3mm{sup 3}) for water/fat imaging. Flip angle (10°), acquisition bandwidth (250kHz), FOV (480×480×144mm{sup 3}) and reconstructed voxel size (0.94×0.94×1.5mm{sup 3}) were kept the same for both sequences. Soft tissue and fat tissue were first segmented on the reconstructed water/fat image. A tissue mask was created by combining the segmented water/fat masks, which was then applied on the fast-GRE image (MRFGRE). A second mask was created to remove the Gibbs artifacts present in regions in close vicinity to the phantom. MRFGRE data was smoothed with a 3D anisotropic diffusion filter for noise reduction, after which cortical bone and air was separated using a threshold determined from the histogram. Results: There is signal in the cortical bone region in the 3D GRE images, indicating the possibility of separating cortical bone and air based on signal intensity from short-TE MR image. The acquisition time for the 3D fast-GRE sequence was 17s, which can be reduced to less than 10s with parallel imaging. The attenuation image created from water-fat-bone segmentation is visually similar compared to reference CT. Conclusion: Cortical bone and air can be separated based on intensity in MR image with a short-TE 3D fast-GRE sequence. Further research is required
Szczepura, K; Hogg, P
2014-01-01
Objective: To measure the organ dose and calculate effective dose from CT attenuation correction (CTAC) acquisitions from four commonly used gamma camera single photon emission CT/CT systems. Methods: CTAC dosimetry data was collected using thermoluminescent dosemeters on GE Healthcare's Infinia™ Hawkeye™ (GE Healthcare, Buckinghamshire, UK) four- and single-slice systems, Siemens Symbia™ T6 (Siemens Healthcare, Erlangen, Germany) and the Philips Precedence (Philips Healthcare, Amsterdam, Netherlands). Organ and effective dose from the administration of 99mTc-tetrofosmin and 99mTc-sestamibi were calculated using International Commission of Radiological Protection reports 80 and 106. Using these data, the lifetime biological risk was calculated. Results: The Siemens Symbia gave the lowest CTAC dose (1.8 mSv) followed by the GE Infinia Hawkeye single-slice (1.9 mSv), GE Infinia Hawkeye four-slice (2.5 mSv) and Philips Precedence v. 3.0. Doses were significantly lower than the calculated doses from radiopharmaceutical administration (11 and 14 mSv for 99mTc-tetrofosmin and 99mTc-sestamibi, respectively). Overall lifetime biological risks were lower, which suggests that using CTAC data posed minimal risk to the patient. Comparison of data for breast tissue demonstrated a higher risk than that from the radiopharmaceutical administration. Conclusion: CTAC doses were confirmed to be much lower than those from radiopharmaceutical administration. The localized nature of the CTAC exposure compared to the radiopharmaceutical biological distribution indicated dose and risk to the breast to be higher. Advances in knowledge: This research proved that CTAC is a comparatively low-dose acquisition. However, it has been shown that there is increased risk for breast tissue especially in the younger patients. As per legislation, justification is required and CTAC should only be used in situations that demonstrate sufficient net benefit. PMID:24998249
Frolov, Alexei M.; Wardlaw, David M.
2014-09-14
Mass-dependent and field shift components of the isotopic shift are determined to high accuracy for the ground 1{sup 1}S−states of some light two-electron Li{sup +}, Be{sup 2+}, B{sup 3+}, and C{sup 4+} ions. To determine the field components of these isotopic shifts we apply the Racah-Rosental-Breit formula. We also determine the lowest order QED corrections to the isotopic shifts for each of these two-electron ions.
Brady, S; Shulkin, B
2015-06-15
Purpose: To develop ultra-low dose computed tomography (CT) attenuation correction (CTAC) acquisition protocols for pediatric positron emission tomography CT (PET CT). Methods: A GE Discovery 690 PET CT hybrid scanner was used to investigate the change to quantitative PET and CT measurements when operated at ultra-low doses (10–35 mAs). CT quantitation: noise, low-contrast resolution, and CT numbers for eleven tissue substitutes were analyzed in-phantom. CT quantitation was analyzed to a reduction of 90% CTDIvol (0.39/3.64; mGy) radiation dose from baseline. To minimize noise infiltration, 100% adaptive statistical iterative reconstruction (ASiR) was used for CT reconstruction. PET images were reconstructed with the lower-dose CTAC iterations and analyzed for: maximum body weight standardized uptake value (SUVbw) of various diameter targets (range 8–37 mm), background uniformity, and spatial resolution. Radiation organ dose, as derived from patient exam size specific dose estimate (SSDE), was converted to effective dose using the standard ICRP report 103 method. Effective dose and CTAC noise magnitude were compared for 140 patient examinations (76 post-ASiR implementation) to determine relative patient population dose reduction and noise control. Results: CT numbers were constant to within 10% from the non-dose reduced CTAC image down to 90% dose reduction. No change in SUVbw, background percent uniformity, or spatial resolution for PET images reconstructed with CTAC protocols reconstructed with ASiR and down to 90% dose reduction. Patient population effective dose analysis demonstrated relative CTAC dose reductions between 62%–86% (3.2/8.3−0.9/6.2; mSv). Noise magnitude in dose-reduced patient images increased but was not statistically different from pre dose-reduced patient images. Conclusion: Using ASiR allowed for aggressive reduction in CTAC dose with no change in PET reconstructed images while maintaining sufficient image quality for co
Paulus, Daniel H.; Quick, Harald H.; Geppert, Christian; Fenchel, Matthias; Zhan, Yiqiang; Hermosillo, Gerardo; Faul, David; Boada, Fernando; Friedman, Kent P.; Koesters, Thomas
2016-01-01
In routine whole-body PET/MR hybrid imaging, attenuation correction (AC) is usually performed by segmentation methods based on a Dixon MR sequence providing up to 4 different tissue classes. Because of the lack of bone information with the Dixon-based MR sequence, bone is currently considered as soft tissue. Thus, the aim of this study was to evaluate a novel model-based AC method that considers bone in whole-body PET/MR imaging. Methods The new method (“Model”) is based on a regular 4-compartment segmentation from a Dixon sequence (“Dixon”). Bone information is added using a model-based bone segmentation algorithm, which includes a set of prealigned MR image and bone mask pairs for each major body bone individually. Model was quantitatively evaluated on 20 patients who underwent whole-body PET/MR imaging. As a standard of reference, CT-based μ-maps were generated for each patient individually by nonrigid registration to the MR images based on PET/CT data. This step allowed for a quantitative comparison of all μ-maps based on a single PET emission raw dataset of the PET/MR system. Volumes of interest were drawn on normal tissue, soft-tissue lesions, and bone lesions; standardized uptake values were quantitatively compared. Results In soft-tissue regions with background uptake, the average bias of SUVs in background volumes of interest was 2.4% ± 2.5% and 2.7% ± 2.7% for Dixon and Model, respectively, compared with CT-based AC. For bony tissue, the −25.5% ± 7.9% underestimation observed with Dixon was reduced to −4.9% ± 6.7% with Model. In bone lesions, the average underestimation was −7.4% ± 5.3% and −2.9% ± 5.8% for Dixon and Model, respectively. For soft-tissue lesions, the biases were 5.1% ± 5.1% for Dixon and 5.2% ± 5.2% for Model. Conclusion The novel MR-based AC method for whole-body PET/MR imaging, combining Dixon-based soft-tissue segmentation and model-based bone estimation, improves PET quantification in whole-body hybrid PET
Kato, Hiroki; Shimosegawa, Eku; Fujino, Koichi; Hatazawa, Jun
2016-01-01
Background Integrated SPECT/CT enables non-uniform attenuation correction (AC) using built-in CT instead of the conventional uniform AC. The effect of CT-based AC on voxel-based statistical analyses of brain SPECT findings has not yet been clarified. Here, we assessed differences in the detectability of regional cerebral blood flow (CBF) reduction using SPECT voxel-based statistical analyses based on the two types of AC methods. Subjects and Methods N-isopropyl-p-[123I]iodoamphetamine (IMP) CBF SPECT images were acquired for all the subjects and were reconstructed using 3D-OSEM with two different AC methods: Chang’s method (Chang’s AC) and the CT-based AC method. A normal database was constructed for the analysis using SPECT findings obtained for 25 healthy normal volunteers. Voxel-based Z-statistics were also calculated for SPECT findings obtained for 15 patients with chronic cerebral infarctions and 10 normal subjects. We assumed that an analysis with a higher specificity would likely produce a lower mean absolute Z-score for normal brain tissue, and a more sensitive voxel-based statistical analysis would likely produce a higher absolute Z-score for in old infarct lesions, where the CBF was severely decreased. Results The inter-subject variation in the voxel values in the normal database was lower using CT-based AC, compared with Chang’s AC, for most of the brain regions. The absolute Z-score indicating a SPECT count reduction in infarct lesions was also significantly higher in the images reconstructed using CT-based AC, compared with Chang’s AC (P = 0.003). The mean absolute value of the Z-score in the 10 intact brains was significantly lower in the images reconstructed using CT-based AC than in those reconstructed using Chang’s AC (P = 0.005). Conclusions Non-uniform CT-based AC by integrated SPECT/CT significantly improved sensitivity and the specificity of the voxel-based statistical analyses for regional SPECT count reductions, compared with
Li, Y.; Krieger, J.B. ); Norman, M.R. ); Iafrate, G.J. )
1991-11-15
The optimized-effective-potential (OEP) method and a method developed recently by Krieger, Li, and Iafrate (KLI) are applied to the band-structure calculations of noble-gas and alkali halide solids employing the self-interaction-corrected (SIC) local-spin-density (LSD) approximation for the exchange-correlation energy functional. The resulting band gaps from both calculations are found to be in fair agreement with the experimental values. The discrepancies are typically within a few percent with results that are nearly the same as those of previously published orbital-dependent multipotential SIC calculations, whereas the LSD results underestimate the band gaps by as much as 40%. As in the LSD---and it is believed to be the case even for the exact Kohn-Sham potential---both the OEP and KLI predict valence-band widths which are narrower than those of experiment. In all cases, the KLI method yields essentially the same results as the OEP.
Radiometric correction of scatterometric wind measurements
NASA Technical Reports Server (NTRS)
1995-01-01
Use of a spaceborne scatterometer to determine the ocean-surface wind vector requires accurate measurement of radar backscatter from ocean. Such measurements are hindered by the effect of attenuation in the precipitating regions over sea. The attenuation can be estimated reasonably well with the knowledge of brightness temperatures observed by a microwave radiometer. The NASA SeaWinds scatterometer is to be flown on the Japanese ADEOS2. The AMSR multi-frequency radiometer on ADEOS2 will be used to correct errors due to attenuation in the SeaWinds scatterometer measurements. Here we investigate the errors in the attenuation corrections. Errors would be quite small if the radiometer and scatterometer footprints were identical and filled with uniform rain. However, the footprints are not identical, and because of their size one cannot expect uniform rain across each cell. Simulations were performed with the SeaWinds scatterometer (13.4 GHz) and AMSR (18.7 GHz) footprints with gradients of attenuation. The study shows that the resulting wind speed errors after correction (using the radiometer) are small for most cases. However, variations in the degree of overlap between the radiometer and scatterometer footprints affect the accuracy of the wind speed measurements.
NASA Astrophysics Data System (ADS)
Li, Yusheng; Defrise, Michel; Metzler, Scott D.; Matej, Samuel
2015-08-01
In positron emission tomography (PET) imaging, attenuation correction with accurate attenuation estimation is crucial for quantitative patient studies. Recent research showed that the attenuation sinogram can be determined up to a scaling constant utilizing the time-of-flight information. The TOF-PET data can be naturally and efficiently stored in a histo-image without information loss, and the radioactive tracer distribution can be efficiently reconstructed using the DIRECT approaches. In this paper, we explore transmission-less attenuation estimation from TOF-PET histo-images. We first present the TOF-PET histo-image formation and the consistency equations in the histo-image parameterization, then we derive a least-squares solution for estimating the directional derivatives of the attenuation factors from the measured emission histo-images. Finally, we present a fast solver to estimate the attenuation factors from their directional derivatives using the discrete sine transform and fast Fourier transform while considering the boundary conditions. We find that the attenuation histo-images can be uniquely determined from the TOF-PET histo-images by considering boundary conditions. Since the estimate of the attenuation directional derivatives can be inaccurate for LORs tangent to the patient boundary, external sources, e.g. a ring or annulus source, might be needed to give an accurate estimate of the attenuation gradient for such LORs. The attenuation estimation from TOF-PET emission histo-images is demonstrated using simulated 2D TOF-PET data.
Attenberger, Ulrike; Catana, Ciprian; Chandarana, Hersh; Catalano, Onofrio A; Friedman, Kent; Schonberg, Stefan A; Thrall, James; Salvatore, Marco; Rosen, Bruce R; Guimaraes, Alexander R
2015-08-01
Simultaneous data collection for positron emission tomography and magnetic resonance imaging (PET/MR) is now a reality. While the full benefits of concurrently acquiring PET and MR data and the potential added clinical value are still being evaluated, initial studies have identified several important potential pitfalls in the interpretation of fluorodeoxyglucose (FDG) PET/MRI in oncologic whole-body imaging, the majority of which being related to the errors in the attenuation maps created from the MR data. The purpose of this article was to present such pitfalls and artifacts using case examples, describe their etiology, and discuss strategies to overcome them. Using a case-based approach, we will illustrate artifacts related to (1) Inaccurate bone tissue segmentation; (2) Inaccurate air cavities segmentation; (3) Motion-induced misregistration; (4) RF coils in the PET field of view; (5) B0 field inhomogeneity; (6) B1 field inhomogeneity; (7) Metallic implants; (8) MR contrast agents. PMID:26025348
Ge, Nan; Chevalier, Stéphane; Hinebaugh, James; Yip, Ronnie; Lee, Jongmin; Antonacci, Patrick; Kotaka, Toshikazu; Tabuchi, Yuichiro; Bazylak, Aimy
2016-03-01
Synchrotron X-ray radiography, due to its high temporal and spatial resolutions, provides a valuable means for understanding the in operando water transport behaviour in polymer electrolyte membrane fuel cells. The purpose of this study is to address the specific artefact of imaging sample movement, which poses a significant challenge to synchrotron-based imaging for fuel cell diagnostics. Specifically, the impact of the micrometer-scale movement of the sample was determined, and a correction methodology was developed. At a photon energy level of 20 keV, a maximum movement of 7.5 µm resulted in a false water thickness of 0.93 cm (9% higher than the maximum amount of water that the experimental apparatus could physically contain). This artefact was corrected by image translations based on the relationship between the false water thickness value and the distance moved by the sample. The implementation of this correction method led to a significant reduction in false water thickness (to ∼0.04 cm). Furthermore, to account for inaccuracies in pixel intensities due to the scattering effect and higher harmonics, a calibration technique was introduced for the liquid water X-ray attenuation coefficient, which was found to be 0.657 ± 0.023 cm(-1) at 20 keV. The work presented in this paper provides valuable tools for artefact compensation and accuracy improvements for dynamic synchrotron X-ray imaging of fuel cells. PMID:26917148
Szidarovszky, Tamás; Császár, Attila G.
2015-01-07
The total partition functions Q(T) and their first two moments Q{sup ′}(T) and Q{sup ″}(T), together with the isobaric heat capacities C{sub p}(T), are computed a priori for three major MgH isotopologues on the temperature range of T = 100–3000 K using the recent highly accurate potential energy curve, spin-rotation, and non-adiabatic correction functions of Henderson et al. [J. Phys. Chem. A 117, 13373 (2013)]. Nuclear motion computations are carried out on the ground electronic state to determine the (ro)vibrational energy levels and the scattering phase shifts. The effect of resonance states is found to be significant above about 1000 K and it increases with temperature. Even very short-lived states, due to their relatively large number, have significant contributions to Q(T) at elevated temperatures. The contribution of scattering states is around one fourth of that of resonance states but opposite in sign. Uncertainty estimates are given for the possible error sources, suggesting that all computed thermochemical properties have an accuracy better than 0.005% up to 1200 K. Between 1200 and 2500 K, the uncertainties can rise to around 0.1%, while between 2500 K and 3000 K, a further increase to 0.5% might be observed for Q{sup ″}(T) and C{sub p}(T), principally due to the neglect of excited electronic states. The accurate thermochemical data determined are presented in the supplementary material for the three isotopologues of {sup 24}MgH, {sup 25}MgH, and {sup 26}MgH at 1 K increments. These data, which differ significantly from older standard data, should prove useful for astronomical models incorporating thermodynamic properties of these species.
NASA Astrophysics Data System (ADS)
Zhang, T.; Zhou, L.; Tong, S.
2015-12-01
The absolute determination of the Cu isotope ratio in NIST SRM 3114 based on a regression mass bias correction model is performed for the first time with NIST SRM 944 Ga as the calibrant. A value of 0.4471±0.0013 (2SD, n=37) for the 65Cu/63Cu ratio was obtained with a value of +0.18±0.04 ‰ (2SD, n=5) for δ65Cu relative to NIST 976.The availability of the NIST SRM 3114 material, now with the absolute value of the 65Cu/63Cu ratio and a δ65Cu value relative to NIST 976 makes it suitable as a new candidate reference material for Cu isotope studies. In addition, a protocol is described for the accurate and precise determination of δ65Cu values of geological reference materials. Purification of Cu from the sample matrix was performed using the AG MP-1M Bio-Rad resin. The column recovery for geological samples was found to be 100±2% (2SD, n=15).A modified method of standard-sample bracketing with internal normalization for mass bias correction was employed by adding natural Ga to both the sample and the solution of NIST SRM 3114, which was used as the bracketing standard. An absolute value of 0.4471±0.0013 (2SD, n=37) for 65Cu/63Cu quantified in this study was used to calibrate the 69Ga/71Ga ratio in the two adjacent bracketing standards of SRM 3114,their average value of 69Ga/71Ga was then used to correct the 65Cu/63Cu ratio in the sample. Measured δ65Cu values of 0.18±0.04‰ (2SD, n=20),0.13±0.04‰ (2SD, n=9),0.08±0.03‰ (2SD, n=6),0.01±0.06‰(2SD, n=4) and 0.26±0.04‰ (2SD, n=7) were obtained for five geological reference materials of BCR-2,BHVO-2,AGV-2,BIR-1a,and GSP-2,respectively,in agreement with values obtained in previous studies.
Control algorithms for dynamic attenuators
Hsieh, Scott S.; Pelc, Norbert J.
2014-06-15
modulation) without increasing peak variance. The 15-element piecewise-linear dynamic attenuator reduces dose by an average of 42%, and the perfect attenuator reduces dose by an average of 50%. Improvements in peak variance are several times larger than improvements in mean variance. Heuristic control eliminates the need for a prescan. For the piecewise-linear attenuator, the cost of heuristic control is an increase in dose of 9%. The proposed iterated WMV minimization produces results that are within a few percent of the true solution. Conclusions: Dynamic attenuators show potential for significant dose reduction. A wide class of dynamic attenuators can be accurately controlled using the described methods.
Modeling transmission and scatter for photon beam attenuators.
Ahnesjö, A; Weber, L; Nilsson, P
1995-11-01
The development of treatment planning methods in radiation therapy requires dose calculation methods that are both accurate and general enough to provide a dose per unit monitor setting for a broad variety of fields and beam modifiers. The purpose of this work was to develop models for calculation of scatter and transmission for photon beam attenuators such as compensating filters, wedges, and block trays. The attenuation of the beam is calculated using a spectrum of the beam, and a correction factor based on attenuation measurements. Small angle coherent scatter and electron binding effects on scattering cross sections are considered by use of a correction factor. Quality changes in beam penetrability and energy fluence to dose conversion are modeled by use of the calculated primary beam spectrum after passage through the attenuator. The beam spectra are derived by the depth dose effective method, i.e., by minimizing the difference between measured and calculated depth dose distributions, where the calculated distributions are derived by superposing data from a database for monoenergetic photons. The attenuator scatter is integrated over the area viewed from the calculation point of view using first scatter theory. Calculations are simplified by replacing the energy and angular-dependent cross-section formulas with the forward scatter constant r2(0) and a set of parametrized correction functions. The set of corrections include functions for the Compton energy loss, scatter attenuation, and secondary bremsstrahlung production. The effect of charged particle contamination is bypassed by avoiding use of dmax for absolute dose calibrations. The results of the model are compared with scatter measurements in air for copper and lead filters and with dose to a water phantom for lead filters for 4 and 18 MV. For attenuated beams, downstream of the buildup region, the calculated results agree with measurements on the 1.5% level. The accuracy was slightly less in situations
Modeling of polychromatic attenuation using computed tomography reconstructed images
NASA Technical Reports Server (NTRS)
Yan, C. H.; Whalen, R. T.; Beaupre, G. S.; Yen, S. Y.; Napel, S.
1999-01-01
This paper presents a procedure for estimating an accurate model of the CT imaging process including spectral effects. As raw projection data are typically unavailable to the end-user, we adopt a post-processing approach that utilizes the reconstructed images themselves. This approach includes errors from x-ray scatter and the nonidealities of the built-in soft tissue correction into the beam characteristics, which is crucial to beam hardening correction algorithms that are designed to be applied directly to CT reconstructed images. We formulate this approach as a quadratic programming problem and propose two different methods, dimension reduction and regularization, to overcome ill conditioning in the model. For the regularization method we use a statistical procedure, Cross Validation, to select the regularization parameter. We have constructed step-wedge phantoms to estimate the effective beam spectrum of a GE CT-I scanner. Using the derived spectrum, we computed the attenuation ratios for the wedge phantoms and found that the worst case modeling error is less than 3% of the corresponding attenuation ratio. We have also built two test (hybrid) phantoms to evaluate the effective spectrum. Based on these test phantoms, we have shown that the effective beam spectrum provides an accurate model for the CT imaging process. Last, we used a simple beam hardening correction experiment to demonstrate the effectiveness of the estimated beam profile for removing beam hardening artifacts. We hope that this estimation procedure will encourage more independent research on beam hardening corrections and will lead to the development of application-specific beam hardening correction algorithms.
An analytical approach to quantitative reconstruction of non-uniform attenuated brain SPECT.
Liang, Z; Ye, J; Harrington, D P
1994-11-01
An analytical approach to quantitative brain SPECT (single-photon-emission computed tomography) with non-uniform attenuation is developed. The approach formulates accurately the projection-transform equation as a summation of primary- and scatter-photon contributions. The scatter contribution can be estimated using the multiple-energy-window samples and removed from the primary-energy-window data by subtraction. The approach models the primary contribution as a convolution of the attenuated source and the detector-response kernel at a constant depth from the detector with the central-ray approximation. The attenuated Radon transform of the source can be efficiently deconvolved using the depth-frequency relation. The approach inverts exactly the attenuated Radon transform by Fourier transforms and series expansions. The performance of the analytical approach was studied for both uniform- and non-uniform-attenuation cases, and compared to the conventional FBP (filtered-backprojection) method by computer simulations. A patient brain X-ray image was acquired by a CT (computed-tomography) scanner and converted to the object-specific attenuation map for 140 keV energy. The mathematical Hoffman brain phantom was used to simulate the emission source and was resized such that it was completely surrounded by the skull of the CT attenuation map. The detector-response kernel was obtained from measurements of a point source at several depths in air from a parallel-hole collimator of a SPECT camera. The projection data were simulated from the object-specific attenuating source including the depth-dependent detector response. Quantitative improvement (>5%) in reconstructing the data was demonstrated with the nonuniform attenuation compensation, as compared to the uniform attenuation correction and the conventional FBP reconstruction. The commuting time was less than 5 min on an HP/730 desktop computer for an image array of 1282*32 from 128 projections of 128*32 size. PMID
General relationships between ultrasonic attenuation and dispersion
NASA Technical Reports Server (NTRS)
Odonnell, M.; Jaynes, E. T.; Miller, J. G.
1978-01-01
General relationships between the ultrasonic attenuation and dispersion are presented. The validity of these nonlocal relationships hinges only on the properties of causality and linearity, and does not depend upon details of the mechanism responsible for the attenuation and dispersion. Approximate, nearly local relationships are presented and are demonstrated to predict accurately the ultrasonic dispersion in solutions of hemoglobin from the results of attenuation measurements.
Evaluation of QNI corrections in porous media applications
NASA Astrophysics Data System (ADS)
Radebe, M. J.; de Beer, F. C.; Nshimirimana, R.
2011-09-01
Qualitative measurements using digital neutron imaging has been the more explored aspect than accurate quantitative measurements. The reason for this bias is that quantitative measurements require correction for background and material scatter, and neutron spectral effects. Quantitative Neutron Imaging (QNI) software package has resulted from efforts at the Paul Scherrer Institute, Helmholtz Zentrum Berlin (HZB) and Necsa to correct for these effects, while the sample-detector distance (SDD) principle has previously been demonstrated as a measure to eliminate material scatter effect. This work evaluates the capabilities of the QNI software package to produce accurate quantitative results on specific characteristics of porous media, and its role to nondestructive quantification of material with and without calibration. The work further complements QNI abilities by the use of different SDDs. Studies of effective %porosity of mortar and attenuation coefficient of water using QNI and SDD principle are reported.
Improved Background Corrections for Uranium Holdup Measurements
Oberer, R.B.; Gunn, C.A.; Chiang, L.G.
2004-06-21
In the original Generalized Geometry Holdup (GGH) model, all holdup deposits were modeled as points, lines, and areas[1, 5]. Two improvements[4] were recently made to the GGH model and are currently in use at the Y-12 National Security Complex. These two improvements are the finite-source correction CF{sub g} and the self-attenuation correction. The finite-source correction corrects the average detector response for the width of point and line geometries which in effect, converts points and lines into areas. The result of a holdup measurement of an area deposit is a density-thickness which is converted to mass by multiplying it by the area of the deposit. From the measured density-thickness, the true density-thickness can be calculated by correcting for the material self-attenuation. Therefore the self-attenuation correction is applied to finite point and line deposits as well as areas. This report demonstrates that the finite-source and self-attenuation corrections also provide a means to better separate the gamma rays emitted by the material from the gamma rays emitted by background sources for an improved background correction. Currently, the measured background radiation is attenuated for equipment walls in the case of area deposits but not for line and point sources. The measured background radiation is not corrected for attenuation by the uranium material. For all of these cases, the background is overestimated which causes a negative bias in the measurement. The finite-source correction and the self-attenuation correction will allow the correction of the measured background radiation for both the equipment attenuation and material attenuation for area sources as well as point and line sources.
Quantitative SPECT reconstruction using CT-derived corrections
NASA Astrophysics Data System (ADS)
Willowson, Kathy; Bailey, Dale L.; Baldock, Clive
2008-06-01
A method for achieving quantitative single-photon emission computed tomography (SPECT) based upon corrections derived from x-ray computed tomography (CT) data is presented. A CT-derived attenuation map is used to perform transmission-dependent scatter correction (TDSC) in conjunction with non-uniform attenuation correction. The original CT data are also utilized to correct for partial volume effects in small volumes of interest. The accuracy of the quantitative technique has been evaluated with phantom experiments and clinical lung ventilation/perfusion SPECT/CT studies. A comparison of calculated values with the known total activities and concentrations in a mixed-material cylindrical phantom, and in liver and cardiac inserts within an anthropomorphic torso phantom, produced accurate results. The total activity in corrected ventilation-subtracted perfusion images was compared to the calibrated injected dose of [99mTc]-MAA (macro-aggregated albumin). The average difference over 12 studies between the known and calculated activities was found to be -1%, with a range of ±7%.
Trinquier, Anne; Touboul, Mathieu; Walker, Richard J
2016-02-01
Determination of the (182)W/(184)W ratio to a precision of ± 5 ppm (2σ) is desirable for constraining the timing of core formation and other early planetary differentiation processes. However, WO3(-) analysis by negative thermal ionization mass spectrometry normally results in a residual correlation between the instrumental-mass-fractionation-corrected (182)W/(184)W and (183)W/(184)W ratios that is attributed to mass-dependent variability of O isotopes over the course of an analysis and between different analyses. A second-order correction using the (183)W/(184)W ratio relies on the assumption that this ratio is constant in nature. This may prove invalid, as has already been realized for other isotope systems. The present study utilizes simultaneous monitoring of the (18)O/(16)O and W isotope ratios to correct oxide interferences on a per-integration basis and thus avoid the need for a double normalization of W isotopes. After normalization of W isotope ratios to a pair of W isotopes, following the exponential law, no residual W-O isotope correlation is observed. However, there is a nonideal mass bias residual correlation between (182)W/(i)W and (183)W/(i)W with time. Without double normalization of W isotopes and on the basis of three or four duplicate analyses, the external reproducibility per session of (182)W/(184)W and (183)W/(184)W normalized to (186)W/(183)W is 5-6 ppm (2σ, 1-3 μg loads). The combined uncertainty per session is less than 4 ppm for (183)W/(184)W and less than 6 ppm for (182)W/(184)W (2σm) for loads between 3000 and 50 ng. PMID:26751903
NASA Astrophysics Data System (ADS)
Chimot, J.; Vlemmix, T.; Veefkind, J. P.; de Haan, J. F.; Levelt, P. F.
2015-08-01
The Ozone Monitoring Instrument (OMI) instrument has provided daily global measurements of tropospheric NO2 for more than a decade. Numerous studies have drawn attention to the complexities related to measurements of tropospheric NO2 in the presence of aerosols. Fine particles affect the OMI spectral measurements and the length of the average light path followed by the photons. However, they are not explicitly taken into account in the current OMI tropospheric NO2 retrieval chain. Instead, the operational OMI O2-O2 cloud retrieval algorithm is applied both to cloudy scenes and to cloud free scenes with aerosols present. This paper describes in detail the complex interplay between the spectral effects of aerosols, the OMI O2-O2 cloud retrieval algorithm and the impact on the accuracy of the tropospheric NO2 retrievals through the computed Air Mass Factor (AMF) over cloud-free scenes. Collocated OMI NO2 and MODIS Aqua aerosol products are analysed over East China, in industrialized area. In addition, aerosol effects on the tropospheric NO2 AMF and the retrieval of OMI cloud parameters are simulated. Both the observation-based and the simulation-based approach demonstrate that the retrieved cloud fraction linearly increases with increasing Aerosol Optical Thickness (AOT), but the magnitude of this increase depends on the aerosol properties and surface albedo. This increase is induced by the additional scattering effects of aerosols which enhance the scene brightness. The decreasing effective cloud pressure with increasing AOT represents primarily the absorbing effects of aerosols. The study cases show that the actual aerosol correction based on the implemented OMI cloud model results in biases between -20 and -40 % for the DOMINO tropospheric NO2 product in cases of high aerosol pollution (AOT ≥ 0.6) and elevated particles. On the contrary, when aerosols are relatively close to the surface or mixed with NO2, aerosol correction based on the cloud model results in
Rocklin, Gabriel J.; Mobley, David L.; Dill, Ken A.; Hünenberger, Philippe H.
2013-01-01
The calculation of a protein-ligand binding free energy based on molecular dynamics (MD) simulations generally relies on a thermodynamic cycle in which the ligand is alchemically inserted into the system, both in the solvated protein and free in solution. The corresponding ligand-insertion free energies are typically calculated in nanoscale computational boxes simulated under periodic boundary conditions and considering electrostatic interactions defined by a periodic lattice-sum. This is distinct from the ideal bulk situation of a system of macroscopic size simulated under non-periodic boundary conditions with Coulombic electrostatic interactions. This discrepancy results in finite-size effects, which affect primarily the charging component of the insertion free energy, are dependent on the box size, and can be large when the ligand bears a net charge, especially if the protein is charged as well. This article investigates finite-size effects on calculated charging free energies using as a test case the binding of the ligand 2-amino-5-methylthiazole (net charge +1 e) to a mutant form of yeast cytochrome c peroxidase in water. Considering different charge isoforms of the protein (net charges −5, 0, +3, or +9 e), either in the absence or the presence of neutralizing counter-ions, and sizes of the cubic computational box (edges ranging from 7.42 to 11.02 nm), the potentially large magnitude of finite-size effects on the raw charging free energies (up to 17.1 kJ mol−1) is demonstrated. Two correction schemes are then proposed to eliminate these effects, a numerical and an analytical one. Both schemes are based on a continuum-electrostatics analysis and require performing Poisson-Boltzmann (PB) calculations on the protein-ligand system. While the numerical scheme requires PB calculations under both non-periodic and periodic boundary conditions, the latter at the box size considered in the MD simulations, the analytical scheme only requires three non-periodic PB
Rocklin, Gabriel J.; Mobley, David L.; Dill, Ken A.; Hünenberger, Philippe H.
2013-11-14
The calculation of a protein-ligand binding free energy based on molecular dynamics (MD) simulations generally relies on a thermodynamic cycle in which the ligand is alchemically inserted into the system, both in the solvated protein and free in solution. The corresponding ligand-insertion free energies are typically calculated in nanoscale computational boxes simulated under periodic boundary conditions and considering electrostatic interactions defined by a periodic lattice-sum. This is distinct from the ideal bulk situation of a system of macroscopic size simulated under non-periodic boundary conditions with Coulombic electrostatic interactions. This discrepancy results in finite-size effects, which affect primarily the charging component of the insertion free energy, are dependent on the box size, and can be large when the ligand bears a net charge, especially if the protein is charged as well. This article investigates finite-size effects on calculated charging free energies using as a test case the binding of the ligand 2-amino-5-methylthiazole (net charge +1 e) to a mutant form of yeast cytochrome c peroxidase in water. Considering different charge isoforms of the protein (net charges −5, 0, +3, or +9 e), either in the absence or the presence of neutralizing counter-ions, and sizes of the cubic computational box (edges ranging from 7.42 to 11.02 nm), the potentially large magnitude of finite-size effects on the raw charging free energies (up to 17.1 kJ mol{sup −1}) is demonstrated. Two correction schemes are then proposed to eliminate these effects, a numerical and an analytical one. Both schemes are based on a continuum-electrostatics analysis and require performing Poisson-Boltzmann (PB) calculations on the protein-ligand system. While the numerical scheme requires PB calculations under both non-periodic and periodic boundary conditions, the latter at the box size considered in the MD simulations, the analytical scheme only requires three non
Body Deformation Correction for SPECT Imaging
Gu, Songxiang; McNamara, Joseph E.; Mitra, Joyeeta; Gifford, Howard C.; Johnson, Karen; Gennert, Michael A.; King, Michael A.
2010-01-01
Patient motion degrades the quality of SPECT studies. Body bend and twist are types of patient deformation, which may occur during SPECT imaging, and which has been generally ignored in SPECT motion correction strategies. To correct for these types of motion, we propose a deformation model and its inclusion within an iterative reconstruction algorithm. Two experiments were conducted to investigate the applicability of our model. In the first experiment, the return of the postmotion-compensation locations of markers on the body-surface of a volunteer to approximate their original coordinates is used to examine our method of estimating the parameters of our model and the parameters’ use in undoing deformation. The second experiment employed simulated projections of the MCAT phantom formed using an analytical projector which includes attenuation and distance-dependent resolution to investigate applications of our model in reconstruction. We demonstrate in the simulation studies that twist and bend can significantly degrade SPECT image quality visually. Our correction strategy is shown to be able to greatly diminish the degradation seen in the slices, provided the parameters are estimated accurately. We view this work as a first step towards being able to estimate and correct patient deformation based on information obtained from marker tracking data. PMID:20336188
NASA Astrophysics Data System (ADS)
Chimot, J.; Vlemmix, T.; Veefkind, J. P.; de Haan, J. F.; Levelt, P. F.
2016-02-01
The Ozone Monitoring Instrument (OMI) has provided daily global measurements of tropospheric NO2 for more than a decade. Numerous studies have drawn attention to the complexities related to measurements of tropospheric NO2 in the presence of aerosols. Fine particles affect the OMI spectral measurements and the length of the average light path followed by the photons. However, they are not explicitly taken into account in the current operational OMI tropospheric NO2 retrieval chain (DOMINO - Derivation of OMI tropospheric NO2) product. Instead, the operational OMI O2 - O2 cloud retrieval algorithm is applied both to cloudy and to cloud-free scenes (i.e. clear sky) dominated by the presence of aerosols. This paper describes in detail the complex interplay between the spectral effects of aerosols in the satellite observation and the associated response of the OMI O2 - O2 cloud retrieval algorithm. Then, it evaluates the impact on the accuracy of the tropospheric NO2 retrievals through the computed Air Mass Factor (AMF) with a focus on cloud-free scenes. For that purpose, collocated OMI NO2 and MODIS (Moderate Resolution Imaging Spectroradiometer) Aqua aerosol products are analysed over the strongly industrialized East China area. In addition, aerosol effects on the tropospheric NO2 AMF and the retrieval of OMI cloud parameters are simulated. Both the observation-based and the simulation-based approach demonstrate that the retrieved cloud fraction increases with increasing Aerosol Optical Thickness (AOT), but the magnitude of this increase depends on the aerosol properties and surface albedo. This increase is induced by the additional scattering effects of aerosols which enhance the scene brightness. The decreasing effective cloud pressure with increasing AOT primarily represents the shielding effects of the O2 - O2 column located below the aerosol layers. The study cases show that the aerosol correction based on the implemented OMI cloud model results in biases
Global Attenuation Model of the Upper Mantle
NASA Astrophysics Data System (ADS)
Adenis, A.; Debayle, E.; Ricard, Y. R.
2015-12-01
We present a three-dimensional shear attenuation model based on a massive surface wave data-set (372,629 Rayleigh waveforms analysed in the period range 50-300s by Debayle and Ricard, 2012). For each seismogram, this approach yields depth-dependent path average models of shear velocity and quality factor, and a set of fundamental and higher-mode dispersion and attenuation curves. We combine these attenuation measurements in a tomographic inversion after a careful rejection of the noisy data. We first remove data likely to be biased by a poor knowledge of the source. Then we assume that waves corresponding to events having close epicenters and recorded at the same station sample the same elastic and anelastic structure, we cluster the corresponding rays and average the attenuation measurements. Logarithms of the attenuations are regionalized using the non-linear east square formalism of Tarantola and Valette (1982), resulting in attenuation tomographic maps between 50s and 300s. After a first inversion, outlyers are rejected and a second inversion yields a moderate variance reduction of about 20%. We correct the attenuation curves for focusing effect using the linearized ray theory of Woodhouse and Wong (1986). Accounting for focussing effects allows building tomographic maps with variance reductions reaching 40%. In the period range 120-200s, the root mean square of the model perturbations increases from about 5% to 20%. Our 3-D attenuation models present strong agreement with surface tectonics at period lower than 200s. Areas of low attenuation are located under continents and areas of high attenuation are associated with oceans. Surprisingly, although mid oceanic ridges are located in attenuating regions, their signature, even if enhanced by focusing corrections, remains weaker than in the shear velocity models. Synthetic tests suggests that regularisation contributes to damp the attenuation signature of ridges, which could therefore be underestimated.
NASA Technical Reports Server (NTRS)
Dickinson, R. M.; Hardy, J. C.
1969-01-01
Radio frequency attenuator, having negligible insertion loss at minimum attenuation, can be used for making precise antenna gain measurements. It is small in size compared to a rotary-vane attenuator.
Mao, Ye-Wei; Kong, Xu; Lin, Lin E-mail: xkong@ustc.edu.cn
2014-07-01
Variations in the attenuation law have a significant impact on observed spectral energy distributions for galaxies. As one important observational property for galaxies at ultraviolet and infrared wavelength bands, the correlation between infrared-to-ultraviolet luminosity ratio and ultraviolet color index (or ultraviolet spectral slope), i.e., the IRX-UV relation (or IRX-β relation), offered a widely used formula for correcting dust attenuation in galaxies, but the usability appears to be in doubt now because of considerable dispersion in this relation found by many studies. In this paper, on the basis of spectral synthesis modeling and spatially resolved measurements of four nearby spiral galaxies, we provide an interpretation of the deviation in the IRX-UV relation with variations in the attenuation law. From both theoretical and observational viewpoints, two components in the attenuation curve, the linear background and the 2175 Å bump, are suggested to be the parameters in addition to the stellar population age (addressed in the first paper of this series) in the IRX-UV function; different features in the attenuation curve are diagnosed for the galaxies in our sample. Nevertheless, it is often difficult to ascertain the attenuation law for galaxies in actual observations. Possible reasons for preventing the successful detection of the parameters in the attenuation curve are also discussed in this paper, including the degeneracy of the linear background and the 2175 Å bump in observational channels, the requirement for young and dust-rich systems to study, and the difficulty in accurate estimates of dust attenuations at different wavelength bands.
SPECT Compton-scattering correction by analysis of energy spectra.
Koral, K F; Wang, X Q; Rogers, W L; Clinthorne, N H; Wang, X H
1988-02-01
The hypothesis that energy spectra at individual spatial locations in single photon emission computed tomographic projection images can be analyzed to separate the Compton-scattered component from the unscattered component is tested indirectly. An axially symmetric phantom consisting of a cylinder with a sphere is imaged with either the cylinder or the sphere containing 99mTc. An iterative peak-erosion algorithm and a fitting algorithm are given and employed to analyze the acquired spectra. Adequate separation into an unscattered component and a Compton-scattered component is judged on the basis of filtered-backprojection reconstruction of corrected projections. In the reconstructions, attenuation correction is based on the known geometry and the total attenuation cross section for water. An independent test of the accuracy of separation is not made. For both algorithms, reconstructed slices for the cold-sphere, hot-surround phantom have the correct shape as confirmed by simulation results that take into account the measured dependence of system resolution on depth. For the inverse phantom, a hot sphere in a cold surround, quantitative results with the fitting algorithm are accurate but with a particular number of iterations of the erosion algorithm are less good. (A greater number of iterations would improve the 26% error with the algorithm, however.) These preliminary results encourage us to believe that a method for correcting for Compton-scattering in a wide variety of objects can be found, thus helping to achieve quantitative SPECT. PMID:3258023
Hargrove, Douglas L.
2004-09-14
A portable, hand-held meter used to measure direct current (DC) attenuation in low impedance electrical signal cables and signal attenuators. A DC voltage is applied to the signal input of the cable and feedback to the control circuit through the signal cable and attenuators. The control circuit adjusts the applied voltage to the cable until the feedback voltage equals the reference voltage. The "units" of applied voltage required at the cable input is the system attenuation value of the cable and attenuators, which makes this meter unique. The meter may be used to calibrate data signal cables, attenuators, and cable-attenuator assemblies.
Assimilation of attenuated data from X-band network radars using ensemble Kalman filter
NASA Astrophysics Data System (ADS)
Cheng, Jing
To use reflectivity data from X-band radars for quantitative precipitation estimation and storm-scale data assimilation, the effect of attenuation must be properly accounted for. Traditional approaches try to make correction to the attenuated reflectivity first before using the data. An alternative, theoretically more attractive approach builds the attenuation effect into the reflectivity observation operator of a data assimilation system, such as an ensemble Kalman filter (EnKF), allowing direct assimilation of the attenuated reflectivity and taking advantage of microphysical state estimation using EnKF methods for a potentially more accurate solution. This study first tests the approach for the CASA (Center for Collaborative Adaptive Sensing of the Atmosphere) X-band radar network configuration through observing system simulation experiments (OSSE) for a quasi-linear convective system (QLCS) that has more significant attenuation than isolated storms. To avoid the problem of potentially giving too much weight to fully attenuated reflectivity, an analytical, echo-intensity-dependent model for the observation error (AEM) is developed and is found to improve the performance of the filter. By building the attenuation into the forward observation operator and combining it with the application of AEM, the assimilation of attenuated CASA observations is able to produce a reasonably accurate analysis of the QLCS inside CASA radar network coverage. Compared with foregoing assimilation of radar data with weak radar reflectivity or assimilating only radial velocity data, our method can suppress the growth of spurious echoes while obtaining a more accurate analysis in the terms of root-mean-square (RMS) error. Sensitivity experiments are designed to examine the effectiveness of AEM by introducing multiple sources of observation errors into the simulated observations. The performance of such an approach in the presence of resolution-induced model error is also evaluated and
Mehranian, Abolfazl; Zaidi, Habib
2015-09-01
It has recently been shown that the attenuation map can be estimated from time-of-flight (TOF) PET emission data using joint maximum likelihood reconstruction of attenuation and activity (MLAA). In this work, we propose a novel MRI-guided MLAA algorithm for emission-based attenuation correction in whole-body PET/MR imaging. The algorithm imposes MR spatial and CT statistical constraints on the MLAA estimation of attenuation maps using a constrained Gaussian mixture model (GMM) and a Markov random field smoothness prior. Dixon water and fat MR images were segmented into outside air, lung, fat and soft-tissue classes and an MR low-intensity (unknown) class corresponding to air cavities, cortical bone and susceptibility artifacts. The attenuation coefficients over the unknown class were estimated using a mixture of four Gaussians, and those over the known tissue classes using unimodal Gaussians, parameterized over a patient population. To eliminate misclassification of spongy bones with surrounding tissues, and thus include them in the unknown class, we heuristically suppressed fat in water images and also used a co-registered bone probability map. The proposed MLAA-GMM algorithm was compared with the MLAA algorithms proposed by Rezaei and Salomon using simulation and clinical studies with two different tracer distributions. The results showed that our proposed algorithm outperforms its counterparts in suppressing the cross-talk and scaling problems of activity and attenuation and thus produces PET images of improved quantitative accuracy. It can be concluded that the proposed algorithm effectively exploits the MR information and can pave the way toward accurate emission-based attenuation correction in TOF PET/MRI. PMID:25769148
Seismic attenuation anisotropy in reservoir sedimentary rocks
Best, A.I.
1994-12-31
Seismic attenuation is a fundamental property of reservoir sedimentary rocks; it is strongly related to reservoir permeability. Knowledge of its variation with lithology, with burial depth, and with wave propagation direction is vital for understanding the attenuation mechanism. Given this information, realistic theoretical models may be constructed for predicting attenuation, and hence permeability, over a wide frequency range. Accurate ultrasonic attenuation measurements were made in the laboratory over a range of effective pressures on sandstone samples with different amounts of humic organic matter. The organic matter formed fine laminations along the bedding planes of the sandstones. The results show that the sandstones are highly attenuating at 5 MPa mainly because of the presence of grain contact microcracks giving rise to squirt flow; at 40 MPa, when most of the microcracks are closed, the clean sandstones are poorly attenuating, but the organic-rich sandstones remain highly attenuating. It is postulated that the compliant organic matter is responsible for causing squirt flow at high and at low pressures. The results also show that the maximum attenuation occurs when the particle motion of the propagating wave is perpendicular to the planes of the organic matter laminations. These results are consistent with the squirt flow theory of Akbar et al (1993) for compressional waves.
ERIC Educational Resources Information Center
Rom, Mark Carl
2011-01-01
Grades matter. College grading systems, however, are often ad hoc and prone to mistakes. This essay focuses on one factor that contributes to high-quality grading systems: grading accuracy (or "efficiency"). I proceed in several steps. First, I discuss the elements of "efficient" (i.e., accurate) grading. Next, I present analytical results…
Christie, Alan M.; Snyder, Kurt I.
1985-01-01
A pressure surge attenuation system for pipes having a fluted region opposite crushable metal foam. As adapted for nuclear reactor vessels and heads, crushable metal foam is disposed to attenuate pressure surges.
Accurate monotone cubic interpolation
NASA Technical Reports Server (NTRS)
Huynh, Hung T.
1991-01-01
Monotone piecewise cubic interpolants are simple and effective. They are generally third-order accurate, except near strict local extrema where accuracy degenerates to second-order due to the monotonicity constraint. Algorithms for piecewise cubic interpolants, which preserve monotonicity as well as uniform third and fourth-order accuracy are presented. The gain of accuracy is obtained by relaxing the monotonicity constraint in a geometric framework in which the median function plays a crucial role.
Accurate Finite Difference Algorithms
NASA Technical Reports Server (NTRS)
Goodrich, John W.
1996-01-01
Two families of finite difference algorithms for computational aeroacoustics are presented and compared. All of the algorithms are single step explicit methods, they have the same order of accuracy in both space and time, with examples up to eleventh order, and they have multidimensional extensions. One of the algorithm families has spectral like high resolution. Propagation with high order and high resolution algorithms can produce accurate results after O(10(exp 6)) periods of propagation with eight grid points per wavelength.
Tracer attenuation in groundwater
NASA Astrophysics Data System (ADS)
Cvetkovic, Vladimir
2011-12-01
The self-purifying capacity of aquifers strongly depends on the attenuation of waterborne contaminants, i.e., irreversible loss of contaminant mass on a given scale as a result of coupled transport and transformation processes. A general formulation of tracer attenuation in groundwater is presented. Basic sensitivities of attenuation to macrodispersion and retention are illustrated for a few typical retention mechanisms. Tracer recovery is suggested as an experimental proxy for attenuation. Unique experimental data of tracer recovery in crystalline rock compare favorably with the theoretical model that is based on diffusion-controlled retention. Non-Fickian hydrodynamic transport has potentially a large impact on field-scale attenuation of dissolved contaminants.
Attenuation Tomography of the Upper Mantle
NASA Astrophysics Data System (ADS)
Adenis, A.; Debayle, E.; Ricard, Y. R.
2014-12-01
We present a 3-D model of surface wave attenuation in the upper mantle. The model is constrained by a large data set of fundamental and higher Rayleigh mode observations. This data set consists of about 1,800,000 attenuation curves measured in the period range 50-300s by Debayle and Ricard (2012). A careful selection allows us to reject data for which measurements are likely biased by the poor knowledge of the scalar seismic moment or by a ray propagation too close to a node of the source radiation pattern. For each epicenter-station path, elastic focusing effects due to seismic heterogeneities are corrected using DR2012 and the data are turned into log(1/Q). The selected data are then combined in a tomographic inversion using the non-linear least square formalism of Tarantola and Valette (1982). The obtained attenuation maps are in agreement with the surface tectonic for periods and modes sensitive to the top 200km of the upper mantle. Low attenuation regions correlate with continental shields while high attenuation regions are located beneath young oceanic regions. The attenuation pattern becomes more homogeneous at depths greater than 200 km and the maps are dominated by a high quality factor signature beneath slabs. We will discuss the similarities and differences between the tomographies of seismic velocities and of attenuations.
Soliman, Khaled; Alenezi, Ahmed
2015-02-01
The aim of the study was to measure the actual dose at 1 m from the patients per unit activity with the aim of providing a more accurate prediction of the dose levels around radioiodine patients in the hospital, as well as to compare our results with the literature. In this work the demonstration of a patient body tissue attenuation factor is verified by comparing the dose rates measured from the patients with those measured from the unshielded radioiodine capsules immediately after administration of the radioactivity. The normalized dose rate per unit activity is therefore proposed as an operational quantity that can be used to predict exposure rates to staff and patients' relatives. The average dose rate measured from our patient per unit activity was 38.4±11.8 μSv/h/GBq. The calculated attenuation correction factor based on our measurements was 0.55±0.17. The calculated dose rate from a radioiodine therapy patient should normally include a factor accounting for patient body tissue attenuation and scatter. The attenuation factor is currently neglected and not applied in operational radiation protection. Realistic estimation of radiation dose levels from radioiodine therapy patients when properly performed will reduce the operational cost and optimize institutional radiation protection practice. It is recommended to include patient attenuation factors in risk assessment exercises - in particular, when accurate estimates of total effective doses to exposed individuals are required when direct measurements are not possible. The information provided about patient attenuation might benefit radiation protection specialists and regulators. PMID:25279710
Correction of WindScat Scatterometric Measurements by Combining with AMSR Radiometric Data
NASA Technical Reports Server (NTRS)
Song, S.; Moore, R. K.
1996-01-01
The Seawinds scatterometer on the advanced Earth observing satellite-2 (ADEOS-2) will determine surface wind vectors by measuring the radar cross section. Multiple measurements will be made at different points in a wind-vector cell. When dense clouds and rain are present, the signal will be attenuated, thereby giving erroneous results for the wind. This report describes algorithms to use with the advanced mechanically scanned radiometer (AMSR) scanning radiometer on ADEOS-2 to correct for the attenuation. One can determine attenuation from a radiometer measurement based on the excess brightness temperature measured. This is the difference between the total measured brightness temperature and the contribution from surface emission. A major problem that the algorithm must address is determining the surface contribution. Two basic approaches were developed for this, one using the scattering coefficient measured along with the brightness temperature, and the other using the brightness temperature alone. For both methods, best results will occur if the wind from the preceding wind-vector cell can be used as an input to the algorithm. In the method based on the scattering coefficient, we need the wind direction from the preceding cell. In the method using brightness temperature alone, we need the wind speed from the preceding cell. If neither is available, the algorithm can work, but the corrections will be less accurate. Both correction methods require iterative solutions. Simulations show that the algorithms make significant improvements in the measured scattering coefficient and thus is the retrieved wind vector. For stratiform rains, the errors without correction can be quite large, so the correction makes a major improvement. For systems of separated convective cells, the initial error is smaller and the correction, although about the same percentage, has a smaller effect.
Iterative Beam Hardening Correction for Multi-Material Objects
Zhao, Yunsong; Li, Mengfei
2015-01-01
In this paper, we propose an iterative beam hardening correction method that is applicable for the case with multiple materials. By assuming that the materials composing scanned object are known and that they are distinguishable by their linear attenuation coefficients at some given energy, the beam hardening correction problem is converted into a nonlinear system problem, which is then solved iteratively. The reconstructed image is the distribution of linear attenuation coefficient of the scanned object at a given energy. So there are no beam hardening artifacts in the image theoretically. The proposed iterative scheme combines an accurate polychromatic forward projection with a linearized backprojection. Both forward projection and backprojection have high degree of parallelism, and are suitable for acceleration on parallel systems. Numerical experiments with both simulated data and real data verifies the validity of the proposed method. The beam hardening artifacts are alleviated effectively. In addition, the proposed method has a good tolerance on the error of the estimated x-ray spectrum. PMID:26659554
Iterative Beam Hardening Correction for Multi-Material Objects.
Zhao, Yunsong; Li, Mengfei
2015-01-01
In this paper, we propose an iterative beam hardening correction method that is applicable for the case with multiple materials. By assuming that the materials composing scanned object are known and that they are distinguishable by their linear attenuation coefficients at some given energy, the beam hardening correction problem is converted into a nonlinear system problem, which is then solved iteratively. The reconstructed image is the distribution of linear attenuation coefficient of the scanned object at a given energy. So there are no beam hardening artifacts in the image theoretically. The proposed iterative scheme combines an accurate polychromatic forward projection with a linearized backprojection. Both forward projection and backprojection have high degree of parallelism, and are suitable for acceleration on parallel systems. Numerical experiments with both simulated data and real data verifies the validity of the proposed method. The beam hardening artifacts are alleviated effectively. In addition, the proposed method has a good tolerance on the error of the estimated x-ray spectrum. PMID:26659554
Aerosol effects and corrections in the Halogen Occultation Experiment
NASA Technical Reports Server (NTRS)
Hervig, Mark E.; Russell, James M., III; Gordley, Larry L.; Daniels, John; Drayson, S. Roland; Park, Jae H.
1995-01-01
The eruptions of Mt. Pinatubo in June 1991 increased stratospheric aerosol loading by a factor of 30, affecting chemistry, radiative transfer, and remote measurements of the stratosphere. The Halogen Occultation Experiment (HALOE) instrument on board Upper Atmosphere Research Satellite (UARS) makes measurements globally for inferring profiles of NO2, H2O, O3, HF, HCl, CH4, NO, and temperature in addition to aerosol extinction at five wavelengths. Understanding and removing the aerosol extinction is essential for obtaining accurate retrievals from the radiometer channels of NO2, H2O and O3 in the lower stratosphere since these measurements are severely affected by contaminant aerosol absorption. If ignored, aerosol absorption in the radiometer measurements is interpreted as additional absorption by the target gas, resulting in anomalously large mixing ratios. To correct the radiometer measurements for aerosol effects, a retrieved aerosol extinction profile is extrapolated to the radiometer wavelengths and then included as continuum attenuation. The sensitivity of the extrapolation to size distribution and composition is small for certain wavelength combinations, reducing the correction uncertainty. The aerosol corrections extend the usable range of profiles retrieved from the radiometer channels to the tropopause with results that agree well with correlative measurements. In situations of heavy aerosol loading, errors due to aerosol in the retrieved mixing ratios are reduced to values of about 15, 25, and 60% in H2O, O3, and NO2, respectively, levels that are much less than the correction magnitude.
NASA Astrophysics Data System (ADS)
Itano, Wayne M.; Ramsey, Norman F.
1993-07-01
The paper discusses current methods for accurate measurements of time by conventional atomic clocks, with particular attention given to the principles of operation of atomic-beam frequency standards, atomic hydrogen masers, and atomic fountain and to the potential use of strings of trapped mercury ions as a time device more stable than conventional atomic clocks. The areas of application of the ultraprecise and ultrastable time-measuring devices that tax the capacity of modern atomic clocks include radio astronomy and tests of relativity. The paper also discusses practical applications of ultraprecise clocks, such as navigation of space vehicles and pinpointing the exact position of ships and other objects on earth using the GPS.
Accurate quantum chemical calculations
NASA Technical Reports Server (NTRS)
Bauschlicher, Charles W., Jr.; Langhoff, Stephen R.; Taylor, Peter R.
1989-01-01
An important goal of quantum chemical calculations is to provide an understanding of chemical bonding and molecular electronic structure. A second goal, the prediction of energy differences to chemical accuracy, has been much harder to attain. First, the computational resources required to achieve such accuracy are very large, and second, it is not straightforward to demonstrate that an apparently accurate result, in terms of agreement with experiment, does not result from a cancellation of errors. Recent advances in electronic structure methodology, coupled with the power of vector supercomputers, have made it possible to solve a number of electronic structure problems exactly using the full configuration interaction (FCI) method within a subspace of the complete Hilbert space. These exact results can be used to benchmark approximate techniques that are applicable to a wider range of chemical and physical problems. The methodology of many-electron quantum chemistry is reviewed. Methods are considered in detail for performing FCI calculations. The application of FCI methods to several three-electron problems in molecular physics are discussed. A number of benchmark applications of FCI wave functions are described. Atomic basis sets and the development of improved methods for handling very large basis sets are discussed: these are then applied to a number of chemical and spectroscopic problems; to transition metals; and to problems involving potential energy surfaces. Although the experiences described give considerable grounds for optimism about the general ability to perform accurate calculations, there are several problems that have proved less tractable, at least with current computer resources, and these and possible solutions are discussed.
Foltyn, Stephen R.
1988-01-01
The disclosure relates to low loss, high power variable attenuators comprng one or more transmissive and/or reflective multilayer dielectric filters. The attenuator is particularly suitable to use with unpolarized lasers such as excimer lasers. Beam attenuation is a function of beam polarization and the angle of incidence between the beam and the filter and is controlled by adjusting the angle of incidence the beam makes to the filter or filters. Filters are selected in accordance with beam wavelength.
Foltyn, S.R.
1987-05-29
The disclosure relates to low loss, high power variable attenuators comprising one or more transmissive and/or reflective multilayer dielectric filters. The attenuator is particularly suitable to use with unpolarized lasers such as excimer lasers. Beam attenuation is a function of beam polarization and the angle of incidence between the beam and the filter and is controlled by adjusting the angle of incidence the beam makes to the filter or filters. Filters are selected in accordance with beam wavelength. 9 figs.
Accurate ab Initio Spin Densities
2012-01-01
We present an approach for the calculation of spin density distributions for molecules that require very large active spaces for a qualitatively correct description of their electronic structure. Our approach is based on the density-matrix renormalization group (DMRG) algorithm to calculate the spin density matrix elements as a basic quantity for the spatially resolved spin density distribution. The spin density matrix elements are directly determined from the second-quantized elementary operators optimized by the DMRG algorithm. As an analytic convergence criterion for the spin density distribution, we employ our recently developed sampling-reconstruction scheme [J. Chem. Phys.2011, 134, 224101] to build an accurate complete-active-space configuration-interaction (CASCI) wave function from the optimized matrix product states. The spin density matrix elements can then also be determined as an expectation value employing the reconstructed wave function expansion. Furthermore, the explicit reconstruction of a CASCI-type wave function provides insight into chemically interesting features of the molecule under study such as the distribution of α and β electrons in terms of Slater determinants, CI coefficients, and natural orbitals. The methodology is applied to an iron nitrosyl complex which we have identified as a challenging system for standard approaches [J. Chem. Theory Comput.2011, 7, 2740]. PMID:22707921
Correlation Attenuation Due to Measurement Error: A New Approach Using the Bootstrap Procedure
ERIC Educational Resources Information Center
Padilla, Miguel A.; Veprinsky, Anna
2012-01-01
Issues with correlation attenuation due to measurement error are well documented. More than a century ago, Spearman proposed a correction for attenuation. However, this correction has seen very little use since it can potentially inflate the true correlation beyond one. In addition, very little confidence interval (CI) research has been done for…
Quantitative fully 3D PET via model-based scatter correction
Ollinger, J.M.
1994-05-01
We have investigated the quantitative accuracy of fully 3D PET using model-based scatter correction by measuring the half-life of Ga-68 in the presence of scatter from F-18. The inner chamber of a Data Spectrum cardiac phantom was filled with 18.5 MBq of Ga-68. The outer chamber was filled with an equivalent amount of F-18. The cardiac phantom was placed in a 22x30.5 cm elliptical phantom containing anthropomorphic lung inserts filled with a water-Styrofoam mixture. Ten frames of dynamic data were collected over 13.6 hours on Siemens-CTI 953B scanner with the septa retracted. The data were corrected using model-based scatter correction, which uses the emission images, transmission images and an accurate physical model to directly calculate the scatter distribution. Both uncorrected and corrected data were reconstructed using the Promis algorithm. The scatter correction required 4.3% of the total reconstruction time. The scatter fraction in a small volume of interest in the center of the inner chamber of the cardiac insert rose from 4.0% in the first interval to 46.4% in the last interval as the ratio of F-18 activity to Ga-68 activity rose from 1:1 to 33:1. Fitting a single exponential to the last three data points yields estimates of the half-life of Ga-68 of 77.01 minutes and 68.79 minutes for uncorrected and corrected data respectively. Thus, scatter correction reduces the error from 13.3% to 1.2%. This suggests that model-based scatter correction is accurate in the heterogeneous attenuating medium found in the chest, making possible quantitative, fully 3D PET in the body.
NASA Technical Reports Server (NTRS)
Waegell, Mordecai J.; Palacios, David M.
2011-01-01
Jitter_Correct.m is a MATLAB function that automatically measures and corrects inter-frame jitter in an image sequence to a user-specified precision. In addition, the algorithm dynamically adjusts the image sample size to increase the accuracy of the measurement. The Jitter_Correct.m function takes an image sequence with unknown frame-to-frame jitter and computes the translations of each frame (column and row, in pixels) relative to a chosen reference frame with sub-pixel accuracy. The translations are measured using a Cross Correlation Fourier transformation method in which the relative phase of the two transformed images is fit to a plane. The measured translations are then used to correct the inter-frame jitter of the image sequence. The function also dynamically expands the image sample size over which the cross-correlation is measured to increase the accuracy of the measurement. This increases the robustness of the measurement to variable magnitudes of inter-frame jitter
Landing gear noise attenuation
NASA Technical Reports Server (NTRS)
Moe, Jeffrey W. (Inventor); Whitmire, Julia (Inventor); Kwan, Hwa-Wan (Inventor); Abeysinghe, Amal (Inventor)
2011-01-01
A landing gear noise attenuator mitigates noise generated by airframe deployable landing gear. The noise attenuator can have a first position when the landing gear is in its deployed or down position, and a second position when the landing gear is in its up or stowed position. The noise attenuator may be an inflatable fairing that does not compromise limited space constraints associated with landing gear retraction and stowage. A truck fairing mounted under a truck beam can have a compliant edge to allow for non-destructive impingement of a deflected fire during certain conditions.
Giordano, S.
1963-11-12
A high peak power level r-f attenuator that is readily and easily insertable along a coaxial cable having an inner conductor and an outer annular conductor without breaking the ends thereof is presented. Spaced first and second flares in the outer conductor face each other with a slidable cylindrical outer conductor portion therebetween. Dielectric means, such as water, contact the cable between the flares to attenuate the radio-frequency energy received thereby. The cylindrical outer conductor portion is slidable to adjust the voltage standing wave ratio to a low level, and one of the flares is slidable to adjust the attenuation level. An integral dielectric container is also provided. (AFC)
GPR measurements of attenuation in concrete
Eisenmann, David Margetan, Frank J. Pavel, Brittney
2015-03-31
Ground-penetrating radar (GPR) signals from concrete structures are affected by several phenomenon, including: (1) transmission and reflection coefficients at interfaces; (2) the radiation patterns of the antenna(s) being used; and (3) the material properties of concrete and any embedded objects. In this paper we investigate different schemes for determining the electromagnetic (EM) attenuation of concrete from measured signals obtained using commercially-available GPR equipment. We adapt procedures commonly used in ultrasonic inspections where one compares the relative strengths of two or more signals having different travel paths through the material of interest. After correcting for beam spread (i.e., diffraction), interface phenomena, and equipment amplification settings, any remaining signal differences are assumed to be due to attenuation thus allowing the attenuation coefficient (say, in dB of loss per inch of travel) to be estimated. We begin with a brief overview of our approach, and then discuss how diffraction corrections were determined for our two 1.6 GHz GPR antennas. We then present results of attenuation measurements for two types of concrete using both pulse/echo and pitch/catch measurement setups.
NASA Astrophysics Data System (ADS)
Hanrieder, N.; Wilbert, S.; Pitz-Paal, R.; Emde, C.; Gasteiger, J.; Mayer, B.; Polo, J.
2015-05-01
Losses of reflected Direct Normal Irradiance due to atmospheric extinction in concentrating solar tower plants can vary significantly with site and time. The losses of the direct normal irradiance between the heliostat field and receiver in a solar tower plant are mainly caused by atmospheric scattering and absorption by aerosol and water vapor concentration in the atmospheric boundary layer. Due to a high aerosol particle number, radiation losses can be significantly larger in desert environments compared to the standard atmospheric conditions which are usually considered in raytracing or plant optimization tools. Information about on-site atmospheric extinction is only rarely available. To measure these radiation losses, two different commercially available instruments were tested and more than 19 months of measurements were collected at the Plataforma Solar de Almería and compared. Both instruments are primarily used to determine the meteorological optical range (MOR). The Vaisala FS11 scatterometer is based on a monochromatic near-infrared light source emission and measures the strength of scattering processes in a small air volume mainly caused by aerosol particles. The Optec LPV4 long-path visibility transmissometer determines the monochromatic attenuation between a light-emitting diode (LED) light source at 532 nm and a receiver and therefore also accounts for absorption processes. As the broadband solar attenuation is of interest for solar resource assessment for Concentrating Solar Power (CSP), a correction procedure for these two instruments is developed and tested. This procedure includes a spectral correction of both instruments from monochromatic to broadband attenuation. That means the attenuation is corrected for the actual, time-dependent by the collector reflected solar spectrum. Further, an absorption correction for the Vaisala FS11 scatterometer is implemented. To optimize the Absorption and Broadband Correction (ABC) procedure, additional
SEISMIC ATTENUATION FOR RESERVOIR CHARACTERIZATION
Joel Walls; M.T. Taner; Naum Derzhi; Gary Mavko; Jack Dvorkin
2003-12-01
We have developed and tested technology for a new type of direct hydrocarbon detection. The method uses inelastic rock properties to greatly enhance the sensitivity of surface seismic methods to the presence of oil and gas saturation. These methods include use of energy absorption, dispersion, and attenuation (Q) along with traditional seismic attributes like velocity, impedance, and AVO. Our approach is to combine three elements: (1) a synthesis of the latest rock physics understanding of how rock inelasticity is related to rock type, pore fluid types, and pore microstructure, (2) synthetic seismic modeling that will help identify the relative contributions of scattering and intrinsic inelasticity to apparent Q attributes, and (3) robust algorithms that extract relative wave attenuation attributes from seismic data. This project provides: (1) Additional petrophysical insight from acquired data; (2) Increased understanding of rock and fluid properties; (3) New techniques to measure reservoir properties that are not currently available; and (4) Provide tools to more accurately describe the reservoir and predict oil location and volumes. These methodologies will improve the industry's ability to predict and quantify oil and gas saturation distribution, and to apply this information through geologic models to enhance reservoir simulation. We have applied for two separate patents relating to work that was completed as part of this project.
Anderson, Gene R.; Armendariz, Marcelino G.; Carson, Richard F.; Bryan, Robert P.; Duckett, III, Edwin B.; Kemme, Shanalyn Adair; McCormick, Frederick B.; Peterson, David W.
2006-04-04
An apparatus and method of attenuating and/or conditioning optical energy for an optical transmitter, receiver or transceiver module is disclosed. An apparatus for attenuating the optical output of an optoelectronic connector including: a mounting surface; an array of optoelectronic devices having at least a first end; an array of optical elements having at least a first end; the first end of the array of optical elements optically aligned with the first end of the array of optoelectronic devices; an optical path extending from the first end of the array of optoelectronic devices and ending at a second end of the array of optical elements; and an attenuator in the optical path for attenuating the optical energy emitted from the array of optoelectronic devices. Alternatively, a conditioner may be adapted in the optical path for conditioning the optical energy emitted from the array of optoelectronic devices.
NASA Technical Reports Server (NTRS)
1995-01-01
Mike Buzzetti designed a fiber optic attenuator while working at Jet Propulsion Laboratory, intended for use in NASA's Deep Space Network. Buzzetti subsequently patented and received an exclusive license to commercialize the device, and founded Nanometer Technologies to produce it. The attenuator functions without introducing measurable back-reflection or insertion loss, and is relatively insensitive to vibration and changes in temperature. Applications include cable television, telephone networks, other signal distribution networks, and laboratory instrumentation.
NASA Astrophysics Data System (ADS)
Wen, Junhai; Lu, Hongbing; Li, Tianfang; Liang, Zhengrong
2003-05-01
In the past decades, analytical (non-iterative) methods have been extensively investigated and developed for the reconstruction of three-dimensional (3D) single-photon emission computed tomography (SPECT). However, it becomes possible only recently when the exact analytic non-uniform attenuation reconstruction algorithm was derived. Based on the explicit inversion formula for the attenuated Radon transform discovered by Novikov (2000), we extended the previous researches of inverting the attenuated Radon transform of parallel-beam collimation geometry to fan-beam and variable focal-length fan-beam (VFF) collimators and proposed an efficient, analytical solution to 3D SPECT reconstruction with VFF collimators, which compensates simultaneously for non-uniform attenuation, scatter, and spatially-variant or distance-dependent resolution variation (DDRV), as well as suppression of signal-dependent non-stationary Poisson noise. In this procedure, to avoid the reconstructed images being corrupted by the presence of severe noise, we apply a Karhune-Loève (K-L) domain adaptive Wiener filter, which accurately treats the non-stationary Poisson noise. The scatter is then removed by our scatter estimation method, which is based on the energy spectrum and modified from the triple-energy-window acquisition protocol. For the correction of DDRV, a distance-dependent deconvolution is adapted to provide a solution that realistically characterizes the resolution kernel in a real SPECT system. Finally image is reconstructed using our VFF non-uniform attenuation inversion formula.
Developing a Short-Period, Fundamental-Mode Rayleigh-Wave Attenuation Model for Asia
NASA Astrophysics Data System (ADS)
Yang, X.; Levshin, A. L.; Barmin, M. P.; Ritzwoller, M. H.
2008-12-01
We are developing a 2D, short-period (12 - 22 s), fundamental-mode Rayleigh-wave attenuation model for Asia. This model can be used to invert for a 3D attenuation model of the Earth's crust and upper mantle as well as to implement more accurate path corrections in regional surface-wave magnitude calculations. The prerequisite for developing a reliable Rayleigh-wave attenuation model is the availability of accurate fundamental-mode Rayleigh-wave amplitude measurements. Fundamental-mode Rayleigh-wave amplitudes could be contaminated by a variety of sources such as multipathing, focusing and defocusing, body wave, higher-mode surface wave, and other noise sources. These contaminations must be reduced to the largest extent possible. To achieve this, we designed a procedure by taking advantage of certain Rayleigh-wave characteristics, such as dispersion and elliptical particle motion, for accurate amplitude measurements. We first analyze the dispersion of the surface-wave data using a spectrogram. Based on the characteristics of the data dispersion, we design a phase-matched filter by using either a manually picked dispersion curve, or a group-velocity-model predicted dispersion curve, or the dispersion of the data, and apply the filter to the seismogram. Intelligent filtering of the seismogram and windowing of the resulting cross-correlation based on the spectrogram analysis and the comparison between the phase-match filtered data spectrum, the raw-data spectrum and the theoretical source spectrum effectively reduces amplitude contaminations and results in reliable amplitude measurements in many cases. We implemented these measuring techniques in a graphic-user-interface tool called Surface Wave Amplitude Measurement Tool (SWAMTOOL). Using the tool, we collected and processed waveform data for 200 earthquakes occurring throughout 2003-2006 inside and around Eurasia. The records from 135 broadband stations were used. After obtaining the Rayleigh-wave amplitude
Timebias corrections to predictions
NASA Technical Reports Server (NTRS)
Wood, Roger; Gibbs, Philip
1993-01-01
The importance of an accurate knowledge of the time bias corrections to predicted orbits to a satellite laser ranging (SLR) observer, especially for low satellites, is highlighted. Sources of time bias values and the optimum strategy for extrapolation are discussed from the viewpoint of the observer wishing to maximize the chances of getting returns from the next pass. What is said may be seen as a commercial encouraging wider and speedier use of existing data centers for mutually beneficial exchange of time bias data.
Smith, Peter D.; Claytor, Thomas N.; Berry, Phillip C.; Hills, Charles R.
2010-10-12
An x-ray detector is disclosed that has had all unnecessary material removed from the x-ray beam path, and all of the remaining material in the beam path made as light and as low in atomic number as possible. The resulting detector is essentially transparent to x-rays and, thus, has greatly reduced internal scatter. The result of this is that x-ray attenuation data measured for the object under examination are much more accurate and have an increased dynamic range. The benefits of this improvement are that beam hardening corrections can be made accurately, that computed tomography reconstructions can be used for quantitative determination of material properties including density and atomic number, and that lower exposures may be possible as a result of the increased dynamic range.
Allodji, Rodrigue S; Thiébaut, Anne C M; Leuraud, Klervi; Rage, Estelle; Henry, Stéphane; Laurier, Dominique; Bénichou, Jacques
2012-12-30
A broad variety of methods for measurement error (ME) correction have been developed, but these methods have rarely been applied possibly because their ability to correct ME is poorly understood. We carried out a simulation study to assess the performance of three error-correction methods: two variants of regression calibration (the substitution method and the estimation calibration method) and the simulation extrapolation (SIMEX) method. Features of the simulated cohorts were borrowed from the French Uranium Miners' Cohort in which exposure to radon had been documented from 1946 to 1999. In the absence of ME correction, we observed a severe attenuation of the true effect of radon exposure, with a negative relative bias of the order of 60% on the excess relative risk of lung cancer death. In the main scenario considered, that is, when ME characteristics previously determined as most plausible from the French Uranium Miners' Cohort were used both to generate exposure data and to correct for ME at the analysis stage, all three error-correction methods showed a noticeable but partial reduction of the attenuation bias, with a slight advantage for the SIMEX method. However, the performance of the three correction methods highly depended on the accurate determination of the characteristics of ME. In particular, we encountered severe overestimation in some scenarios with the SIMEX method, and we observed lack of correction with the three methods in some other scenarios. For illustration, we also applied and compared the proposed methods on the real data set from the French Uranium Miners' Cohort study. PMID:22996087
A method to correct for spectral artifacts in optical-CT dosimetry
Pierquet, Michael; Jordan, Kevin; Oldham, Mark
2011-01-01
The recent emergence of radiochromic dosimeters with low inherent light-scattering presents the possibility of fast 3D dosimetry using broad-beam optical computed tomography (optical-CT). Current broad beam scanners typically employ either a single or a planar array of light-emitting diodes (LED) for the light source. The spectrum of light from LED sources is polychromatic and this, in combination with the non-uniform spectral absorption of the dosimeter, can introduce spectral artifacts arising from preferential absorption of photons at the peak absorption wavelengths in the dosimeter. Spectral artifacts can lead to large errors in the reconstructed attenuation coefficients, and hence dose measurement. This work presents an analytic method for correcting for spectral artifacts which can be applied if the spectral characteristics of the light source, absorbing dosimeter, and imaging detector are known or can be measured. The method is implemented here for a PRESAGE® dosimeter scanned with the DLOS telecentric scanner (Duke Large field-of-view Optical-CT Scanner). Emission and absorption profiles were measured with a commercial spectrometer and spectrophotometer, respectively. Simulations are presented that show spectral changes can introduce errors of 8% for moderately attenuating samples where spectral artifacts are less pronounced. The correction is evaluated by application to a 16 cm diameter PRESAGE® cylindrical dosimeter irradiated along the axis with two partially overlapping 6 × 6 cm fields of different doses. The resulting stepped dose distribution facilitates evaluation of the correction as each step had different spectral contributions. The spectral artifact correction was found to accurately correct the reconstructed coefficients to within ~1.5%, improved from ~7.5%, for normalized dose distributions. In conclusion, for situations where spectral artifacts cannot be removed by physical filters, the method shown here is an effective correction. Physical
A rigid motion correction method for helical computed tomography (CT)
NASA Astrophysics Data System (ADS)
Kim, J.-H.; Nuyts, J.; Kyme, A.; Kuncic, Z.; Fulton, R.
2015-03-01
We propose a method to compensate for six degree-of-freedom rigid motion in helical CT of the head. The method is demonstrated in simulations and in helical scans performed on a 16-slice CT scanner. Scans of a Hoffman brain phantom were acquired while an optical motion tracking system recorded the motion of the bed and the phantom. Motion correction was performed by restoring projection consistency using data from the motion tracking system, and reconstructing with an iterative fully 3D algorithm. Motion correction accuracy was evaluated by comparing reconstructed images with a stationary reference scan. We also investigated the effects on accuracy of tracker sampling rate, measurement jitter, interpolation of tracker measurements, and the synchronization of motion data and CT projections. After optimization of these aspects, motion corrected images corresponded remarkably closely to images of the stationary phantom with correlation and similarity coefficients both above 0.9. We performed a simulation study using volunteer head motion and found similarly that our method is capable of compensating effectively for realistic human head movements. To the best of our knowledge, this is the first practical demonstration of generalized rigid motion correction in helical CT. Its clinical value, which we have yet to explore, may be significant. For example it could reduce the necessity for repeat scans and resource-intensive anesthetic and sedation procedures in patient groups prone to motion, such as young children. It is not only applicable to dedicated CT imaging, but also to hybrid PET/CT and SPECT/CT, where it could also ensure an accurate CT image for lesion localization and attenuation correction of the functional image data.
Robust diffraction correction method for high-frequency ultrasonic tissue characterization
NASA Astrophysics Data System (ADS)
Raju, Balasundar
2001-05-01
The computation of quantitative ultrasonic parameters such as the attenuation or backscatter coefficient requires compensation for diffraction effects. In this work a simple and accurate diffraction correction method for skin characterization requiring only a single focal zone is developed. The advantage of this method is that the transducer need not be mechanically repositioned to collect data from several focal zones, thereby reducing the time of imaging and preventing motion artifacts. Data were first collected under controlled conditions from skin of volunteers using a high-frequency system (center frequency=33 MHz, BW=28 MHz) at 19 focal zones through axial translation. Using these data, mean backscatter power spectra were computed as a function of the distance between the transducer and the tissue, which then served as empirical diffraction correction curves for subsequent data. The method was demonstrated on patients patch-tested for contact dermatitis. The computed attenuation coefficient slope was significantly (p<0.05) lower at the affected site (0.13+/-0.02 dB/mm/MHz) compared to nearby normal skin (0.2+/-0.05 dB/mm/MHz). The mean backscatter level was also significantly lower at the affected site (6.7+/-2.1 in arbitrary units) compared to normal skin (11.3+/-3.2). These results show diffraction corrected ultrasonic parameters can differentiate normal from affected skin tissues.
Feed-forward digital phase and amplitude correction system
Yu, D.U.L.; Conway, P.H.
1994-11-15
Phase and amplitude modifications in repeatable RF pulses at the output of a high power pulsed microwave amplifier are made utilizing a digital feed-forward correction system. A controlled amount of the output power is coupled to a correction system for processing of phase and amplitude information. The correction system comprises circuitry to compare the detected phase and amplitude with the desired phase and amplitude, respectively, and a digitally programmable phase shifter and attenuator and digital logic circuitry to control the phase shifter and attenuator. The phase and amplitude of subsequent are modified by output signals from the correction system. 11 figs.
Feed-forward digital phase and amplitude correction system
Yu, David U. L.; Conway, Patrick H.
1994-01-01
Phase and amplitude modifications in repeatable RF pulses at the output of a high power pulsed microwave amplifier are made utilizing a digital feed-forward correction system. A controlled amount of the output power is coupled to a correction system for processing of phase and amplitude information. The correction system comprises circuitry to compare the detected phase and amplitude with the desired phase and amplitude, respectively, and a digitally programmable phase shifter and attenuator and digital logic circuitry to control the phase shifter and attenuator. The Phase and amplitude of subsequent are modified by output signals from the correction system.
Reconstruction algorithm for polychromatic CT imaging: application to beam hardening correction
NASA Technical Reports Server (NTRS)
Yan, C. H.; Whalen, R. T.; Beaupre, G. S.; Yen, S. Y.; Napel, S.
2000-01-01
This paper presents a new reconstruction algorithm for both single- and dual-energy computed tomography (CT) imaging. By incorporating the polychromatic characteristics of the X-ray beam into the reconstruction process, the algorithm is capable of eliminating beam hardening artifacts. The single energy version of the algorithm assumes that each voxel in the scan field can be expressed as a mixture of two known substances, for example, a mixture of trabecular bone and marrow, or a mixture of fat and flesh. These assumptions are easily satisfied in a quantitative computed tomography (QCT) setting. We have compared our algorithm to three commonly used single-energy correction techniques. Experimental results show that our algorithm is much more robust and accurate. We have also shown that QCT measurements obtained using our algorithm are five times more accurate than that from current QCT systems (using calibration). The dual-energy mode does not require any prior knowledge of the object in the scan field, and can be used to estimate the attenuation coefficient function of unknown materials. We have tested the dual-energy setup to obtain an accurate estimate for the attenuation coefficient function of K2 HPO4 solution.
MNAtoolbox: A Monitored Natural Attenuation Site Screening Program
Borns, David J.; Brady, Patrick V.; Brady, Warren D.; Krupka, Kenneth M.; Spalding, Brian P.; Waters, Robert D.; Zhang, Pengchu
1999-07-12
Screening of sites for the potential application and reliance upon monitored natural attenuation (MNA) can be done using MNAtoolbox, a web-based tool for estimating extent of biodegradation, chemical transformation, and dilution. MNAtoolbox uses site-specific input data, where available (default parameters are taken from the literature), to roughly quantify the nature and extent of attenuation at a particular site. Use of MNAtoolbox provides 3 important elements of site evaluation: (1) Identifies likely attenuation pathways, (2) Clearly identifies sites where MNA is inappropriate, and (3) Evaluates data requirements for subsequent reliance on MNA as a sole or partial corrective action.
Shear wave speed dispersion and attenuation in granular marine sediments.
Kimura, Masao
2013-07-01
The reported compressional wave speed dispersion and attenuation could be explained by a modified gap stiffness model incorporated into the Biot model (the BIMGS model). In contrast, shear wave speed dispersion and attenuation have not been investigated in detail. No measurements of shear wave speed dispersion have been reported, and only Brunson's data provide the frequency characteristics of shear wave attenuation. In this study, Brunson's attenuation measurements are compared to predictions using the Biot-Stoll model and the BIMGS model. It is shown that the BIMGS model accurately predicts the frequency dependence of shear wave attenuation. Then, the shear wave speed dispersion and attenuation in water-saturated silica sand are measured in the frequency range of 4-20 kHz. The vertical stress applied to the sample is 17.6 kPa. The temperature of the sample is set to be 5 °C, 20 °C, and 35 °C in order to change the relaxation frequency in the BIMGS model. The measured results are compared with those calculated using the Biot-Stoll model and the BIMGS model. It is shown that the shear wave speed dispersion and attenuation are predicted accurately by using the BIMGS model. PMID:23862793
Asymmetric scatter kernels for software-based scatter correction of gridless mammography
NASA Astrophysics Data System (ADS)
Wang, Adam; Shapiro, Edward; Yoon, Sungwon; Ganguly, Arundhuti; Proano, Cesar; Colbeth, Rick; Lehto, Erkki; Star-Lack, Josh
2015-03-01
Scattered radiation remains one of the primary challenges for digital mammography, resulting in decreased image contrast and visualization of key features. While anti-scatter grids are commonly used to reduce scattered radiation in digital mammography, they are an incomplete solution that can add radiation dose, cost, and complexity. Instead, a software-based scatter correction method utilizing asymmetric scatter kernels is developed and evaluated in this work, which improves upon conventional symmetric kernels by adapting to local variations in object thickness and attenuation that result from the heterogeneous nature of breast tissue. This fast adaptive scatter kernel superposition (fASKS) method was applied to mammography by generating scatter kernels specific to the object size, x-ray energy, and system geometry of the projection data. The method was first validated with Monte Carlo simulation of a statistically-defined digital breast phantom, which was followed by initial validation on phantom studies conducted on a clinical mammography system. Results from the Monte Carlo simulation demonstrate excellent agreement between the estimated and true scatter signal, resulting in accurate scatter correction and recovery of 87% of the image contrast originally lost to scatter. Additionally, the asymmetric kernel provided more accurate scatter correction than the conventional symmetric kernel, especially at the edge of the breast. Results from the phantom studies on a clinical system further validate the ability of the asymmetric kernel correction method to accurately subtract the scatter signal and improve image quality. In conclusion, software-based scatter correction for mammography is a promising alternative to hardware-based approaches such as anti-scatter grids.
Radiofrequency attenuator and method
Warner, Benjamin P.; McCleskey, T. Mark; Burrell, Anthony K.; Agrawal, Anoop; Hall, Simon B.
2009-01-20
Radiofrequency attenuator and method. The attenuator includes a pair of transparent windows. A chamber between the windows is filled with molten salt. Preferred molten salts include quarternary ammonium cations and fluorine-containing anions such as tetrafluoroborate (BF.sub.4.sup.-), hexafluorophosphate (PF.sub.6.sup.-), hexafluoroarsenate (AsF.sub.6.sup.-), trifluoromethylsulfonate (CF.sub.3SO.sub.3.sup.-), bis(trifluoromethylsulfonyl)imide ((CF.sub.3SO.sub.2).sub.2N.sup.-), bis(perfluoroethylsulfonyl)imide ((CF.sub.3CF.sub.2SO.sub.2).sub.2N.sup.-) and tris(trifluoromethylsulfonyl)methide ((CF.sub.3SO.sub.2).sub.3C.sup.-). Radicals or radical cations may be added to or electrochemically generated in the molten salt to enhance the RF attenuation.
Radiofrequency attenuator and method
Warner, Benjamin P.; McCleskey, T. Mark; Burrell, Anthony K.; Agrawal, Anoop; Hall, Simon B.
2009-11-10
Radiofrequency attenuator and method. The attenuator includes a pair of transparent windows. A chamber between the windows is filled with molten salt. Preferred molten salts include quarternary ammonium cations and fluorine-containing anions such as tetrafluoroborate (BF.sub.4.sup.-), hexafluorophosphate (PF.sub.6.sup.-), hexafluoroarsenate (AsF.sub.6.sup.-), trifluoromethylsulfonate (CF.sub.3SO.sub.3.sup.-), bis(trifluoromethylsulfonyl)imide ((CF.sub.3SO.sub.2).sub.2N.sup.-), bis(perfluoroethylsulfonyl)imide ((CF.sub.3CF.sub.2SO.sub.2).sub.2N.sup.-) and tris(trifluoromethylsulfonyl)methide ((CF.sub.3SO.sub.2).sub.3 C.sup.-). Radicals or radical cations may be added to or electrochemically generated in the molten salt to enhance the RF attenuation.
Seismic attenuation in Florida
Bellini, J.J.; Bartolini, T.J.; Lord, K.M.; Smith, D.L. . Dept. of Geology)
1993-03-01
Seismic signals recorded by the expanded distribution of earthquake seismograph stations throughout Florida and data from a comprehensive review of record archives from stations GAI contribute to an initial seismic attenuation model for the Florida Plateau. Based on calculations of surface particle velocity, a pattern of attenuation exists that appears to deviate from that established for the remainder of the southeastern US. Most values suggest greater seismic attenuation within the Florida Plateau. However, a separate pattern may exist for those signals arising from the Gulf of Mexico. These results have important implications for seismic hazard assessments in Florida and may be indicative of the unique lithospheric identity of the Florida basement as an exotic terrane.
Detection and measurement of gamma-ray self-attenuation in plutonium residues
Prettyman, T.H.; Foster, L.A.; Estep, R.J.
1996-09-01
A new method to correct for self-attenuation in gamma-ray assays of plutonium is presented. The underlying assumptions of the technique are based on a simple but accurate physical model of plutonium residues, particularly pyrochemical salts, in which it is assumed that the plutonium is divided into two portions, each of which can be treated separately from the standpoint of gamma-ray analysis: a portion that is in the form of plutonium metal shot; and a dilute portion that is mixed with the matrix. The performance of the technique is evaluated using assays of plutonium residues by tomographic gamma scanning at the Los Alamos Plutonium Facility. The ability of the method to detect saturation conditions is examined.
Johnson, D
1940-03-22
IN a recently published volume on "The Origin of Submarine Canyons" the writer inadvertently credited to A. C. Veatch an excerpt from a submarine chart actually contoured by P. A. Smith, of the U. S. Coast and Geodetic Survey. The chart in question is Chart IVB of Special Paper No. 7 of the Geological Society of America entitled "Atlantic Submarine Valleys of the United States and the Congo Submarine Valley, by A. C. Veatch and P. A. Smith," and the excerpt appears as Plate III of the volume fist cited above. In view of the heavy labor involved in contouring the charts accompanying the paper by Veatch and Smith and the beauty of the finished product, it would be unfair to Mr. Smith to permit the error to go uncorrected. Excerpts from two other charts are correctly ascribed to Dr. Veatch. PMID:17839404
SU-E-I-07: An Improved Technique for Scatter Correction in PET
Lin, S; Wang, Y; Lue, K; Lin, H; Chuang, K
2014-06-01
Purpose: In positron emission tomography (PET), the single scatter simulation (SSS) algorithm is widely used for scatter estimation in clinical scans. However, bias usually occurs at the essential steps of scaling the computed SSS distribution to real scatter amounts by employing the scatter-only projection tail. The bias can be amplified when the scatter-only projection tail is too small, resulting in incorrect scatter correction. To this end, we propose a novel scatter calibration technique to accurately estimate the amount of scatter using pre-determined scatter fraction (SF) function instead of the employment of scatter-only tail information. Methods: As the SF depends on the radioactivity distribution and the attenuating material of the patient, an accurate theoretical relation cannot be devised. Instead, we constructed an empirical transformation function between SFs and average attenuation coefficients based on a serious of phantom studies with different sizes and materials. From the average attenuation coefficient, the predicted SFs were calculated using empirical transformation function. Hence, real scatter amount can be obtained by scaling the SSS distribution with the predicted SFs. The simulation was conducted using the SimSET. The Siemens Biograph™ 6 PET scanner was modeled in this study. The Software for Tomographic Image Reconstruction (STIR) was employed to estimate the scatter and reconstruct images. The EEC phantom was adopted to evaluate the performance of our proposed technique. Results: The scatter-corrected image of our method demonstrated improved image contrast over that of SSS. For our technique and SSS of the reconstructed images, the normalized standard deviation were 0.053 and 0.182, respectively; the root mean squared errors were 11.852 and 13.767, respectively. Conclusion: We have proposed an alternative method to calibrate SSS (C-SSS) to the absolute scatter amounts using SF. This method can avoid the bias caused by the insufficient
The importance of accurate atmospheric modeling
NASA Astrophysics Data System (ADS)
Payne, Dylan; Schroeder, John; Liang, Pang
2014-11-01
This paper will focus on the effect of atmospheric conditions on EO sensor performance using computer models. We have shown the importance of accurately modeling atmospheric effects for predicting the performance of an EO sensor. A simple example will demonstrated how real conditions for several sites in China will significantly impact on image correction, hyperspectral imaging, and remote sensing. The current state-of-the-art model for computing atmospheric transmission and radiance is, MODTRAN® 5, developed by the US Air Force Research Laboratory and Spectral Science, Inc. Research by the US Air Force, Navy and Army resulted in the public release of LOWTRAN 2 in the early 1970's. Subsequent releases of LOWTRAN and MODTRAN® have continued until the present. Please verify that (1) all pages are present, (2) all figures are correct, (3) all fonts and special characters are correct, and (4) all text and figures fit within the red margin lines shown on this review document. Complete formatting information is available at http://SPIE.org/manuscripts Return to the Manage Active Submissions page at http://spie.org/submissions/tasks.aspx and approve or disapprove this submission. Your manuscript will not be published without this approval. Please contact author_help@spie.org with any questions or concerns. The paper will demonstrate the importance of using validated models and local measured meteorological, atmospheric and aerosol conditions to accurately simulate the atmospheric transmission and radiance. Frequently default conditions are used which can produce errors of as much as 75% in these values. This can have significant impact on remote sensing applications.
Accurate measurement of unsteady state fluid temperature
NASA Astrophysics Data System (ADS)
Jaremkiewicz, Magdalena
2016-07-01
In this paper, two accurate methods for determining the transient fluid temperature were presented. Measurements were conducted for boiling water since its temperature is known. At the beginning the thermometers are at the ambient temperature and next they are immediately immersed into saturated water. The measurements were carried out with two thermometers of different construction but with the same housing outer diameter equal to 15 mm. One of them is a K-type industrial thermometer widely available commercially. The temperature indicated by the thermometer was corrected considering the thermometers as the first or second order inertia devices. The new design of a thermometer was proposed and also used to measure the temperature of boiling water. Its characteristic feature is a cylinder-shaped housing with the sheath thermocouple located in its center. The temperature of the fluid was determined based on measurements taken in the axis of the solid cylindrical element (housing) using the inverse space marching method. Measurements of the transient temperature of the air flowing through the wind tunnel using the same thermometers were also carried out. The proposed measurement technique provides more accurate results compared with measurements using industrial thermometers in conjunction with simple temperature correction using the inertial thermometer model of the first or second order. By comparing the results, it was demonstrated that the new thermometer allows obtaining the fluid temperature much faster and with higher accuracy in comparison to the industrial thermometer. Accurate measurements of the fast changing fluid temperature are possible due to the low inertia thermometer and fast space marching method applied for solving the inverse heat conduction problem.