Fast transmission CT for determining attenuation maps using a collimated line source, rotatable air-copper-lead attenuators and fan-beam collimation

We describe a technique using a line source and a rotatable air-copper-lead assembly to acquire gamma transmission computed tomographic (TCT) data for determining attenuation maps to compensate SPECT emission scans. The technique minimizes problems associated with discriminating 99mTc transmission and 201Tl emission photons and requires only a modest increase in total study time. A 99mTc line source and a stacked foil ("multislat") collimator are placed near the focal line of a fan-beam collimator (114 cm focal length) mounted on one detector of a triple-camera SPECT system. We acquired TCT data of plastic rod and anthropomorphic thorax phantoms to investigate the capability of the line source and rotatable air-copper-lead attenuators to determine attenuation maps. The data were acquired with and without 5.4 MBq (145 microCi) of 201Tl placed in the myocardial chamber of the thorax phantom. Phantoms also were scanned using a curved transmission slab source mounted to a parallel-hole collimator. Fan-beam TCT images have improved resolution compared with parallel-beam TCT images. Two patient scans also were performed to evaluate the clinical usefulness of fan-beam TCT. The rotatable air-copper-lead attenuator method eliminates contamination of emission data by transmission photons and reduces spill-over of emission data into the transmission energy window for some cases. Results show the feasibility of using fast, sequential or interlaced transmission scans of a line source within a rotatable air-copper-lead attenuator assembly to obtain accurate attenuation maps for SPECT attenuation compensation.     

Magnetic resonance imaging of the bone marrow in hematological malignancies

The objective of this study was to determine the feasibility of using a fast (short-duration) transmission computed tomogram (TCT), acquired immediately before or after the emission CT, to correct for photon attenuation in cardiac SPECT. METHODS: The asymmetric fanbeam geometry with a 99mTc line source was used to acquire TCTs after conventional cardiac emission CT imaging on a triple-head SPECT system. The TCTs were reconstructed to generate patient-specific attenuation maps, which were used with an iterative maximum likelihood algorithm to reconstruct attenuation-corrected cardiac SPECT studies. The results of attenuation correction based on TCTs as short as 1 min were compared with long-duration transmission imaging for a phantom and several human studies. RESULTS: Attenuation correction based on asymmetric fanbeam TCT significantly improves the uniformity of images of a uniform tracer distribution in a cardiac-thorax phantom configured to simulate a large patient. By using a high-activity line source and a rapid camera rotation, a suitable attenuation map for this phantom can be obtained from a 4-min TCT. A similar result is obtained for patients with thorax widths of <40 cm. CONCLUSION: A sequential imaging protocol for acquiring a fast TCT can be used for attenuation correction of cardiac SPECT imaging. The sequential TCT can be acquired without significantly extending the duration of the imaging study. This method provides a way to perform attenuation correction on existing triple-head SPECT systems without extensively modifying the system.     

Transmission scanning in emission tomography

Attenuation correction in single-photon (SPET) and positron emission (PET) tomography is now accepted as a vital component for the production of artefact-free, quantitative data. The most accurate attenuation correction methods are based on measured transmission scans acquired before, during, or after the emission scan. Alternative methods use segmented images, assumed attenuation coefficients or consistency criteria to compensate for photon attenuation in reconstructed images. This review examines the methods of acquiring transmission scans in both SPET and PET and the manner in which these data are used. While attenuation correction gives an exact correction in PET, as opposed to an approximate one in SPET, the magnitude of the correction factors required in PET is far greater than in SPET. Transmission scans also have a number of other potential applications in emission tomography apart from attenuation correction, such as scatter correction, inter-study spatial co-registration and alignment, and motion detection and correction. The ability to acquire high-quality transmission data in a practical clinical protocol is now an essential part of the practice of nuclear medicine.     

Simultaneous acquisition of iodine-123 emission and technetium-99m transmission data for quantitative brain single-photon emission tomographic imaging

The aim of this study was to obtain quantitative iodine-123 brain single-photon emission tomographic (SPET) images with scatter and attenuation correction. We used a triple-headed SPET gamma camera system equipped with fan-beam collimators with a technetium-99m line transmission source placed at one of the focal lines of the fan-beam collimators. Four energy windows were employed for data acquisition: (a) 126-132 keV, (b) 132-143 keV, (c) 143-175 keV and (d) 175-186 keV. A simultaneous transmission-emission computed tomography scan (TCT-ECT) was carried out for a brain phantom containing 123I solution. The triple energy window scatter correction was applied to the 123I ECT data measured by means of the windows (b), (c) and (d) acquired by two detectors. Attenuation maps were reconstructed from 99mTc TCT data measured by means of the windows (a), (b) and (c) acquired by one detector. Chang's iterative attenuation correction method using the attenuation maps was applied to the 123I ECT images. In the phantom study cross-calibrated SPET values obtained with the simultaneous mode were almost equal to those obtained with the sequential mode, and they were close to the true value, within an error range of 5.5%. In the human study corrected images showed a higher grey-to-white matter count ratio and relatively higher uptake in the cerebellum, basal ganglia and thalamus than uncorrected images. We conclude that this correction method provides improved quantification and quality of SPET images and that the method is clinically practical because it requires only a single scan with a 99mTc external source.     

Multiple line source array for SPECT transmission scans: simulation, phantom and patient studies

Accurate attenuation and scatter corrections in quantitative SPECT studies require attenuation maps of the density distribution in the scanned object. These can be obtained from simultaneous emission/transmission scans. METHODS: A new method has been developed using a multiple line source array (MLA) for transmission scans, and its performance has been investigated using computer simulations and experimental data. The activity in the central lines of the MLA was higher than at the edges of the system, so that more transmission photons would be directed toward the thicker parts of the human body. A series of transmission-only and simultaneous emission/transmission studies were performed for different phantom configurations and human subjects. Attenuation maps were generated and used in reconstruction of attenuation-corrected emission images. RESULTS: The mu coefficients for attenuation maps obtained using the MLA system and simulated and experimental data display no artifacts and are qualitatively and quantitatively correct. For phantoms, the agreement between the measured and the true value of mu for water was found to be better than 4%. The attenuation-corrected emission images for the phantom studies demonstrate that the activity in the heart can be accurately reconstructed. A significant qualitative improvement was also obtained when the attenuation correction was used on patient data. CONCLUSION: Our results indicate that the MLA transmission source can be used in simultaneous transmission/emission imaging to generate accurate attenuation maps. These maps allow for performing an object-specific, attenuation correction of the emission images.     

Importance of bone attenuation in brain SPECT quantification

The purpose of this study was to determine the effects of nonuniform attenuation on relative quantification in brain SPECT and to compare the ability of the Chang and Sorenson uniform attenuation corrections (UACs) to achieve volumetric relative quantification. METHODS: Three head phantoms (dry human skull, Rando and Radiology Support Devices (RSD) phantoms) were compared with a human head using a gamma camera transmission CT (gammaTCT) SPECT system and x-ray CT. Subsequently, the RSD phantom's brain reservoir was filled with a uniform water solution of 99mTc, and SPECT and gammaTCT data were acquired using fanbeam collimation. The attenuating effects of bone, scalp and head-holder in individual projections were determined by an analytical projection technique using the SPECT and gammaTCT reconstructions. The Chang UAC used brain and head contours that were segmented from the gammaTCT reconstruction to demarcate its attenuation map, whereas the Sorenson UAC fit slice-specific ellipses to the SPECT projection data. For each UAC, volumetric relative quantification was measured with varying attenuation coefficients (mus) of the attenuation map. RESULTS: Gamma camera transmission CT and x-ray CT scans showed that the dry skull and Rando phantoms suffered from a dried trabecular bone compartment. The RSD phantom most closely reproduced the attenuation coefficients of the human gammaTCT and x-ray CT scans. The analytical projections showed that the attenuating effects of bone, scalp and head-holder were nonuniform across the projections and accounted for 18%-37% of the total count loss. Volumetric relative quantification was best achieved with the Chang (zero iterations) attenuation correction using the head contour and mu = 0.075 cm(-1); however, cortical activity was found to be 10% higher than cerebellar activity. For all UACs, the optimal choices of mu were experimentally found to be lower than the recommended 0.12 cm(-1) for brain tissue. This result is theoretically supported here. CONCLUSION: The magnitude of errors resulting from uniform attenuation corrections can be greater than the magnitudes of regional cerebral blood flow deficits in patients with dementia, as compared with normal controls. This suggests that nonuniform attenuation correction in brain SPECT imaging must be applied to accurately estimate regional cerebral blood flow.     

Significance of nonuniform attenuation correction in quantitative brain SPECT imaging

The purposes of this study were to develop a method for nonuniform attenuation correction of 123I emission brain images based on transmission imaging with a longer-lived isotope (i.e., 57Co) and to evaluate the relative improvement in quantitative SPECT images achieved with nonuniform attenuation correction. METHODS: Emission and transmission SPECT scans were acquired on three different sets of studies: a heterogeneous brain phantom filled with 1231 to simulate the distribution of dopamine transporters labeled with 2beta-carbomethoxy-3beta-(4-123I-iodophenyl)tropane (123I-beta-CIT); nine healthy human control subjects who underwent transmission scanning using two separate line sources (57Co and 123I); and a set of eight patients with Parkinson's disease and five healthy control subjects who received both emission and transmission scans after injection of 123I-beta-CIT. Attenuation maps were reconstructed using a Bayesian transmission reconstruction algorithm, and attenuation correction was performed using Chang's postprocessing method. The spatial distribution of errors within the brain was obtained from attenuation correction factors computed from uniform and nonuniform attenuation maps and was visualized on a pixel-by-pixel basis as an error image. RESULTS: For the heterogeneous brain phantom, the uniform attenuation correction had errors of 2%-6.5% for regions corresponding to striatum and background, whereas nonuniform attenuation correction was within 1%. Analysis of 123I transmission images of the nine healthy human control subjects showed differences between uniform and nonuniform attenuation correction to be in the range of 6.4%-16.0% for brain regions of interest (ROIs). The human control subjects who received transmission scans only were used to generate a curvilinear function to convert 57Co attenuation values into those for 123I, based on a pixel-by-pixel comparison of two coregistered transmission images for each subject. These values were applied to the group of patients and healthy control subjects who received transmission 57Co scans and emission 123I scans after injection of 123I-beta-CIT. In comparison to nonuniform attenuation correction as the gold standard, uniform attenuation with the ellipse drawn around the transmission image caused an approximately 5% error, whereas placement of the ellipse around the emission image caused a 15% error. CONCLUSION: Nonuniform attenuation correction allowed a moderate improvement in the measurement of absolute activity in individual brain ROIs. When images were analyzed as target-to-background activity ratios, as is commonly performed with 123I-beta-CIT, these outcome measures showed only small differences when Parkinson's disease patients and healthy control subjects were compared using nonuniform, uniform or even no attenuation correction.     

A simple method for the study of cytosolic content of oligonucleotides in cells

BACKGROUND: Nonuniform attenuation in the thorax can generate artifacts in single-photon emission computed tomographic myocardial perfusion studies that mimic coronary artery disease. In this article we present both phantom and simulation data, as well as clinical data, in support of an emission-based method that provides reliable correction for attenuation effects without the need for a transmission measurement. METHODS AND RESULTS: The attenuation map is derived from the measured distribution of 99mTc-labeled macroaggregated albumin in the lungs and a radioactive binder wrapped about the thorax. This information is acquired as part of a dual-isotope acquisition during the rest 201Tl study. Segmentation is used to define the interiors of lung and body compartments, which are assigned a single attenuation coefficient for each of the two tissue types. The appropriateness of this approach was investigated by examining the measured attenuation coefficients in a group of 80 individuals (40 male, 40 female) from positron emission tomographic transmission studies. The correction technique was evaluated with computer simulations, a physical phantom, and clinical data acquired from 20 patients. Analysis of the positron emission tomographic data found a small SD in the mean attenuation coefficients for the body (<5%) and lungs (<15%). The application of emission-based attenuation-correction technique produced a substantial reduction in the magnitude of the attenuation artifact in images obtained from both the phantom and the simulation studies. The emission-based attenuation-correction technique was easily applied to myocardial perfusion studies, where it had a significant effect, resulting in changes in interpretation for nine of 20 patients. CONCLUSIONS: The results of this study provide strong support for the concept that an attenuation map can be generated with fixed attenuation values in place of those that are directly measured. Thus the emission-based attenuation-correction technique can be considered an inexpensive alternative to transmission-based correction methods. Because the emission-based correction technique does not require any additional hardware, it has the major advantage of being applicable to all single-photon emission computed tomographic systems.     

Application of models of working at the interface between primary care and mental health services in Israel

A practical method for scatter and attenuation compensation was employed in thallium-201 myocardial single-photon emission tomography (SPET or ECT) with the triple-energy-window (TEW) technique and an iterative attenuation correction method by using a measured attenuation map. The map was reconstructed from technetium-99m transmission CT (TCT) data. A dual-headed SPET gamma camera system equipped with parallel-hole collimators was used for ECT/TCT data acquisition and a new type of external source named "sheet line source" was designed for TCT data acquisition. This sheet line source was composed of a narrow long fluoroplastic tube embedded in a rectangular acrylic board. After injection of 99mTc solution into the tube by an automatic injector, the board was attached in front of the collimator surface of one of the two detectors. After acquiring emission and transmission data separately or simultaneously, we eliminated scattered photons in the transmission and emission data with the TEW method, and reconstructed both images. Then, the effect of attenuation in the scatter-corrected ECT images was compensated with Chang's iterative method by using measured attenuation maps. Our method was validated by several phantom studies and clinical cardiac studies. The method offered improved homogeneity in distribution of myocardial activity and accurate measurements of myocardial tracer uptake. We conclude that the above correction method is feasible because a new type of 99mTc external source may not produce truncation in TCT images and is cost-effective and easy to prepare in clinical situations.     

The relative contributions of scatter and attenuation corrections toward improved brain SPECT quantification

Mounting evidence indicates that scatter and attenuation are major confounds to objective diagnosis of brain disease by quantitative SPECT. There is considerable debate, however, as to the relative importance of scatter correction (SC) and attenuation correction (AC), and how they should be implemented. The efficacy of SC and AC for 99mTc brain SPECT was evaluated using a two-compartment fully tissue-equivalent anthropomorphic head phantom. Four correction schemes were implemented: uniform broad-beam AC, non-uniform broad-beam AC, uniform SC + AC, and non-uniform SC + AC. SC was based on non-stationary deconvolution scatter subtraction, modified to incorporate a priori knowledge of either the head contour (uniform SC) or transmission map (non-uniform SC). The quantitative accuracy of the correction schemes was evaluated in terms of contrast recovery, relative quantification (cortical:cerebellar activity), uniformity ((coefficient of variation of 230 macro-voxels) x 100%), and bias (relative to a calibration scan). Our results were: uniform broad-beam (mu = 0.12 cm(-1)) AC (the most popular correction): 71% contrast recovery, 112% relative quantification, 7.0% uniformity, +23% bias. Non-uniform broad-beam (soft tissue mu = 0.12 cm(-1)) AC: 73%, 114%, 6.0%, +21%, respectively. Uniform SC + AC: 90%, 99%, 4.9%, +12%, respectively. Non-uniform SC + AC: 93%, 101%, 4.0%, +10%, respectively. SC and AC achieved the best quantification; however, non-uniform corrections produce only small improvements over their uniform counterparts. SC + AC was found to be superior to AC; this advantage is distinct and consistent across all four quantification indices.     

Attenuation compensation for cardiac single-photon emission computed tomographic imaging: Part 1. Impact of attenuation and methods of estimating attenuation maps

Attenuation is believed to be one of the major causes of false-positive cardiac single-photon emission computed tomographic (SPECT) perfusion images. This article reviews the physics of attenuation, the artifacts produced by attenuation, and the need for scatter correction in combination with attenuation correction. The review continues with a comparison of the various configurations for transmission imaging that could be used to estimate patient specific attenuation maps, and an overview of how these are being developed for use on multiheaded SPECT systems, including discussions of truncation, noise, and spatial resolution of the estimated attenuation maps. Ways of estimating patient specific attenuation maps besides transmission imaging are also discussed.     

201Tl myocardial SPECT. First experiences with a simultaneous transmission-emission acquisition protocol for patient-specific attenuated correction

AIM: In this study our first clinical experiences with simultaneous transmission and emission acquisition in 201 TI myocardial SPECT (T/E-SPECT) are discussed. METHODS: The non-uniform attenuation (AK) was carried out with a triple-head camera (PRISM 3000, Picker Inc.) correction equipped with fanbeam collimators. A line source of 750 MBq 99mTc was used to construct the transmission profile. Prior to investigation patients got 80-120 MBq 201TI-chloride intravenously injected. RESULTS: The study comprises the evaluation of 40 patients, derived from the clinical routine. The investigation followed an usual one day protocol. Our results using T/E-SPECT reveal an almost equilibrated activity distribution between anterior and posterior myocardial wall. CONCLUSION: For this reason it is to be expected that T/E-SPECT provides more reliable information about the posterior myocardial wall, than the usual SPECT technique without attenuation correction.     

Evaluation of attenuation corrections using Monte Carlo simulated lung SPECT

SPECT (single photon emission computed tomography) images are distorted by photon attenuation. The effect is complex in the thoracic region due to different tissue densities. This study compares the effect on the image homogeneity of two different methods of attenuation correction in lung SPECT; one pre-processing and one post-processing method. This study also investigates the impact of attenuation correction parameters such as lung contour, body contour, density of the lung tissue and effective attenuation coefficient. The Monte Carlo technique was used to simulate SPECT studies of a digital thorax phantom containing a homogeneous activity distribution in the lung. Homogeneity in reconstructed images was calculated as the coefficient of variation (CV). The isolated effect of the attenuation correction was assessed by normalizing pixel values from the attenuation corrected lung by pixel values from the lung with no attenuation effects. Results show that the CV decreased from 12.8% with no attenuation correction to 4.4% using the post-processing method and true densities in the thoracic region. The impact of variations in the definition of the body contour was found to be marginal while the corresponding effect of variations in the lung contour was substantial.     

Oculo-facio-cardio-dental (OFCD) syndrome

OBJECTIVE: This study examined the effects on SPECT quantitation caused by erroneous size and position of the attenuation map and inaccurate pixel size used in the Chang algorithm. METHODS: Projection data of a three-dimensional head phantom were simulated with a uniform attenuation coefficient of 0.15/cm for the inside of the phantom. Images were reconstructed using the filtered backprojection algorithm without attenuation compensation and the Chang algorithm with different attenuation maps. Quantitative comparison then was performed between the reconstructed images and the phantom. RESULTS: The pixel values obtained for noisy data by using the first-order Chang algorithm with an accurate attenuation map were less than 10% different from the true values and the left-right asymmetry was under 5%. Small errors in the geometric parameters of the attenuation map, however, caused considerable quantitative inaccuracy in the reconstructed image. For example, a 0.64-cm error in the size of the map caused 10% deviation from the true value and a 0.64-cm shift of the position of the map towards the left produced 10% left-right pixel value asymmetry. CONCLUSION: The accuracy of the Chang algorithm critically depends on the geometric parameters. For a uniform attenuator with symmetric geometry, such as the human brain, a true left-right symmetry in the pixel value can be altered significantly by a small error in the geometric parameters, while symmetry can be maintained with no attenuation compensation.     

Correction of partial volume effects in myocardial SPECT

BACKGROUND: Marked partial volume effects occur in myocardial single photon emission computed tomographic (SPECT) studies because of limited resolution in imaging the myocardial wall and contractile motion of the heart. Little work has been undertaken to develop correction techniques for SPECT except for efforts to improve the reconstructed resolution. Our purpose was to examine the extent of the problem and propose a correction method. METHODS AND RESULTS: A potential correction method, developed initially for positron emission tomography, involved estimation of extravascular density by means of subtracting vascular density derived in a blood pool study from total density derived from a transmission study. Provided partial volume errors are the same for transmission and emission data, activity per gram of extravascular tissue can be obtained by means of dividing the perfusion regional data by extravascular density for the same region. Simulations were designed to assess the importance of partial volume errors and the use of extravascular density to correct the errors. Recovery coefficients for the myocardium were estimated by means of simulation of the beating heart on the basis of published values for ventricular dimensions. Resolution for transmission with a scanning line source system was compared with emission resolution. The effect of spillover on measured partial volume losses was assessed, and a method for matching spillover for emission and extravascular density was demonstrated. Correction for partial volume effects was demonstrated for a phantom with variable wall thickness. Significant variation in recovery coefficient was demonstrated between posterior and septal walls for individual patients independent of heart size. Filtering was necessary to account for the difference in transmission resolution measured in the axial direction. Spillover effects had a significant influence on the measured recovery for small objects; however, for a specific reconstruction algorithm and defined region size, correction was implemented to match the spillover effects for emission and extravascular density. Use of extravascular density for correction of partial volume loss, for ordered subsets expectation maximization reconstruction with compensation for resolution, was demonstrated to be accurate to within 10%. CONCLUSIONS: The feasibility of correcting partial volume effects with extravascular density was demonstrated. Correction is effective provided care is taken to match both resolution and spillover for emission and extravascular density.     

Three-dimensional PET emission scan registration and transmission scan synthesis

The duration of a positron emission tomography (PET) imaging scan can be reduced if the transmission scan of one patient which is used for emission correction can be synthesized by using the reference transmission scan of another patient. In this paper, we propose a new intersubjects PET emission scan registration method and PET transmission synthesis method by using the boundary information of the body or brain scan of the PET emission scans. The PET emission scans have poor image quality and different intensity statistics so that we preprocess the emission scans to have similar histogram and then apply the point distribution model (PDM) [15] to extract the contours of the emission scan. The extracted boundary contour of every slice is used to reconstruct the three-dimensional (3-D) surface of the reference set and the target set. Our registration is 3-D surface-based which uses the normal flow method [17] to find the correspondence vector field between two 3-D reconstructed surfaces. Since it is difficult to analyze internal organ using the PET emission scan imaging without correction, we assume that the deformation of internal organ is homogeneous. With the corresponding vector field between the two emission scans and the transmission scan of the reference set, we can synthesize the transmission scan of the target set.     

Pulmonary perfusion and density gradients in healthy volunteers

The goal of this study was to measure regional pulmonary perfusion using SPECT and transmission tomography for attenuation correction and density measurements. METHODS: Regional pulmonary perfusion was studied after intravenous injection of radiolabeled particles in 10 supine healthy volunteers using SPECT. Transmission tomography was used to correct for attenuation, measure lung density and delineate the lungs. The effects of attenuation correction on pulmonary perfusion gradients were investigated. RESULTS: In perfusion measurements not corrected for attenuation, we found significant perfusion gradients in the direction of gravity but also significant gradients at isogravitational level. After correction for attenuation, the gravitational gradient was significantly greater than before correction, and gradients at isogravitational level were no longer observed. Perfusion in the ventral lung zone was half of that in the dorsal lung zone. Mean lung density was 0.28 +/- 0.03 g/ml, and density showed a significant increase in the direction of gravity and at isogravitational level. CONCLUSION: We found that SPECT perfusion studies of the lung not corrected for attenuation gave a false impression of nongravitational gradients and underestimate the gradient that is gravity-dependent. Transmission tomography, used for attenuation correction, also quantifies lung density and shows gravity dependent and nondependent density gradients.     

Relativistic coupled-cluster calculations for open-shell atoms

A procedure for patient repositioning and compensation for misalignment between transmission and emission data in positron emission tomography (PET) heart studies has been developed. Following the transmission scan (TR1), patients are moved from the scanner bed for the administration of the tracer, and repositioned when ready for the emission scan (EM1). A short postinjection transmission scan (TR2) is performed at the end of the EM1 study. TR1 and TR2 images are compared to recognize misalignment between transmission and emission studies. TR1 sinograms are compensated for misalignment to allow for a proper attenuation correction. The procedure has been tested on phantom and [18F]FDG PET heart studies. Misalignments down to 2.5 mm translation and 1 degree rotation in the transaxial plane and 4 mm in the axial direction can be recognized and compensated for. The procedure is suitable for clinical purposes, allowing reduction of patient time on the scanner bed, increased patient comfort and significant increase of patient throughput.     

Evaluation of exercise tolerance of patients with heart failure and its clinical significance

A new method of PET attenuation using post-injection transmission scan is presented, which is especially useful in 18F-FDG static studies. The transmission scan is acquired right before the emission scan, which is used to subtract the emission component from the transmission data. When the effect of measurement condition upon the image noise was evaluated with a 20 cm diameter cylindrical phantom, an increase in the injection dose inflated the noise and caused artifacts. There was an optimum dose that minimized the image noise. As the external source activity increased, the image noise decreased, and the optimum dose increased linearly, which enabled estimation of the optimum injection dose under a given external source. When the total (emission plus transmission) scan time was fixed, longer emission scan resulted in better images than longer transmission scan.     

Correction for photon attenuation in SPECT: analytical framework, average attenuation factors, and a new hybrid approach

We provide a unified framework for the investigation of convergence properties of the iterative algorithms for photon attenuation correction in SPECT, including the iterative Chang method--a commonly used approach in which an average attenuation factor calculated over all projection angles is employed in a pointwise correction scheme. A new average attenuation factor calculated along the projection line is introduced, which can compensate exactly for the attenuation effect in the case of a uniform activity distribution. We propose a new hybrid approach in which we use this new average attenuation factor initially and then shift to the iterative Chang method in later iterations. This hybrid approach was evaluated in a simulation study by use of a computer-generated phantom with both non-uniform activity and non-uniform attenuation distributions. The results demonstrate that this hybrid approach improves the convergence speed of the iterative Chang method and produces reconstructed images of high quality.