Photopeak Window Research Articles

The effects of compensation for scatter, lead X-rays, and high-energy contamination on tumor detectability and activity estimation in Ga-67 imaging

Compton scatter, lead X-rays, and high-energy contamination are major factors affecting image quality in Ga-67 imaging. Scattered photons detected in one photopeak window include photons exiting the patient at energies within the photopeak, as well as higher energy photons which have interacted in the collimator and crystal and lost energy. Furthermore, lead X-rays can be detected in the main energy photopeak (93 keV). We have previously developed two energy-based methods, based on artificial neural networks (ANN) and on a generalized spectral (GS) approach to compensate for scatter, high-energy contamination, and lead X-rays in Ga-67 imaging. For comparison, we considered also the projections that would be acquired in the clinic using the optimal energy windows (WIN) we have reported previously for tumor detection and estimation tasks for the 93, 185, and 300 keV photopeaks. The aim of the present study is to evaluate under realistic conditions the impact of these phenomena and their compensation on tumor detection and estimation tasks in Ga-67 imaging. ANN and GS were compared on the basis of performance of a three-channel Hotelling observer (CHO), in detecting the presence of a spherical tumor of unknown size embedded in an anatomic background as well as on the basis of estimation of tumor activity. Projection datasets of spherical tumors ranging from 2 to 6 cm in diameter, located at several sites in an anthropomorphic torso phantom, were simulated using a Monte Carlo program that modeled all photon interactions in the patient as well as in the collimator and the detector for all decays between 91 and 888 keV. One hundred realistic noise realizations were generated from each very-low-noise simulated projection dataset. The presence of scatter degraded both CHO signal-to-noise ratio (SNR) and estimation accuracy. On average, the presence of scatter led to a 12% reduction in CHO SNR. Correcting for scatter further diminished CHO SNR but to a lesser extent with ANN (5% reduction) than with GS (12%). Both scatter corrections improved performance in activity estimation. ANN yielded better precision (1.8% relative standard deviation) than did GS (4%) but greater average bias (5.1% with ANN, 3.6% with GS).

A 3D gantry single photon emission tomograph with hemispherical coverage for dedicated breast imaging

Abstract A novel tomographic gantry was designed, built and initially evaluated for single photon emission imaging of metabolically active lesions in the pendant breast and near chest wall. Initial emission imaging measurements with breast lesions of various uptake ratios are presented. Methods: A prototype tomograph was constructed utilizing a compact gamma camera having a field-of-view of Results: As iteration number increased for the tomographically measured data at all polar angles, contrasts increased while signal-to-noise ratios (SNRs) decreased in the expected way with OSEM reconstruction. The rollover between contrast improvement and SNR degradation of the lesion occurred at two to three iterations. The reconstructed tomographic data yielded SNRs with or without scatter correction that were >9 times better than the planar scans. There was up to a factor of ∼2.5 increase in total primary and scatter contamination in the photopeak window with increasing tilt angle from 15° to 45°, consistent with more direct line-of-sight of myocardial and liver activity with increased camera polar angle. Conclusion: This new, ultra-compact, dedicated tomographic imaging system has the potential of providing valuable, fully 3D functional information about small, otherwise indeterminate breast lesions as an adjunct to diagnostic mammography.

Parameterization of Pb X-ray contamination in simultaneous Tl-201 and Tc-99m dual-isotope imaging

In simultaneous Tl-201 and Tc-99m dual isotope imaging, Pb X-rays are a significant source of contamination in the Tl image. In order to characterize this contamination, we simulated images of line sources in the Tc and Tl photopeak windows using an experimentally verified Monte Carlo program. The line sources were placed at various distances from the collimator face and emitted photons with energies from 88 to 140.5 keV. The Ph X-ray contamination line source response function in the Tl window was fitted well by a Gaussian plus an exponential function. The width of these two functions changed linearly with distance. Parameterization of the Ph X-ray response was done by simultaneously fitting the Ph X-ray response functions at various distances with fitting functions that were determined empirically to model the distance dependence of the Gaussian and exponential components of the Ph X-ray response. The parameterized model of Ph X-ray contamination is useful in developing methods to model Ph X-ray contamination in Tl-201 data by Tc-99m. This Ph X-ray contamination model can be used in iterative reconstruction-based cross-talk compensation for simultaneous Tc/Tl dual isotope imaging.

A spatially varying Compton scatter correction for SPECT utilizing theintegral Klein-Nishina cross section

An algorithm correcting for the fraction of scattered events in SPECT andplanar imaging is proposed. The algorithm utilizes a pixel-based multi-channelanalyser for data acquisition. The method was designed to operate on a locallevel by three subtraction steps: (a) Subtracting a modified Klein-Nishinasingle scatter distribution, pixel by pixel, from the events obtainedexperimentally in the upper half of the photo-peak window. (b) Subtracting amirrored distribution of the unscattered events hence obtained from that oftotal events in the lower half of the window, thus giving the scatterdistribution in this part of the window. (c) Subtracting the sum of the scatterdistributions in both window halves from the corresponding sum of total eventsin order to obtain the unscattered photons within the photo-peak window.The accuracy of the method was validated experimentally, using a new rCBFphantom allowing for imaging in matter corresponding to soft tissue andapproximately in air, respectively. After correction for photon scattering andattenuation, the regional difference in SPECT values in soft tissue equivalentmatter and in low-density matter (simulating air) was only 1.5±7.2%(mean ± 1 SD), thus indicating a high accuracy of the correctionmethod. Provided that an accurate and stable pixel peak-alignment routine isavailable, the method can be applied using a minimum of three windows.

Dual-window scatter correction and energy window setting in cerebralblood flow SPECT: a Monte Carlo study

The image quality in SPECT studies of the regional cerebral blood flow (rCBF)performed with 99mTc-HMPAO is degraded by scattered photons. Thefinite energy resolution of the gamma camera makes the detection of scatteredphotons unavoidable, and this is observed in the image as an impaired contrastbetween grey and white matter structures.In this work, a Monte Carlo simulated SPECT study of a realistic voxel-basedbrain phantom was used to evaluate the resulting contrast-to-noise ratio for anumber of energy window settings, with and without the dual-window scattercorrection. Values of the scaling factor k, used to obtain the fraction ofscattered photons in the photopeak window, were estimated for each energywindow.The use of a narrower, asymmetric, energy discrimination window improved thecontrast, with a subsequent increase in statistical noise due to the lowernumber of counts. The photopeak-window setting giving the best contrast-to-noiseratio was found to be the same whether or not scatter correction was applied.Its value was 17% centred at 142 keV. At the optimum photopeak-window setting,the contrast was improved by using scatter correction, but the contrast-to-noiseratio was made worse.

Comparison of correction techniques for simultaneous201Tl/99mTc myocardial perfusion SPECT imaging: a dogstudy

We compared two correction methods for simultaneous 201Tl/99mTc dual-isotope single-photon emission computed tomography (SPECT).Both approaches use the information from the third energy window placedbetween the photopeak windows of the 201Tl and 99mTc. The first approach, described by Moore et al, corrects only for thecontribution of the 99mTc to the 201Tl primary70 keV window. We developed the three-window transformation dual-isotopecorrection method, which is a simultaneous cross-talk correction. The twocorrection methods were compared in a simultaneous 201Tl/99mTc sestamibi cardiac dog study. Three separate acquisitions wereperformed in this dog study: two single-isotope and one dual-isotopeacquisition. The 201Tl single-isotope images were used asreferences. The total number of counts, and the contrast between the leftventricular cavity (LVC) and the myocardium, were used in 70 keV short-axisslices as parameters for evaluating the results of the dual-isotope correctionmethods. Three consecutive short-axis slices were used to calculate averagedcontrast and the averaged total number of counts. The total number of thecounts was 667 000 ± 500 and 414 500 ± 400 counts for the dual-isotope(201Tl + 99mTc) and single-isotope (201Tl-only) 70 keV images, respectively. The corrected dual-isotope images had514 700 ± 700 and 368 000 ± 600 counts for Moore's correction and ourapproach, respectively. Moore's method improved contrast in the dual-isotope70 keV image to 0.14 ± 0.03 from 0.11 ± 0.02, which was the value in the70 keV non-corrected dual-isotope image. Our method improved the same contrastto 0.22 ± 0.03. The contrast in the 201Tl single-isotope 70 keVimage was 0.28 ± 0.02. Both methods improved the 70 keV dual-isotope images.However, our approach provided slightly better images than Moore's correctionwhen compared with 201Tl-only 70 keV images.

Energy-based compensation for nonuniform attenuation in Ga-67 SPECT imaging

Describes a method to estimate unattenuated projection images directly from Ga-67 SPECT data acquired over 360/spl deg/ at 3 photopeak energies. The algorithm compensates to third order for the effects of nonuniform attenuation, thereby yielding unattenuated projections with <3% accuracy, provided that scattered photons have been successfully subtracted from each energy window. Previously, the authors demonstrated an approach for using prior information to control noise in the estimated projections. They describe here an accelerated, robust algorithm for implementing these constraints. They have also evaluated the accuracy of the method for a region-of-interest (ROI) activity estimation task as a function of the residual error following scatter subtraction, and under conditions of nonstationary spatial resolution. When the maximum residual scatter subtraction errors at 93, 185, and 300 keV, respectively, were assumed to be 8%, 4%, and 2%, all ROI estimates were biased less than /spl plusmn/7% after attenuation compensation. Projection inconsistencies arising from distance-dependent geometric response and energy-dependent collimator penetration increased the standard deviation of ROI estimates (over location) to /spl sim/10%; however, methods of correcting for distance-dependent collimator response can probably be adapted well to Ga-67 imaging. If scatter can be adequately removed from the 3 Ga-67 photopeak windows, this technique is expected to provide reliable compensation for nonuniform attenuation with no need for an independently measured attenuation map.

The influence of backscatter material on Tc and line source responses

SPECT projections are contaminated by scatter, resulting in reduced image contrast and quantitative errors. When tissue is present behind the source, some of the detected photons backscatter via this tissue. Particularly in dual-isotope SPECT and in combined emission-transmission SPECT, backscatter constitutes a major part of the down-scatter contamination in lower-energy windows. In this paper, the effects of backscatter material were investigated. Planar images of and line sources between varying numbers of Perspex slabs were analysed using the photopeak windows and various scatter windows. In the photopeak window no significant change in total counts due to backscatter material was measured. In the photopeak window an increase of about 10% in total counts was observed. In the scatter windows an even more explicit influence of backscatter material was measured. For instance, at a forward depth of 10 cm, total counts of a source detected in the 72 keV window eventually doubled with increasing backscatter material, compared with the situation without backscatter material. The backscatter contribution plateaued when more than 5-10 cm of scatter material was placed behind the source. In conclusion, backscatter should be taken into account, particularly in model-based down-scatter correction methods in dual-isotope SPECT and combined emission-transmission SPECT.

Energy dependence of nonstationary scatter subtraction--restoration in high resolution PET.

In previous works, the determination of object and detector scatter kernels from line source measurements was described and their application in scatter correction was investigated. It was also shown that low energy data contains a large fraction of useful events (true and detector scatter events). In the present work, data acquired in multispectral mode was summed in single broad energy windows of lower energy thresholds varying from 129 to 516 keV in steps of 43 keV and a constant upper energy threshold of 645 keV. Line-source projections were fitted by extracting the object and detector scatter kernels as a function of energy threshold. These kernels were then used to process scatter by the nonstationary convolution subtraction-restoration method in phantom images. After scatter correction, the sensitivity is found to increase by up to 64% at the lower threshold of 129 keV, relative to the conventional photopeak energy window (344-645 keV). Whereas contrast and spatial resolution are degraded as the energy discriminator is lowered, such degradation is fully recovered by the scatter correction. As a result of scatter correction, the noise increases insignificantly in hot regions but substantially in cold regions, in proportion of the amount of scatter.

Quantitative /sup 131/I SPECT with triple energy window Compton scatter correction

In this work accuracy of quantitative /sup 131/I SPECT with triple energy window (TEW) scatter correction is evaluated by phantom measurements. The application is tumor imaging of B-cell lymphoma patients treated with /sup 131/I radioimmunotherapy. The TEW method is a pixel by pixel correction where the scatter fraction in the photopeak window is estimated by linear interpolation between two adjacent narrow sub-windows. For the phantom measurements performed in this work the TEW estimate of scattered counts was close to one half of the total photopeak window counts. Quantification procedure includes marker based X-ray CT-SPECT image fusion to determine object boundaries and to generate attenuation maps. TEW scatter correction significantly reduces the effect of background activity on reconstructed counts within an object, but it still exists due to the finite spatial resolution of the system. Therefore, a background dependent calibration factor had to be used to achieve good quantitative accuracy. Quantitative accuracy with TEW correction was 5% and 14% for a tumor and lung respectively of a physical phantom with non-uniform activity and non-uniform scattering medium. With no scatter subtraction but using a background dependent calibration the quantitative accuracy was 7% and 18% for the tumor and lung respectively.

High resolution CsI(Tl)/Si-PIN detector development for breast imaging

High resolution multi-element (8/spl times/8) imaging arrays with collimators, size matched to discrete CsI(TI) scintillator arrays and Si-PIN photodetector arrays (PDA's) were developed as prototypes for larger arrays for breast imaging. Photodetector pixels were each 1.5/spl times/1.5 mm/sup 2/ with 0.25 mm gaps. A 16-element quadrant of the detector was evaluated with a segmented CsI(TI) scintillator array (1.5/spl times/1.5/spl times/6 mm/sup 3/ segments with 0.25 mm septa) coupled to the silicon array. The scintillator thickness of 6 mm corresponds to >85% total gamma efficiency at 140 keV. Pixel energy resolution of <8% FWHM was obtained for Tc-99m (140 keV). Electronic noise was 41 e/sup -/ RMS corresponding to a 3% FWHM contribution to the 140 keV photopeak. Detection efficiency uniformity (/spl plusmn//spl sigma/%) measured with a Tc-99m flood source was 4.3% for a /spl sim/10% energy photopeak window. Spatial resolution was 1.53 mm FWHM and pitch was 1.75 mm as measured from the Co-57 (122 keV) line spread function. Signal to background was 34 and contrast ([max-min]/[max+min]) was 0.94. The energy resolution and spatial characteristics of the new imaging detector exceed those of other scintillator based imaging detectors. A camera based on this technology will allow: (1) Improved Compton scatter rejection; (2) Detector positioning in close proximity to the breast to increase signal to noise; (3) Improved spatial resolution; and (4) Improved efficiency compared to high resolution collimated gamma cameras for the anticipated compressed breast geometries.

Deadtime correction for two multihead Anger cameras in 131I dual-energy-window-acquisition mode.

Two side-by-side energy windows, one at the photopeak and one at lower energy, are sometimes employed in quantitative SPECT studies. We measured the count-rate losses at moderately high activities of 131I for two multihead Anger cameras in such a dual-window-acquisition mode by imaging a decaying source composed of two hot spheres within a warm cylinder successively over a total of 23 days. The window locations were kept fixed and the paralyzable model was assumed. In addition, for the Picker Prism 3000 XP camera, the source was viewed from three different angles separated by 120 degrees and the final results are from an average over these three angles. For the Picker camera, the fits to the data from the individual windows are good (the mean of the squared correlation coefficient equals 0.98) while for the Siemens Multispect camera fits to the data from head 1 and from the lower-energy, monitor window are relatively poor. Therefore, with the Siemens camera the data from the two windows are combined for deadtime computation. Repeated autopeaking might improve the fits. At the maximum count rate, corresponding to a total activity of 740 MBq (20 mCi) in the phantom, the multiplicative deadtime correction factor is considerably larger for the Picker than for the Siemens camera. For the Picker camera, it is 1.11, 1.12, and 1.12 for heads 1-3 with the photopeak window and 1.10 for all heads with the lower-energy monitor window. For the Siemens camera, the combined-window deadtime correction factor is 1.02 for head 1 and 1.03 for head 2. Differences between the deadtime correction factor for focal activity and for the total activity do not support the hypothesis of count misplacement between foci of activity at these count rates. Therefore, the total-image dead time correction is recommended for any and all parts of the image.

Effect of Dual and Triple Energy Window Scatter Correction Methods on Image Quality in Liver Scintigraphy

To investigate the effect of two scatter correction methods on lesion detectability for both planar and tomographic hepatic imaging. All planar and tomographic acquisitions involved simultaneous collection of photons in the main photopeak window (126-154 keV) and three additional windows (92-116, 116-126 and 154-164 keV). Uncorrected and corrected for scatter images were obtained from the same acquisition data. The dual energy window (DEW) and the triple energy window (TEW) scatter compensation methods were used to obtain two sets of corrected images. The DEW method was implemented with main photopeak window 126-154 keV, Compton scatter window 92-126 keV and scatter multiplier k = 0.5. A modified TEW method was also applied with main photopeak window 126-154 keV and scatter subwindows 116-126 keV and 154-164 keV. Phantoms were used to study the effect of scatter correction on contrast and signal-to-noise ratio. The observer's ability to identify lesions was studied on uncorrected and corrected for scatter patient images. In planar imaging, both scatter compensation methods yielded contrast enhancement. However signal to noise ratio (SNR) was degraded to 0.63 and 0.67 when DEW and TEW were applied respectively. In SPECT images, contrast was increased by a factor of 2.4 and 1.7, while SNR was degraded to 0.60 and 0.64 when DEW and TEW methods were used respectively. Scatter correction using DEW and TEW methods may improve observer's ability to distinguish lesions in planar (p < 0.05 for both methods) and SPECT (p < 0.05 for both methods) liver studies.

Analysis of the reconstructibility and noise properties of scattered photons in Tc SPECT

Since scattered photons carry degraded spatial information, scatter is typically considered a source of contamination in SPECT. However, with the advent of scatter modelling methods and reconstruction-based scatter compensation (RBSC), it may be possible to utilize scattered data in a productive manner. In this work we analyse the reconstructibility of scattered photon projection data and investigate the potential for using scattered photons to reduce the noise levels of SPECT images. We have simulated projection data for an elliptical phantom containing three cold rods in a uniform background of activity. A variety of photopeak and scatter energy windows were formed, as well as corresponding RBSC transfer matrices. Each statistically weighted matrix was decomposed using SVD and analysed in terms of reconstructibility and noise properties. Results indicate that scattered photons contain sufficient information to reconstruct the source activity, but the scatter-only matrices are very poorly conditioned. We have also evaluated several methods of utilizing scattered events via RBSC, and compared them with other, idealized methods of handling scatter. It was found that scattered photons can be used productively when photopeak and non-photopeak data are separated through the use of multiple energy windows. The RBSC methods outperformed ideal scatter subtraction, but fell short of methods which assume perfect discrimination between scattered and primary events. The knowledge gained by this study may help guide future research and lead to better approaches to handling scatter in SPECT.

Effects of detector sensitivity on image quality in multidetector SPECT

The common imaging practice of each detector on a multidetector SPECT imaging system to acquire only a fraction of the total number of projections in a 360/spl deg/ projection set introduces the possibility of imbalances in system response which can cause artifacts and changes in image quality. Here we evaluate the effect of variations in detector sensitivity caused by differences in the position of the photopeak window, window width, and different collimators on reconstructed image quality. Analytical simulations of cardiac phantom geometries and experimental measurements with Data Spectrum's Cardiac and Deluxe SPECT phantoms on a three detector SPECT imaging system were used in the evaluation. The relative efficiency of one of three detectors was changed over a series of values between 100 and 0%, while the efficiency of the remaining two detectors was maintained at 100%. The results show that decreased activity and geometric distortions occur along directions perpendicular to the projections with altered sensitivity. The degree of distortion and decrease of activity depends on the geometry of the activity distribution and the reduction in sensitivity. The relative count ratios in the regions with decreased activity varied over a range of 1.0 to 0.59 as the detector sensitivity is decreased from 100 to 0% of the full energy window. Variations in detector sensitivity can degrade spatial resolution and uniformity and create artifacts in reconstructed tomographic slices.

Evaluation of the effect of scatter correction on lesion detection in hepatic SPECT imaging

Various methods have been proposed to correct for the problems presented by scatter in images from single-photon computed tomography (SPECT). While improvement in quantitative accuracy has been reported with a variety of measures, the effect on the accuracy of lesion detection requires analysis of observer performance. Experiments were designed to evaluate the class of methods that correct for scatter by subtracting counts. An anthropomorphic phantom was used with Monte Carlo simulation to simulate liver imaging with labeled antibodies. The lesion was a 2.5-cm-diameter, spherical, cold tumor in the liver, a large, warm background. Ramp-filtered back-projection and non-iterative Chang attenuation compensation were used to approximate clinical practice. Perfect scatter rejection, defined as images containing only primary (non-scattered) photons, was selected as the ideal case. These images were compared with uncorrected images for conditions of both low and high scatter fractions (SF, the scatter-to-primary ratio), typical of Tc-99m and In-111, respectively. In addition, the dual photopeak window (DPW) method was tested in order to evaluate a non-ideal subtraction correction. Receiver operating characteristic (ROC) experiments were conducted under signal-known-exactly conditions, with the area under the curve, A/sub z/, used as the index of accuracy. A statistically significant difference in detection was found only in a few cases, when scatter rejection was compared with no correction. Subtraction of a scatter estimate changes both contrast and noise, such that an improvement in quantitative accuracy does not necessarily provide improvement in detection accuracy. Corrections that approach scatter rejection conditions may offer some improvement in detection, particularly for cases of high SFs.

An investigation of the filtering of TEW scatter estimates used to compensate for scatter with ordered subset reconstructions

With the Triple Energy Window (TEW) scatter correction method, scatter is estimated by interpolation between narrow energy windows placed on either side (or from a single window just below) the photopeak window. The use of narrow windows results in a noisy estimate which requires filtering to prevent significant amplification of noise in the corrected images. Using Monte Carlo simulated projections of a digital model of the anatomy of the torso, we investigated two-dimensional pre-reconstruction filtering of the scatter estimate by the Wiener low-pass filter, and Butterworth low-pass filters with various cutoff frequencies. The scatter estimate was either subtracted from the photopeak window projection or used directly in ordered-subset maximum-likelihood (OS-ML) reconstruction. Using the normalized mean square error (NMSE) between the estimated and true slices as the criterion, it was observed that: (1) low-pass filtering of the TEW scatter estimate dramatically decreases the NMSE compared to that of no filtering of this estimate; (2) the cutoff frequency of the Butterworth filter used to filter the scatter estimate is lower than that typically used for photopeak window images; (3) the cutoff frequency of the Butterworth filter has a broad range of values over which the NMSE is near its minimum value; (4) the cutoff frequency at which the Butterworth reaches its minimum value depends on the number of counts in the TEW window(s), and source distribution; (5) the Wiener low-pass filter adapts to produce a low, but not necessarily the minimum, NMSE; and (6) the inclusion of the scatter estimate directly into OS-ML reconstruction results in a lower NMSE than subtraction of the scatter estimate from the photopeak window prior to reconstruction.

Attenuation correction strategies for multi-energy photon emitters using SPECT

The aim of this study was to investigate whether the photopeak window projections from different energy photons can be combined into a single window for reconstruction or if it is better to not combine the projections due to differences in the attenuation maps required for each photon energy. The mathematical cardiac torso (MCAT) phantom was modified to simulate the uptake of Ga-67 in the human body. Four spherical hot tumors were placed in locations which challenged attenuation correction. An analytical 3D projector with attenuation and detector response included was used to generate projection sets. Data were reconstructed using filtered backprojection (FBP) reconstruction with Butterworth filtering in conjunction with one iteration of Chang attenuation correction, and with 5 and 10 iterations of ordered-subset maximum-likelihood expectation-maximization (ML-OS) reconstruction. To serve as a standard for comparison, the projection sets obtained from the two energies were first reconstructed separately using their own attenuation maps. The emission data obtained from both energies were added and reconstructed using the following attenuation strategies: (1) the 93 keV attenuation map for attenuation correction, (2) the 185 keV attenuation map for attenuation correction, (3) using a weighted mean obtained from combining the 93 keV and 185 keV maps, and (4) an ordered subset approach which combines both energies. The central count ratio (CCR) and total count ratio (TCR) were used to compare the performance of the different strategies. Compared to the standard method, results indicate an overestimation with strategy 1, an under-estimation with strategy 2 and comparable results with strategies 3 and 4. In all strategies, the CCR's of sphere 4 (in proximity to the liver, spleen and backbone) were under-estimated, although TCR's were comparable to that of the other locations. The weighted mean and ordered subset strategies for attenuation correction were of comparable accuracy to reconstruction of the windows separately. They are recommended for multi-energy photon SPECT imaging quantitation when there is a need to combine the acquisitions of multiple windows.

Estimation of attenuation maps from scatter and photopeak window single photon-emission computed tomographic images of technetium 99m-labeled sestamibi

Background. In single photon-emission computed tomographic imaging of the chest, nonuniform attenuation correction requires use of a patient-specific attenuation map. The aim of this study was to determine whether an estimate of the regions of the lungs and nonpulmonary tissues of the chest could be obtained by segmenting the photopeak and Compton scatter window images in a phantom and in patients to estimate patient-specific attenuation maps. Methods and Results. The photopeak and scatter window slices from 16 consecutive 99mTc-labeled sestamibi perfusion studies were segmented interactively. In these studies, visually reasonable regions could be obtained by estimating a “cold” lung region from scatter window data with additional anatomic information of the myocardium region, the backbone and sternum locations, the liver, and the rib cage from the photopeak window data. In an anthropomorphic torso phantom study and a patient study, comparison was made between the attenuation maps based on segmentation of the emission images and transmission imaging with a slant-hole collimator. It was determined that good agreement in the estimation of the body regions can be achieved with segmentation of the emission images in both the phantom and patient data. Attenuation correction using the maximum-likelihood expectation maximization method was performed on the phantom and the patient data. In both studies, attenuation correction with the segmented attenuation map improved uniformity of the inferior wall region in comparison with the other walls. Conclusions. The estimation of patient-specific attenuation maps by segmenting the scatter and photopeak window slices of 99mTc-labeled sestamibi studies may be a way of reducing the loss of specificity due to attenuation artifacts. The potential limitations on the accuracy of correction inherent in the method due to the estimation of the regions and assignment of the attenuation coefficients need to be determined further, and the method needs to be further automated before it can be considered for routine clinical use.

Scatter correction for first-pass radiocardiography.

The effect of the dual window scatter subtraction method was tested for first-pass radiocardiography using 99Tcm. The photopeak window of 126-154 keV (20%) and Compton window of 93-125 keV (30%) were used. The set of scattered images was multiplied by a factor of 0.4 and subtracted from the primary images. The size of the correction was tested by recalculation among a group of 16 patients sent for cardiac output studies for various reasons. The average of the percentage increase following correction was 4.0% (range -1.1 to 11.7%). The size of scatter correction was rather small in individual patients, but consistent for the group of patients as a whole. There was a statistically significant difference between the raw and corrected cardiac output values.