Convolution Subtraction Research Articles

A Cross-Level Iterative Subtraction Network for Camouflaged Object Detection

Camouflaged object detection (COD) is a challenging task, aimed at segmenting objects that are similar in color and texture to their background. Sufficient multi-scale feature fusion is crucial for accurately segmenting object regions. However, most methods usually focus on information compensation, overlooking the difference between features, which is important for distinguishing the object from the background. To this end, we propose the cross-level iterative subtraction network (CISNet), which integrates information from cross-layer features and enhances details through iteration mechanisms. CISNet involves a cross-level iterative structure (CIS) for feature complementarity, where texture information is used to enrich high-level features and semantic information is used to enhance low-level features. In particular, we present a multi-scale strip convolution subtraction (MSCSub) module within CIS to extract difference information between cross-level features and fuse multi-scale features, which improves the feature representation and guides accurate segmentation. Furthermore, an enhanced guided attention (EGA) module is presented to refine features by deeply mining local context information and capturing a broader range of relationships between different feature maps in a top-down manner. Extensive experiments conducted on four benchmark datasets demonstrate that our model outperforms the state-of-the-art COD models in all evaluation metrics.

Reduction in camera-specific variability in [(123)I]FP-CIT SPECT outcome measures by image reconstruction optimized for multisite settings: impact on age-dependence of the specific binding ratio in the ENC-DAT database of healthy controls.

Quantitative estimates of dopamine transporter availability, determined with [(123)I]FP-CIT SPECT, depend on the SPECT equipment, including both hardware and (reconstruction) software, which limits their use in multicentre research and clinical routine. This study tested a dedicated reconstruction algorithm for its ability to reduce camera-specific intersubject variability in [(123)I]FP-CIT SPECT. The secondary aim was to evaluate binding in whole brain (excluding striatum) as a reference for quantitative analysis. Of 73 healthy subjects from the European Normal Control Database of [(123)I]FP-CIT recruited at six centres, 70 aged between 20 and 82years were included. SPECT images were reconstructed using the QSPECT software package which provides fully automated detection of the outer contour of the head, camera-specific correction for scatter and septal penetration by transmission-dependent convolution subtraction, iterative OSEM reconstruction including attenuation correction, and camera-specific "to kBq/ml" calibration. LINK and HERMES reconstruction were used for head-to-head comparison. The specific striatal [(123)I]FP-CIT binding ratio (SBR) was computed using the Southampton method with binding in the whole brain, occipital cortex or cerebellum as the reference. The correlation between SBR and age was used as the primary quality measure. The fraction of SBR variability explained by age was highest (1) with QSPECT, independently of the reference region, and (2) with whole brain as the reference, independently of the reconstruction algorithm. QSPECT reconstruction appears to be useful for reduction of camera-specific intersubject variability of [(123)I]FP-CIT SPECT in multisite and single-site multicamera settings. Whole brain excluding striatal binding as the reference provides more stable quantitative estimates than occipital or cerebellar binding.

Three-dimensional SPECT reconstruction with transmission-dependent scatter correction

The quality of single-photon emission computed tomography (SPECT) imaging is hampered by attenuation, collimator blurring, and scatter. Correction for all of these three factors is required for accurate reconstruction, but unfortunately, reconstruction-based compensation often leads to clinically unacceptable long reconstruction times. Especially, efficient scatter correction has proved to be difficult to achieve. The objective of this article was to extend the well-known transmission-dependent convolution subtraction (TDCS) scatter-correction approach into a rapid reconstruction-based scatter-compensation method and to include it into a fast 3D reconstruction algorithm with attenuation and collimator-blurring corrections. Ordered subsets expectation maximization algorithm with attenuation, collimator blurring, and accelerated transmission-dependent scatter compensation were implemented. The new reconstruction method was compared with TDCS-based scatter correction and with one other transmission-dependent scatter-correction method using Monte Carlo simulated projection data of (99m)Tc-ECD and (123)I-FP-CIT brain studies. The new reconstruction-based scatter compensation outperformed the other two scatter-correction methods in terms of quantitative accuracy and contrast measured with normalized mean-squared error, gray-to-white matter and striatum-to-background ratios, and also in visual quality. Highest accuracy was achieved when all the corrections (i.e., attenuation, collimator blurring, and scatter) were applied. The developed 3D reconstruction algorithm with transmission-dependent scatter compensation is a promising alternative to accurate and efficient SPECT reconstruction.

Absolute quantitation of myocardial blood flow with 201Tl and dynamic SPECT in canine: optimisation and validation of kinetic modelling

201Tl has been extensively used for myocardial perfusion and viability assessment. Unlike 99mTc-labelled agents, such as 99mTc-sestamibi and 99mTc-tetrofosmine, the regional concentration of 201Tl varies with time. This study is intended to validate a kinetic modelling approach for in vivo quantitative estimation of regional myocardial blood flow (MBF) and volume of distribution of 201Tl using dynamic SPECT. Dynamic SPECT was carried out on 20 normal canines after the intravenous administration of 201Tl using a commercial SPECT system. Seven animals were studied at rest, nine during adenosine infusion, and four after beta-blocker administration. Quantitative images were reconstructed with a previously validated technique, employing OS-EM with attenuation-correction, and transmission-dependent convolution subtraction scatter correction. Measured regional time-activity curves in myocardial segments were fitted to two- and three-compartment models. Regional MBF was defined as the influx rate constant (K(1)) with corrections for the partial volume effect, haematocrit and limited first-pass extraction fraction, and was compared with that determined from radio-labelled microspheres experiments. Regional time-activity curves responded well to pharmacological stress. Quantitative MBF values were higher with adenosine and decreased after beta-blocker compared to a resting condition. MBFs obtained with SPECT (MBF(SPECT)) correlated well with the MBF values obtained by the radio-labelled microspheres (MBF(MS)) (MBF(SPECT) = -0.067 + 1.042 x MBF(MS), p < 0.001). The three-compartment model provided better fit than the two-compartment model, but the difference in MBF values between the two methods was small and could be accounted for with a simple linear regression. Absolute quantitation of regional MBF, for a wide physiological flow range, appears to be feasible using 201Tl and dynamic SPECT.

A Comparative Evaluation of Scatter Correction Techniques in High-Resolution Detectors Based on PSPMTs and Scintillator Arrays

Single photon emission computed tomography images suffer from low contrast as a result of photon scatter. The standard method for excluding the scatter component in pixelized scintillators is the application of an energy window around the central photopeak channel of each crystal cell, but small angle scattered photons still appear in the photopeak window, and they are included in the reconstructed images. A number of scatter correction techniques have been proposed in order to estimate the scatter component, but they have not yet been applied in pixelized scintillators, where most groups use the standard one-photopeak window for scatter correction. In this paper, the author have assessed three subtraction techniques that use a different approach in order to calculate the scatter component and subtract it from the photopeak image: the dual energy window subtraction technique, the convolution subtraction technique, and a deconvolution technique. All these techniques are compared to the standard method

Correction for scatter and septal penetration using convolution subtraction methods and model-based compensation in 123I brain SPECT imaging—a Monte Carlo study

Scatter and septal penetration deteriorate contrast and quantitative accuracy in single photon emission computed tomography (SPECT). In this study four different correction techniques for scatter and septal penetration are evaluated for 123I brain SPECT. One of the methods is a form of model-based compensation which uses the effective source scatter estimation (ESSE) for modelling scatter, and collimator–detector response (CDR) including both geometric and penetration components. The other methods, which operate on the 2D projection images, are convolution scatter subtraction (CSS) and two versions of transmission dependent convolution subtraction (TDCS), one of them proposed by us. This method uses CSS for correction for septal penetration, with a separate kernel, and TDCS for scatter correction. The corrections are evaluated for a dopamine transporter (DAT) study and a study of the regional cerebral blood flow (rCBF), performed with 123I. The images are produced using a recently developed Monte Carlo collimator routine added to the program SIMIND which can include interactions in the collimator. The results show that the method included in the iterative reconstruction is preferable to the other methods and that the new TDCS version gives better results compared with the other 2D methods.

Clinical evaluation of 2D versus 3D whole-body PET image quality using a dedicated BGO PET scanner

Three-dimensional positron emission tomography (3D PET) results in higher system sensitivity, with an associated increase in the detection of scatter and random coincidences. The objective of this work was to compare, from a clinical perspective, 3D and two-dimensional (2D) acquisitions in terms of whole-body (WB) PET image quality with a dedicated BGO PET system. 2D and 3D WB emission acquisitions were carried out in 70 patients. Variable acquisition parameters in terms of time of emission acquisition per axial field of view (aFOV) and slice overlap between sequential aFOVs were used during the 3D acquisitions. 3D and 2D images were reconstructed using FORE+WLS and OSEM respectively. Scatter correction was performed by convolution subtraction and a model-based scatter correction in 2D and 3D respectively. All WB images were attenuation corrected using segmented transmission scans. Images were blindly assessed by three observers for the presence of artefacts, confidence in lesion detection and overall image quality using a scoring system. Statistically significant differences between 2D and 3D image quality were only obtained for 3D emission acquisitions of 3 min. No statistically significant differences were observed for image artefacts or lesion detectability scores. Image quality correlated significantly with patient weight for both modes of operation. Finally, no differences were seen in image artefact scores for the different axial slice overlaps considered, suggesting the use of five slice overlaps in 3D WB acquisitions. 3D WB imaging using a dedicated BGO-based PET scanner offers similar image quality to that obtained in 2D considering similar overall times of acquisitions.

Transmission-dependent convolution subtraction of /sup 99m/Tc-HMPAO rCBF SPECT-a Monte Carlo study

Transmission-dependent convolution subtraction has been shown to be useful when correcting for malpositioned scattered events in single photon emission computed tomography (SPECT). The method is based on convolution subtraction but includes a matrix of scatter fractions instead of a global scatter fraction. In this study, this method is evaluated for regional cerebral blood flow SPECT with /sup 99m/Tc-hexamethyl propylene-amine oxime (HMPAO) by using Monte Carlo simulations. Different geometries for generating the scatter fractions as a function of the attenuation path length are studied and compared. The most optimal value of the exponential describing the falloff of the monoexponential scatter kernel is determined for each geometry. The method is also compared with convolution subtraction with a global scatter fraction. It is shown that the most optimal of the tested geometries is a homogeneous activity distribution. A scatter kernel with an exponential of 0.15 pixel/sup -1/ is most optimal for this geometry. A comparison with convolution subtraction shows that transmission-dependent convolution subtraction can give more accurate results if used with optimal parameters.

Optimization of the width of the photopeak energy window in the TDCS technique for scatter correction in quantitative SPECT

Transmission-dependent convolution subtraction (TDCS) is a promising technique in quantitative SPECT for subtracting the scatter components from emission images. Usually, a 20% photopeak energy window is used in SPECT acquisitions. To date, no investigation has assessed the effects of energy windows in the subtraction of scatter components from emission images with the TDCS technique. To evaluate the energy dependence of the TDCS technique, we analyzed photopeak energy windows of 5%, 10%, 15%, 20%, 25%, 30%, and 35% using /sup 99m/Tc radionuclide. The scatter fractions for a point source placed in a water phantom were estimated using the triple-energy-window (TEW) technique. These estimates were used to establish the parameters of the TDCS equation as a function of transmission. The estimated parameters were applied to the TDCS technique to subtract the scatter components from data of corresponding energy windows using nonuniform and uniform phantoms. All data were reconstructed with the OSEM algorithm. The energy window dependence of the TDCS technique was verified by comparing the coefficients of variance (COV) of the data from the uniform phantom of each energy window. Ten Poisson deviates were generated from each projection's data from the uniform phantom. The nonuniform phantom results showed that the estimated parameters effectively estimated the scatter component from the projection image for each energy window. The COV for the 5%, 10%, 15%, 20%, 25%, 30%, and 35% windows were 2.10%, 1.76%, 1.44 %, 1.39%, 1.34%, 1.37%, and 1.61%, respectively. In the TDCS technique, any photopeak energy window from 15% to 30% may work almost as well as judged by the COV measured for a uniform phantom in this study. The best window by that measure was the 25% window.

Non-stationary convolution subtraction scatter correction with a dual-exponential scatter kernel for the Hamamatsu SHR-7700 animal PET scanner

A spatially variant convolution subtraction scatter correction was developed for a Hamamatsu SHR-7700 animal PET scanner. This scanner, with retractable septa and a gantry that can be tilted 90°, was designed for studies of conscious monkeys. The implemented dual-exponential scatter kernel takes into account both radiation scattered inside the object and radiation scattered in gantry and detectors. This is necessary because of the relatively large contribution of gantry and detector scatter in this scanner. The correction is used for scatter correction of emission as well as transmission data. Transmission scatter correction using the dual-exponential kernel leads to a measured attenuation coefficient of 0.096 cm−1 in water, compared to 0.089 cm−1 without scatter correction. Scatter correction on both emission and transmission data resulted in a residual correction error of 2.1% in water, as well as improved image contrast and hot spot quantification.

Scatter-to-primary based scatter fractions for transmission-dependent convolution subtraction of SPECT images

In single photon emission computed tomography (SPECT), transmission-dependent convolution subtraction has been shown to be useful when correcting for scattered events. The method is based on convolution subtraction, but includes a matrix of scatter fractions instead of a global scatter fraction. The method can be extended to iteratively improve the scatter estimate, but in this note we show that this requires a modification of the theory to use scatter-to-total scatter fractions for the first iteration only and scatter-to-primary fractions thereafter. To demonstrate this, scatter correction is performed on a Monte Carlo simulated image of a point source of activity in water. The modification of the theory is compared to corrections where the scatter fractions are based on the scatter-to-total ratio, using one and ten iterations. The resulting ratios of subtracted to original counts are compared to the true scatter-to-total ratio of the simulation and the most accurate result is found for our modification of the theory.

A hybrid scatter correction for 3D PET based on an estimation of the distribution of unscattered coincidences: implementation on the ECAT EXACT HR+

We implemented a hybrid scatter-correction method for 3D PET that combines two scatter-correction methods in a complementary way. The implemented scheme uses a method based on the discrimination of the energy of events (the estimation of trues method (ETM)) and an auxiliary method (the single scatter simulation method (SSS1) or the convolution–subtraction method (CONV)) in an attempt to increase the accuracy of the correction over a wider range of acquisitions. The ETM takes into account the scatter from outside the field-of-view (FOV), which is not estimated with the auxiliary method. On the other hand, the auxiliary method accounts for events that have scattered with small angles, which have an energy that cannot be discriminated from that of unscattered events using the ETM.The ETM uses the data acquired in an upper energy window above the photopeak (550–650 keV) to obtain a noisy estimate of the unscattered events in the standard window (350–650 keV). Our implementation uses the auxiliary method to correct the residual scatter in the upper window. After appropriate scaling, the upper window data are subtracted from the total coincidences acquired in the standard window, resulting in the final scatter estimate, after smoothing.In this work we compare the hybrid method with the corrections used by default in the 2D and 3D modes of the ECAT EXACT HR+ using phantom measurements. Generally, the contrast was better with the hybrid method, although the relative errors of quantification were similar. We conclude that hybrid techniques such as the one implemented in this work can provide an accurate, general-purpose and practical way to correct the scatter in 3D PET, taking into account the scatter from outside the FOV.

Impact of attenuation and scatter correction in SPECT for quantification of cerebral blood flow using /sup 99m/Tc-Ethyl Cysteinate Dimer

We investigated the effects of attenuation correction and scatter correction on rCBF values with /sup 99m/Tc-ECD SPECT imaging. Scans were performed on seven subjects, in the presence of /sup 99m/Tc-ECD. Quantitative K/sub 1/ images were computed using the reconstructed images and the input function obtained with the frequent arterial blood sampling method. The images were reconstructed by the ordered subset expectation maximization (OSEM) reconstruction in which uniform and segmented /spl mu/ maps were used for attenuation correction with and without scatter correction. The transmission-dependent convolution subtraction technique was utilized for scatter correction. Segmented and uniform /spl mu/ maps were generated from magnetic resonance (MR) images. We also produced uniform /spl mu/ maps using ECD images obtained at various threshold levels and /spl mu/ values (0.11, 0.15, and 0.172 cm/sup -1/). Scatter correction improved the image contrast dramatically. There were no significant differences between K/sub 1/ images with attenuation and scatter corrections assuming a uniform /spl mu/ map (not 0.15 but 0.172 cm/sup -1/) and those corrected with segmented /spl mu/ maps for most regions. However, in the former images, values were overestimated for deep structures (e.g., overestimation of 9.5% in the striatum and 7.3% in the central semi oval). This small but significant error was also observed in phantom studies and Monte Carlo simulations. We show that the overestimation using uniform /spl mu/ maps is due to the weight of the path length in the bone. Absolute K/sub 1/ values were sensitive to the threshold level when the edge of the brain was determined from the ECD images, but the variation of the estimated K/sub 1/ was /spl plusmn/9.0% when the optimal threshold level was selected. This study suggests that the use of uniform attenuation /spl mu/ maps provides reasonable accuracy, despite a small but significant error in deep structure regions, and that uniform /spl mu/ maps may be provided from the emission data alone in this patient population.

SPECT collimator dependency of scatter and validation of transmission-dependent scatter compensation methodologies

Scatter correction is important for absolute quantification in SPECT. Among the approaches taken to achieve scatter correction, some methods using transmission information for scatter estimation require the scatter models based on the physical characteristics of the scattered photons. The influence of the SPECT collimator on the characteristics of the scattered photon, however, has not been well studied yet. Here, the authors investigate the relationship between scatter and collimator design for the use of the transmission-dependent convolution subtraction (TDCS) method. Monte Carlo simulations with various collimator acceptance angles and experiments with four collimators of two SPECT cameras were performed to obtain the scatter fraction (SF) and the line-spread function (LSF) of Tc-99m. The experiments with I-123 also used six collimators of three SPECT systems to obtain the SF and the LSF. In the simulations, the SF of Tc-99m did not change with the collimator acceptance angles. The LSFs measured for Tc-99m showed an excellent agreement between collimators. In the experiments for I-123, the SF was practically the same for the six collimators, although a small difference could be observed due to the septal penetration. Also, the shape of the LSF was very similar between the collimators, and the counts in the background region of reconstructed images did not differ when using different sets of TDCS parameters. These results reveal that scatter is independent of SPECT collimator design for Tc-99m even I-123, and that the TDCS could be applicable to scatter correction of I-123 using a unique collimator-independent set of parameters.

Design and experimental validation of a quantitative myocardial /sup 201/Tl SPECT system

We have developed a quantitative SPECT system, and evaluated its potential for quantitative assessment of bio-physiological functions in the myocardium particularly with /sup 201/Tl. Our approach included development of a transmission system that provides accurate attenuation /spl mu/ maps, and implementation of ordered-subset EM reconstruction with transmission data based attenuation correction in addition to scatter correction using the transmission-dependent convolution subtraction (TDCS) technique. The transmission system was designed using Monte Carlo simulation to minimize the scatter in the transmission projection data while keeping loss of sensitivity minimal, and was attached to an opposing 2-head gamma camera fitted with parallel beam collimators. Observed /spl mu/ values agreed quantitatively well with the theoretical expected values in both phantoms and human thorax. Phantom experiments with /sup 201/Tl also demonstrated that, with both corrections for attenuation and scatter, observed images were directly proportional to the actual radioactivity distribution for various phantom geometries. Attenuation correction without scatter correction improved images in deep structure, but resulted in significant artifacts in the chest phantom in addition to dependency of observed radioactivity concentrations on the diameter of cylindrical phantoms. Absolute quantitation of bio-physiological functions, which is well established in PET, is shown to be feasible using SPECT, if both quantitative attenuation and scatter corrections are employed.

A phantom study evaluating the quantitative aspect of 3D PET imaging of the brain

Phantom studies are used to develop a reliable quantitative data processing protocol for 3D PET brain scanning for conditions typically encountered in FDG and neuroreceptor brain imaging. These protocols often span several half-lives of the injected radiotracer thus resulting in a greatly varying statistical content of the acquired data over the study duration. Detector normalization, scatter correction and their interplay over a wide range of statistical content of acquired data were evaluated. Overall sensitivity calibration factors were determined after all other quantification corrections were applied to the data. The result is an optimum data processing protocol that includes an iterative convolution subtraction scatter correction method, a normalization procedure that takes into account the geometric properties of the scanner and a region of interest based calibration procedure, applied in this order. This protocol yields a 3D PET quantification accuracy within approximately 3% of independently measured concentration values for scanning conditions that include variation in the number of acquired counts from one million to several hundred millions and variation in size and shape from a 20 cm diameter phantom to a tapered phantom with minimum cross section of . This performance is comparable with that of the 2D acquisition mode.

Monte Carlo evaluation of accuracy and noise properties of two scatter correction methods for /sup 201/Tl cardiac SPECT

Two independent scatter correction techniques, transmission dependent convolution subtraction (TDCS) and triple-energy window (TEW) method, were evaluated in terms of quantitative accuracy and noise properties using Monte Carlo simulation (EGS4). Emission projections (primary, scatter and scatter plus primary) were simulated for three numerical phantoms for /sup 201/Tl. Data were reconstructed with ordered-subset EM algorithm including noise-less transmission data based attenuation correction. Accuracy of TDCS and TEW scatter corrections were assessed by comparison with simulated true primary data. The uniform cylindrical phantom simulation demonstrated better quantitative accuracy with TDCS than with TEW (-2.0% vs. 16.7%) and better S/N (6.48 vs. 5.05). A uniform ring myocardial phantom simulation demonstrated better homogeneity with TDCS than TEW in the myocardium; i.e., anterior-to-posterior wall count ratios were 0.99 and 0.76 with TDCS and TEW, respectively. For the MCAT phantom, TDCS provided good visual and quantitative agreement with simulated true primary image without noticeably increasing the noise after scatter correction. Overall TDCS proved to be more accurate and less noisy than TEW, facilitating quantitative assessment of physiological functions with SPECT.

Monte Carlo and experimental evaluation of accuracy and noise properties of two scatter correction methods for SPECT

Scatter correction is a prerequisite for quantitative SPECT, but potentially increases noise. Monte Carlo simulations (EGS4) and physical phantom measurements were used to compare accuracy and noise properties of two scatter correction techniques: the triple-energy window (TEW), and the transmission dependent convolution subtraction (TDCS) techniques. Two scatter functions were investigated for TDCS: (i) the originally proposed mono-exponential function and (ii) an exponential plus Gaussian scatter function demonstrated to be superior from our Monte Carlo simulations. Signal to noise ratio (S/N) and accuracy were investigated in cylindrical phantoms and a chest phantom. Results from each method were compared to the true primary counts (simulations), or known activity concentrations (phantom studies). was used in all cases. The optimized method overall performed best, with an accuracy of better than 4% for all simulations and physical phantom studies. Maximum errors for TEW and of -30 and -22%, respectively, were observed in the heart chamber of the simulated chest phantom. TEW had the worst S/N ratio of the three techniques. The S/N ratios of the two TDCS methods were similar and only slightly lower than those of simulated true primary data. Thus, accurate quantitation can be obtained with , with a relatively small reduction in S/N ratio.

Transmission-based scatter correction of 180 degrees myocardial single-photon emission tomographic studies.

Meaningful comparison of single-photon emission tomographic (SPET) reconstructions for data acquired over 180 degrees or 360 degrees can only be performed if both attenuation and scatter correction are applied. Convolution subtraction has appeal as a practical method for scatter correction; however, it is limited to data acquired over 360 degrees. A new algorithm is proposed which can be applied equally well to data acquired over 180 degrees or 360 degrees. The method involves estimating scatter based on knowledge of reconstructed transmission data in combination with a reconstructed estimate of the activity distribution, obtained using attenuation correction with broad beam attenuation coefficients. Processing is implemented for planes of activity parallel to the projection images for which a simplified model for the scatter distribution may be applied, based on the measured attenuation. The appropriate broad beam (effective) attenuation coefficients were determined by considering the scatter buildup equation. It was demonstrated that narrow beam attenuation coefficients should be scaled by 0.75 and 0.65 to provide broad beam attenuation coefficients for technetium-99m and thallium-201 respectively. Using a thorax phantom, quantitative accuracy of the new algorithm was compared with conventional transmission-based convolution subtraction (TDCS) for 360 degrees data. Similar heart to lung contrasts were achieved and correction of 180 degrees data yielded a 10.4% error for cardiac activity compared to 5.2% for TDCS. Contrast for myocardium to ventricular cavity was similarly good for scatter-corrected 180 degrees and 360 degrees data, in contrast to attenuation-corrected data, where contrast was significantly reduced. The new algorithm provides a practical method for correction of scatter applicable to 180 degrees myocardial SPET.

Source distribution dependent scatter correction for PVI

Source distribution dependent scatter correction methods which incorporate different amounts of information about the source position and material distribution are developed and tested. The techniques use image to projection integral transformation, incorporating varying degrees of information on the distribution of scattering material, or convolution subtraction methods, with some information about the scattering material included in one of the convolution methods. To test the techniques, they are applied to data generated by Monte Carlo simulations which use geometric shapes or a voxelized density map to model the scattering material. Source position and material distribution are found to have some effect on scatter correction. An image to projection method which incorporates a density map produces accurate scatter correction but is computationally expensive. Simpler methods, both image to projection and convolution, can also provide effective scatter correction.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>