Ordered Subset Expectation Maximization Research Articles

Ordered Subset Expectation Maximum Algorithms Based on Symmetric Structure for Image Reconstruction

In this paper, we propose the symmetric structure of the reconstructed points discretization model to partition and order the subsets of Ordered Subset Expectation Maximum (OSEM) algorithms for image reconstruction and then simplify the calculation of the projection coefficient matrix while satisfying the balancing properties of subsets. The reconstructed points discretization model was utilized to describe the forward and inverse relationships between the reconstructed points and the projection data according to the distance from the point to the ray rather than the intersection length between the square pixel and the ray. This discretization model provides new approaches for improving and constructing the reconstruction algorithms on the basis of the geometry of the model. The experimental results show that the OSEM algorithms based on the reconstructed points discretization model and its geometric symmetry structure can effectively improve the imaging speed and the imaging precision.

Impact of a Bayesian penalized likelihood reconstruction algorithm on image quality in novel digital PET/CT: clinical implications for the assessment of lung tumors

BackgroundThe aim of this study was to evaluate and compare PET image reconstruction algorithms on novel digital silicon photomultiplier PET/CT in patients with newly diagnosed and histopathologically confirmed lung cancer. A total of 45 patients undergoing 18F-FDG PET/CT for initial lung cancer staging were included. PET images were reconstructed using ordered subset expectation maximization (OSEM) with time-of-flight and point spread function modelling as well as Bayesian penalized likelihood reconstruction algorithm (BSREM) with different β-values yielding a total of 7 datasets per patient. Subjective and objective image assessment with all image datasets was carried out, including subgroup analyses for patients with high dose (> 2.0 MBq/kg) and low dose (≤ 2.0 MBq/kg) of 18F-FDG injection regimen.ResultsSubjective image quality ratings were significantly different among all different reconstruction algorithms as well as among BSREM using different β-values only (both p < 0.001). BSREM with a β-value of 600 was assigned the highest score for general image quality, image sharpness, and lesion conspicuity. BSREM reconstructions resulted in higher SUVmax of lung tumors compared to OSEM of up to + 28.0% (p < 0.001). BSREM reconstruction resulted in higher signal-/ and contrast-to-background ratios of lung tumor and higher signal-/ and contrast-to-noise ratio compared to OSEM up to a β-value of 800. Lower β-values (BSREM450) resulted in the best image quality for high dose 18F-FDG injections, whereas higher β-values (BSREM600) lead to the best image quality in low dose 18F-FDG PET/CT (p < 0.05).ConclusionsBSREM reconstruction algorithm used in digital detector PET leads to significant increases of lung tumor SUVmax, signal-to-background ratio, and signal-to-noise ratio, which translates into a higher image quality, tumor conspicuity, and image sharpness.

Effect of sphere size and reconstruction method on 99Tcm-pertechnetate SPECT/CT imaging: a phantom study

Objective To validate the quantitative accuracy of 99Tcm-pertechnetate(99TcmO4-)SPECT/CT imaging and its influencing factors (sphere size, radioactive concentration, reconstruction methods). Methods National Electrical Manufacturers Association (NEMA) International Electrotechnical Commission (IEC) body phantom with 99TcmO4- solution filled in 6 spheres (diameter: 10, 13, 17, 22, 28, 37 mm) and 1 cylinder (target/background ratio: 10∶1) was used to assess the accuracy of uptake quantification. SPECT/CT was used for imaging data acquisition, and volume of interest (VOI) was drawn after the images being corrected and reconstructed. The radioactive concentrations of 6 spheres were measured and compared with the true radioactive concentrations, and quantitative accuracy of each sphere was calculated. The experiment was repeated for 12 times with the same method and different radioactive concentrations. Ordered-subsets expectation maximization (OSEM) reconstruction was performed with 2 or 5 iterations to process imaging data. CT VOI and nuclear medicine (NM) threshold methods were used for the delineation of VOI. Accuracy was compared by paired t test. Results The NM thresholds to get calculated volume close to the true volume were determined to be 40%, 40%, 50%, 70%, 80%, 90% for 6 spheres with diameters of 37, 28, 22, 17, 13, 10 mm, respectively. Comparing 2 methods of VOI delineation (CT VOI and NM threshold), only the accuracy for 17 mm sphere was significantly different (t=5.89, P<0.01). Five iterations group showed higher accuracy than 2 iterations group (t values: 3.19-6.92, all P<0.05). The average accuracies under 12 different radioactive concentrations were 102%, 95%, 80%, 66%, 27%, 20%, respectively, for spheres in order from large size to small size. Conclusion The sphere volume and reconstruction method have effect on quantitative accuracy of 99TcmO4- SPECT/CT imaging, and spheres smaller than 17 mm is also significantly affected by partial volume effect. Key words: Tomography, emission-computed, single-photon; Tomography, X-ray computed; Models, structural; Image processing, computer-assisted; Sodium pertechnetate Tc 99m

An improved OSEM iterative reconstruction algorithm for transmission tomographic gamma scanning

As one of the most advanced non-destructive analytical techniques for nuclear wastes, tomographic Gamma Scanning (TGS) is able to give accurate quantitative and qualitative measurements of nuclear waste barrels. OSEM (Ordered Subsets Expectation Maximization) has been used in transmission TGS image reconstruction on account of its good reconstruction quality and ideal convergence rate. In this paper, an improved method—NMO-OSEM (Non-minimization optimization OSEM) was proposed, it's an iterative algorithm with corrected initial values optimized by non-minimization optimization method. To evaluate its performance, a TGS system is used to perform transmission measurements on barreled nuclear wastes. The results show: ①Compared with the reconstructed images by traditional OSEM under 6 transmission energies (122 keV, 344 keV, 779 keV, 964 keV, 1112 keV, 1408 keV), the improved NMO-OSEM has a great advantage in reducing the artifacts and effectively improving the quality of the reconstructed images. ②the attenuation coefficients values of 72 voxels under 3 emission energies (662 keV, 1173 keV and 1332 keV) reconstructed by the proposed algorithm are more accurate (range of error: 0.22%–13.83%) than reconstructed by traditional OSEM (range of error: 1.31%–32.21%), which proves this method has more stable reconstruction precision and could be regarded as an ideal option for real applications.

Evaluation of MR-based PET Images with a Homogeneous μmap by Using Integrated PET/MR

This study intends to obtain magnetic resonance (MR)-based positron emission tomography (PET) images with a relatively uniform μmap by using a National Electrical Manufacturers Association (NEMA) International Electrotechnical Commission (IEC) body phantom and to evaluate the PET phantom images by applying various reconstruction parameters in accordance with the protocol of the NEMA. In order to obtain a uniform MR-based μmap, we set the initial voltage of the RF transmitter adjustment algorithm to 75 V, and we obtained the attenuation-corrected PET image by applying the two-point vibe Dixon MR sequence. The images reconstructed using the 3D ordered subset expectation maximization (OSEM) algorithm with the point spread function (PSF) showed the highest percentage of contrast recovery and the lowest percentage background variability compared to the other 3D reconstruction algorithms like the Filtered backprojection (FBP), 3D OSEM, and 3D OSEM with PSF algorithms. The 3D OSEM with PSF reconstruction algorithm is confirmed to be effective for maintaining the optimum image quality.

Effects of the attenuation correction and reconstruction method parameters on conventional cardiac dynamic SPECT.

Nuclear cardiology has not witnessed development of new tracers or hardware for many years. Hence there is a need for the development of improvised techniques. Dynamic cardiac single photon emission computed tomography (SPECT) is one such technique that has a potential to overcome the limitations of conventional myocardial SPECT including the absolute quantification of blood flow. The main goal of this study is to evaluate the effect of attenuation correction (AC) on estimation of the washout parameters extracted from dynamic SPECT using a conventional protocol. The effect of the postprocessing on quantitative evaluation of dynamic SPECT is also assessed.A physical phantom was employed to physically simulate the dynamic behavior of a heart in the thorax. Using a dual detector SPECT system, 180° tomographic data in every 90 seconds were acquired. The SPECT data were reconstructed using ordered subset expectation maximization (OSEM) method while different iterations and a Butterworth filter with different cut-off frequencies were applied. Estimated washout parameter of the time activity curves (TACs) was compared with applying AC or without it.Results show that AC can improve the bias of computed washout parameter in normal regions (average bias reduction in normal ROI: 7%). Moreover, the postreconstruction filtering and reducing the number of iterations in reconstructing phase can reduce the variance of the computed washout values in normal regions (from 3.99% for cut-off frequency 0.5 cycle/cm and 32 times update in OSEM to 2.05% for cut-off frequency 0.35 cycle/cm and 16 times update in OSEM). They also reduce the actual size of the defect region (13% reduction in defect extent for above change in reconstruction parameters).According to the results, the AC and postprocessing filtration can directly affect the standard deviation of washout value acquired by cardiac dynamic SPECT. These parameters also showed a direct effect on the defect extent in final results. The study showed that the AC may partly improve the bias of calculated normal washout value. The effect of attenuation correction on the defective washout value could not be answered comprehensively in this paper.

Protocols for Harmonized Quantification and Noise Reduction in Low-Dose Oncologic 18F-FDG PET/CT Imaging.

Oncologic 18F-FDG PET/CT acquisition and reconstruction protocols need to be optimized for both quantitative and detection tasks. To date, most studies have focused on either quantification or noise, leading to quantitative harmonization guidelines or appropriate noise levels. We developed and evaluated protocols that provide harmonized quantitation with optimal amounts of noise as a function of acquisition parameters and body mass. Methods: Multiple image acquisitions (n = 17) of the International Electrotechnical Commission/National Electrical Manufacturers Association PET image-quality phantom were performed with variable counting statistics. Phantom images were reconstructed with 3-dimensional ordered-subset expectation maximization (OSEM3D) and point-spread function (PSF) for harmonized quantification of the contrast recovery coefficient of the maximum pixel value (CRC max ). The lowest counting statistics that resulted in compliance with European Association of Nuclear Medicine recommendations for CRC max and CRC max variability were used as optimization metrics. Image noise in the liver of 48 typical oncologic 18F-FDG PET/CT studies was analyzed with OSEM3D and PSF harmonized reconstructions. We also evaluated 164 additional 18F-FDG PET/CT reconstructed list-mode images to derive analytic expressions that predict image quality and noise variability. Phantom-to-subject translational analysis was used to derive optimized acquisition and reconstruction protocols. Results: For harmonized quantitation levels, PSF reconstructions yielded decreased noise and lower CRC max variability than regular OSEM3D reconstructions, suggesting they could enable a decreased activity regimen for matched performance. Conclusion: PSF reconstruction with a 7-mm postprocessing filter can provide harmonized quantification performance and acceptable image noise levels with injected activity, duration, and mass settings using a 260 MBq⋅s/kg acquisition parameter at scan time. Similarly, OSEM3D with a 5-mm postprocessing filter can provide similar performance with 401 MBq⋅s/kg.

OA165] Optimization of quantitative 99MTC-MAA SPECT/CT imaging for 90Y RADIOEMBOLIZATION: A 3D-printed phantom study

Purpose Radioembolization (RE) with 90Y-microspheres has gained widespread acceptance as a safe and effective technique for treating liver malignancies. Accurate quantification in RE is a key step in treatment planning optimization and it is becoming a pressing issue in the light of the Directive 2013/59/EURATOM, stating that individual dose planning must be enforced in legislation by EU member states by February 2018. The aim of this study was to develop a SPECT/CT imaging protocol for quantitative imaging optimization in RE based on a novel anthropomorphic 3D-printed phantom. Methods and materials A SPECT/CT system provided with a novel software for imaging quantification was used to assess the quantitative accuracy of 99mTc-MAA SPECT imaging for treatment planning in RE. Gamma camera calibration factors and recovery coefficients were determined performing preliminary SPECT/CT acquisitions of a homogeneous and a NEMA/IEC phantom, respectively. The quantification procedure was then validated using a 3D-printed anthropomorphic phantom provided with a fillable liver section and multiple inserts. Raw data reconstruction was performed using two independent software: i) the built-in xSPECT package, using both the Ordered Subset Expectation–Maximization (OSEM) and the Ordered Subset Conjugated Gradient (OSCG) algorithm; ii) the commercially available QSPECT software based on OSEM method. All reconstructed images were manually fused and appropriate regions of interest (ROIs) were drawn on the MIM 6.1.7 system. Results The measured activity concentration is consistent within 3% with the expected activity concentration values. This is in keeping with the overall accuracy ( 10%) stated by the manufacturer. No substantial differences were reported between xSPECT with OSCG or OSEM in terms of relative activity. However, a 20% difference was found between ROIs extracted from images reconstructed with xSPECT and QSPECT using OSEM. Several artifacts were visible in the QSPECT reconstruction, likely due to the limited area used for attenuation and scatter corrections within the software. Conclusions The investigated protocol based on the innovative xSPECT tool allows an accurate absolute quantification within 3%. QSPECT software provides the poorest results with several artifacts as well as intrinsic dishomogeneities.

P088] Optimization of yttrium-90 bremstrahlung imaging for evaluation of microspheres post-therapeutic liver radioembolization distribution

Purpose Post-therapy imaging of 90Y-Bremsstrahlung x-rays using SPECT, although 90Y is not the best isotope for imaging, has the potential of providing a reliable activity distribution of the 90Y-microspheres inside the liver. In literature, several image acquisition parameters and subsequent different reconstruction protocols can be found. With the aim of obtaining an optimized 90Y-Bremsstrahlung SPECT image, two reconstruction methods, Filtered-Backprojection (FBP) and Ordered-Subsets-Expectation-Maximization (OSEM), were applied both to phantoms and post-therapeutic images using several reconstruction parameters. Methods A 90Y-Bremsstrahlung image of a water filled Jaszczak phantom was acquired. A line source containing 222 MBq of 90Y and uniformly distributed activity was placed inside, along the longitudinal axis. The image was acquired using a gamma camera (e.cam, Siemens) equipped with a MEGP parallel-hole collimator and an energy window of 136–184 keV. By modifying the order and critical frequency (FBP) and number of iterations, subsets and Gaussian-filter FWHM (OSEM), SPECT resolution, image contrast, noise and counts per voxel ( C voxel ) were obtained from the phantom images. 24 different reconstruction protocols (10 FBP and 14 OSEM) were used and compared. Results The results of the reconstruction methodologies evaluated showed that in the FBP algorithm, improving the image resolution implies a loss of contrast and an increase of the noise. Regarding the OSEM algorithm, an increase of the noise level occurs with a higher number of iterations, although both contrast and image resolution improved. In addition, the C voxel also depends on the reconstruction algorithm parameters. It was showed that the C voxel is strongly connected with noise, and that increasing the noise also increases the C voxel . Conclusion Although in the past FBP algorithms were thoroughly used as standard methods for SPECT reconstruction, nowadays, iterative methods can be used in an acceptable reconstruction time due to an increase in computer performance and, often, present better quantitative and qualitative results. Still, optimization is required to provide the best trade-off between noise/contrast and image resolution. Nevertheless, the availability of FBP reconstruction algorithms is still desirable due to the higher number of parameters (e.g. different filters).

I179] SPECT/CT quantitative reconstruction techniques

The last years, the clinical interest in quantitative SPECT/CT imaging is increasing. Especially in the rapidly growing field of radionuclide therapy, quantitative imaging is essential to provide nuclear medicine physicians with accurate dosimetric data. As a result, manufacturers now focus on producing SPECT/CT systems that are intrinsically quantitative. SPECT quantification is more challenging than PET quantification. In fact, the correction for photon absorption is relatively straightforward to apply in PET. In addition, in SPECT, we need to consider a variety of radionuclides with different photon energies and energy windows, different collimators and the potential for drifts in performance due to the greater number of moving parts in a SPECT system. With SPECT/CT, the coregistered CT data provide accurate information on the density of the body tissues, which can be used in algorithms that correct for photons that have been Compton-scattered or attenuated within the body. State-of-the-art reconstruction techniques are based on statistical iterative methods such as the ordered-subset maximum-likelihood expectation maximization algorithm (OSEM). In this approach, the physical characteristics of the acquisition process can be completely modeled in the reconstruction. In this presentation, an overview of all factors affecting SPECT image quantification will be presented. The latest developments to achieve quantitative SPECT/CT reconstruction will be discussed, together with calibration issues to finally obtain radioactivity concentrations per unit volume (kBq/ml).

Quantitative performance and optimal regularization parameter in block sequential regularized expectation maximization reconstructions in clinical 68Ga-PSMA PET/MR

BackgroundIn contrast to ordered subset expectation maximization (OSEM), block sequential regularized expectation maximization (BSREM) positron emission tomography (PET) reconstruction algorithms can run until full convergence while controlling image quality and noise. Recent studies with BSREM and 18F-FDG PET reported higher signal-to-noise ratios and higher standardized uptake values (SUV). In this study, we investigate the optimal regularization parameter (β) for clinical 68Ga-PSMA PET/MR reconstructions in the pelvic region applying time-of-flight (TOF) BSREM in comparison to TOF OSEM.Two-minute emission data from the pelvic region of 25 patients who underwent 68Ga-PSMA PET/MR were retrospectively reconstructed. Reference OSEM reconstructions had 28 subsets and 2 iterations. BSREM reconstructions were performed with 15 β values between 150 and 1200. Regions of interest (ROIs) were drawn around lesions and in uniform background. Background SUVmean (average) and SUVstd (standard deviation), and lesion SUVmax (average of 5 hottest voxels) were calculated. Differences were analyzed using the Wilcoxon matched pairs signed-rank test.ResultsA total of 40 lesions were identified in the pelvic region. Background noise (SUVstd) and lesions SUVmax decreased with increasing β. Image reconstructions with β values lower than 400 have higher (p < 0.01) background noise, compared to the reference OSEM reconstructions, and are therefore less useful. Lesions with low activity on images reconstructed with β values higher than 600 have a lower (p < 0.05) SUVmax compared to the reference. These reconstructions are likely visually appealing due to the lower background noise, but the lower SUVmax could possibly render small low-uptake lesions invisible.ConclusionsIn our study, we showed that PET images reconstructed with TOF BSREM in combination with the 68Ga-PSMA tracer result in lower background noise and higher SUVmax values in lesions compared to TOF OSEM. Our study indicates that a β value between 400 and 550 might be the optimal compromise between high SUVmax and low background noise.

Characterization of Noise and Resolution for Quantitative 177Lu SPECT/CT with xSPECT Quant.

Quantitative SPECT/CT imaging forms the basis for internal dosimetry in molecular radiotherapies. While the conversion from counts to activity is typically based on conversion factors individually measured by each site, a recently introduced commercially available reconstruction (xSPECT Quant) offers a standardized and traceable calibration of SPECT/CT systems. The aim of this work was to assess the characteristics of xSPECT Quant in combination with 177Lu as one of the most important radionuclides used in molecular radiotherapies and to compare it to a widely used ordered-subset expectation-maximization reconstruction (Flash3D). Methods: In a series of 177Lu-filled phantom measurements, several important features were investigated for xSPECT Quant and Flash3D. Noise behavior and accuracy of the activity determination were evaluated in a large cylinder. Recovery coefficients were assessed in a hot-sphere phantom with and without background. Additionally, the resolutions were determined in a line source phantom as well as in a matched-filter resolution analysis of the hot-sphere-cold-background phantom. Results: Both reconstruction algorithms improve the spatial resolution at the cost of noise build-up. Despite its slower convergence, Flash3D features a more efficient recovery. Although resolution recovery methods are applied within both reconstructions, partial-volume errors-namely activity overestimation in the object center and spill-out of counts from the object edges-remain of relevance. In contrast to Flash3D, for which only the total number of updates (iterations × subsets) plays a role, the exact subdivision into iterations and subsets affected all characteristics of xSPECT Quant (optimum, 1 subset). The optimal trade-off between noise build-up and resolution improvement was found for 48 iterations and 1 subset, resulting in a quantitative accuracy of 1.2% in the Jaszczak cylinder (xSPECT Quant cross-calibrated to the dose calibrator). Conclusion: If the reconstruction parameters are chosen with care, both examined reconstructions can provide absolute quantitative SPECT images with high image quality (subcentimeter resolution at an acceptable noise build-up) as well as high quantitative accuracy (given a well-calibrated Flash3D conversion from counts to activity concentration). With its standardized (and traceable) activity determination, xSPECT Quant dispenses with site-specific calibration protocols, enabling a reliable activity determination comparable across sites, which is especially useful for multicentric molecular radiotherapy studies.

Fluctuation of quantitative values on acquisition time and the reconstruction conditions in 99mTc-SPECT.

This study aims to carry out a quantitative analysis with high reproducibility using single-photon emission computed tomography/computed tomography (SPECT/CT); we investigated the optimum parameters for the acquisition and the reconstruction. SPECT images were acquired with varying time per view using SPECT phantom (JS-10) and the body phantom of National Electrical Manufacturers Association and International Electrotechnical Commission (Body-phantom), respectively. For the image reconstruction condition, we changed the product of subset and iteration (SI product) and the Gaussian filter using a three-dimensional ordered subset expectation maximization. A combination of no scattering correction and no attenuation correction (SC-/AC-) and a combination of scattering correction and attenuation correction by CT images (SC+/AC+) were performed. The dose linearity, the recovery coefficient, the scatter ratio, and the coefficient of variation were evaluated using JS-10. Using Body-phantom, contrast-to-noise ratios of the hot spheres (13, 17 mm) were calculated. Moreover, the change in the maximum standardized uptake value (SUVmax) and the average SUV (SUVmean) were evaluated for each sphere. From the evaluation results using the JS-10, dose linearity, recovery coefficient, scatter ratio, and coefficient of variation were all good when time per view was 50-150 s, the Gaussian filter was 8-12 mm, and the SI product was 150. From the evaluation results using Body-phantom, comparing the Gaussian filter with 8 mm and 12 mm, the contrast-to-noise ratio was better for 12 mm and the error rate to the change of the scan-time was up to 3.7%. However, SUVmax and SUVmean using 8 mm were closer to the design value of the phantom. It is necessary that Quantitative SPECT be acquired at 50 s or more per view per detection, reconstructed using a three-dimensional ordered subset expectation maximization with SC+/AC+, the SI product is 150 times, and the Gaussian Filter is 8-12 mm. This suggested that the quantitative analysis would be carried out with good reproducibility.

Positron Range Visualization Using the Analytical and Iterative Image Reconstruction Methods From a Preclinical PET Scanner With LYSO Scintillators Coupled to SiPM Detectors: Comparison With Simulated Results

An evaluation of the image quality of the preclinical positron emission tomography scanner consisting of the SensL's SMT C-series silicon photomultiplier (SiPM) arrays coupled with double jaw layer lutetium-yttrium oxyorthosilicate (LYSO) crystals was achieved. The simulated data of a Jaszczak phantom filled with Na-22 and O-15 were compared with reconstructed data of the same phantom when it was filled with Na-22. Two different reconstruction methods were applied in order to visualize any positron range effects that occur within the phantom. The filtered back-projection (FBP) and ordered-subset expectation maximization (OSEM) algorithms provided useful information regarding the spatial resolution of the scanner, showing that these sensors can visually correct the structure of the phantom. The OSEM method appeared to be more reliable than FBP, even when a Compton correction was applied on the data.

A new method for spatial structure detection of complex inner cavities based on 3D γ-photon imaging

This paper presents a new three-dimensional (3D) imaging method for detecting the spatial structure of a complex inner cavity based on positron annihilation and γ-photon detection. This method first marks carrier solution by a certain radionuclide and injects it into the inner cavity where positrons are generated. Subsequently, γ-photons are released from positron annihilation, and the γ-photon detector ring is used for recording the γ-photons. Finally, the two-dimensional (2D) image slices of the inner cavity are constructed by the ordered-subset expectation maximization scheme and the 2D image slices are merged to the 3D image of the inner cavity. To eliminate the artifact in the reconstructed image due to the scattered γ-photons, a novel angle-traversal model is proposed for γ-photon single-scattering correction, in which the path of the single scattered γ-photon is analyzed from a spatial geometry perspective. Two experiments are conducted to verify the effectiveness of the proposed correction model and the advantage of the proposed testing method in detecting the spatial structure of the inner cavity, including the distribution of gas–liquid multi-phase mixture inside the inner cavity. The above two experiments indicate the potential of the proposed method as a new tool for accurately delineating the inner structures of industrial complex parts.

Quantitation of specific binding ratio in 123I-FP-CIT SPECT: accurate processing strategy for cerebral ventricular enlargement with use of 3D-striatal digital brain phantom.

This study aimed to evaluate the effect of ventricular enlargement on the specific binding ratio (SBR) and to validate the cerebrospinal fluid (CSF)-Mask algorithm for quantitative SBR assessment of 123I-FP-CIT single-photon emission computed tomography (SPECT) images with the use of a 3D-striatum digital brain (SDB) phantom. Ventricular enlargement was simulated by three-dimensional extensions in a 3D-SDB phantom comprising segments representing the striatum, ventricle, brain parenchyma, and skull bone. The Evans Index (EI) was measured in 3D-SDB phantom images of an enlarged ventricle. Projection data sets were generated from the 3D-SDB phantoms with blurring, scatter, and attenuation. Images were reconstructed using the ordered subset expectation maximization (OSEM) algorithm and corrected for attenuation, scatter, and resolution recovery. We bundled DaTView (Southampton method) with the CSF-Mask processing software for SBR. We assessed SBR with the use of various coefficients (f factor) of the CSF-Mask. Specific binding ratios of 1, 2, 3, 4, and 5 corresponded to SDB phantom simulations with true values. Measured SBRs > 50% that were underestimated with EI increased compared with the true SBR and this trend was outstanding at low SBR. The CSF-Mask improved 20% underestimates and brought the measured SBR closer to the true values at an f factor of 1.0 despite an increase in EI. We connected the linear regression function (y = - 3.53x + 1.95; r = 0.95) with the EI and f factor using root-mean-square error. Processing with CSF-Mask generates accurate quantitative SBR from dopamine transporter SPECT images of patients with ventricular enlargement.

Optimization of the reconstruction parameters in [123I]FP-CIT SPECT

The aim of this work was to obtain a set of parameters to be applied in [123I]FP-CIT SPECT reconstruction in order to minimize the error between standardized and true values of the specific uptake ratio (SUR) in dopaminergic neurotransmission SPECT studies. To this end, Monte Carlo simulation was used to generate a database of 1380 projection data-sets from 23 subjects, including normal cases and a variety of pathologies. Studies were reconstructed using filtered back projection (FBP) with attenuation correction and ordered subset expectation maximization (OSEM) with correction for different degradations (attenuation, scatter and PSF). Reconstruction parameters to be optimized were the cut-off frequency of a 2D Butterworth pre-filter in FBP, and the number of iterations and the full width at Half maximum of a 3D Gaussian post-filter in OSEM. Reconstructed images were quantified using regions of interest (ROIs) derived from Magnetic Resonance scans and from the Automated Anatomical Labeling map. Results were standardized by applying a simple linear regression line obtained from the entire patient dataset. Our findings show that we can obtain a set of optimal parameters for each reconstruction strategy. The accuracy of the standardized SUR increases when the reconstruction method includes more corrections. The use of generic ROIs instead of subject-specific ROIs adds significant inaccuracies. Thus, after reconstruction with OSEM and correction for all degradations, subject-specific ROIs led to errors between standardized and true SUR values in the range [−0.5, +0.5] in 87% and 92% of the cases for caudate and putamen, respectively. These percentages dropped to 75% and 88% when the generic ROIs were used.

Attenuation correction in myocardial perfusion imaging affects the assessment of infarct size in women with previous inferior infarct.

Myocardial perfusion imaging is a well-established diagnostic tool in patients with known or suspected coronary artery disease. Numerous clinical trials have shown that attenuation correction (AC) in single photon emission computed tomography (SPECT) improves the diagnostic accuracy of myocardial perfusion imaging over non-AC SPECT, differentiating between scar and attenuation artifacts. We have previously shown that attenuation artifacts produce an overestimation of the size of inferior infarcts in the male population. It is assumed that women are less affected by inferior attenuation artifacts than men. The aim of this study is to evaluate the role of AC in the assessment of infarct size in female patients with a history of myocardial inferior infarct. We studied a population of 66 consecutive women, with a history of previous inferior myocardial infarct, by SPECT/computed tomography (CT) with 370+370 MBq of technetium-99m labeled compounds by a 2-day stress-rest protocol. Both AC and uncorrected gated-SPECT/CT studies were reconstructed after scatter and motion correction by ordered-subset expectation maximization iterative reconstruction and resolution recovery. The coregistration of the transmission and emission scans was verified for all patients; any misalignment was realigned manually. Uncorrected and corrected SPECT images were analyzed by software QPS/QGS package using a 17-segment model. For each segment, perfusion and wall motion were quantified using a five-point score according to the American Society of Nuclear Cardiology guidelines. Summed stress, summed rest score (SRS), and summed difference score of the inferior left ventricle wall (inferior, inferoseptal, inferolateral, and apical inferior segments) were calculated. A linear correlation was used to assess the relationship between perfusion and the regional wall motion score as determined by uncorrected gated-SPECT. The results of quantitative analysis of non-AC and CT-AC SPECT images, respectively, were as follows: summed stress score: 9.47±5.01 and 6.58±4.77% (P<0.001); SRS was 6.05±5.02 and 4.14±4.12% (P<0.001); the summed difference score was 2.92±2.74 and 2.52±2.63% (P=NS), respectively. The correlation between corrected and uncorrected SRS and the regional summed wall motion score of the same segment was R=0.31 versus R=0.34. In the female population, like in men, attenuation artifacts affect the calculation of the infarct size of the inferior wall, with overestimation of the infarct size in uncorrected images. The AC regional perfusion score (SRS) better correlates with the regional wall motion score of the inferior wall in women with previous inferior infarct.

Time-of-Flight Information Improved the Detectability of Subcentimeter Spheres Using a Clinical PET/CT Scanner.

Recent advancements in clinical PET/CT scanners have improved the detectability of small lesions. However, the ideal reconstruction parameters for detecting small lesions have not yet been sufficiently clarified. The purpose of this study was to investigate the detectability of subcentimeter spheres using a clinical PET/CT scanner. Methods: We used a clinical PET/CT scanner to obtain the data of a National Electrical Manufacturers Association body phantom consisting of 6 small spheres (inner diameters, 4.0, 5.0, 6.2, 7.9, 10, and 37 mm) containing 18F solution. The background activity was 2.65 kBq/mL, and the sphere-to-background ratio was 8. The PET data obtained for 2 and 120 min were reconstructed using ordered-subsets expectation maximization (OSEM), OSEM + point-spread function (PSF), and OSEM + time-of-flight (TOF) with voxel sizes of 2.04 × 2.04 × 2.00 mm (2-mm voxels) and 4.07 × 4.07 × 3.99 mm (4-mm voxels). A gaussian filter was not used. The image quality was evaluated by visual assessment, as well as by physical assessment of the detectability index and recovery coefficients. Results: According to the visual assessment, the detectability of the spheres improved using TOF and a longer acquisition. Using the OSEM+TOF model, the smallest visually detected spheres were 5 mm in diameter with a 120-min acquisition and 6 mm in diameter with a 2-min acquisition. According to physical assessment, the detectability of spheres 10 mm or smaller using the OSEM+TOF image was superior to that using the OSEM image. In addition, the detectability of each hot sphere and recovery coefficient with 2-mm voxels was superior to that with 4-mm voxels. Although OSEM+PSF images showed less background noise, detectability and the recovery coefficient were not improved for spheres 8 mm or smaller. Conclusion: The TOF model with 2-mm voxels improved the detectability of subcentimeter hot spheres on a clinical PET/CT scanner.

Implementation of GPU accelerated SPECT reconstruction with Monte Carlo-based scatter correction.

Statistical SPECT reconstruction can be very time-consuming especially when compensations for collimator and detector response, attenuation, and scatter are included in the reconstruction. This work proposes an accelerated SPECT reconstruction algorithm based on graphics processing unit (GPU) processing. Ordered subset expectation maximization (OSEM) algorithm with CT-based attenuation modelling, depth-dependent Gaussian convolution-based collimator-detector response modelling, and Monte Carlo-based scatter compensation was implemented using OpenCL. The OpenCL implementation was compared against the existing multi-threaded OSEM implementation running on a central processing unit (CPU) in terms of scatter-to-primary ratios, standardized uptake values (SUVs), and processing speed using mathematical phantoms and clinical multi-bed bone SPECT/CT studies. The difference in scatter-to-primary ratios, visual appearance, and SUVs between GPU and CPU implementations was minor. On the other hand, at its best, the GPU implementation was noticed to be 24 times faster than the multi-threaded CPU version on a normal 128 × 128 matrix size 3 bed bone SPECT/CT data set when compensations for collimator and detector response, attenuation, and scatter were included. GPU SPECT reconstructions show great promise as an every day clinical reconstruction tool.