3D-OSEM Reconstruction Research Articles

Optimization of the Algorithm for the Implementation of Point Spread Function in the 3D-OSEM Reconstruction Algorithm Based on the List-Mode Micro PET Data

Positron emission tomography (PET) is a popular research topic. People are becoming more interested in PET images as they become more widely available. However, the partial volume effect (PVE) in PET images remains one of the most influential factors causing the resolution of PET images to degrade. It is possible to reduce this PVE and achieve better image quality by measuring and modeling the point spread function (PSF) and then accounting for it inside the reconstruction algorithm. In this work, we examined the response characteristics of the MetisTM PET/CT system by acquiring 22Na point source at different locations in the field of view (FOV) of the scanner and reconstructing with small pixel size for images to obtain their radial, tangential, and axial full-width half maximum (FWHM). An image-based model of the PSF model was then obtained by fitting asymmetric two-dimensional Gaussians on the 22Na images. This PSF model determined by FWHM in three directions was integrated into a three-dimensional ordered subsets expectation maximization (3D-OSEM) algorithm based on a list-mode format to form a new PSF-OSEM algorithm. We used both algorithms to reconstruct point source, Derenzo phantom, and mouse PET images and performed qualitative and quantitative analyses. In the point source study, the PSF-OSEM algorithm reduced the FWHM of the point source PET image in three directions to about 0.67 mm, and in the phantom study, the PET image reconstructed by the PSF-OSEM algorithm had better visual effects. At the same time, the quantitative analysis results of the Derenzo phantom were better than the original 3D-OSEM algorithm. In the mouse experiment, the results of qualitative and quantitative analyses showed that the imaging quality of PSF-OSEM algorithm was better than that of 3D-OSEM algorithm. Our results show that adding the PSF model to the 3D-OSEM algorithm in the MetisTM PET/CT system helps to improve the resolution of the image and satisfy the qualitative and quantitative analysis criteria.

Evaluation of standardized uptake value on 131I-6β-iodomethyl-19-norcholesterol scintigraphy for diagnosis of primary aldosteronism and correspondence with adrenal venous sampling.

Adrenal venous sampling (AVS) is a reliable method for lateralization of adrenal hormone secretion, which is important for discriminating between aldosterone-producing adenoma and bilateral adrenal hyperplasia, both of which cause primary aldosteronism (PA). The aim of this study was to evaluate the diagnostic accuracy of the maximum and mean standardized uptake values (SUVmax and SUVmean, respectively) of 131I-6β-iodomethyl-19-norcholesterol (NP-59) single-photon emission computed tomography (SPECT) for PA and its correspondence with AVS. Adrenal NP-59 scintigraphy was performed in 14 patients with suspected PA, and AVS was also performed in 7 of them. SUVmax and SUVmean of the adrenal lesions on the dominant side and their ratios to the values on the non-dominant side (SUVRmax and SUVRmean, respectively) were calculated on SPECT images using ordered-subset conjugate gradient minimization (OSCGM) and three-dimensional ordered-subset expectation maximization (3D-OSEM) reconstruction algorithms. SUVmax and SUVmean on NP-59 SPECT images were significantly higher for aldosterone-producing adenoma than for bilateral adrenal hyperplasia or non-functioning adenoma and slightly superior to SUVRmax and SUVRmean (P = 0.0475 and P = 0.0447 vs. P = 0.124 and P = 0.132, respectively, with OSCGM). The respective areas under the receiver-operating characteristic curve for SUV and SUVR were 0.933 and 0.725 with OSCGM and 0.844 and 0.750 with 3D-OSEM, while SUVmax and SUVRmax had exactly the same diagnostic accuracy as SUVmean and SUVRmean. SUV and SUVR were associated with the diagnostic features on AVS and consistent with lateralization by AVS in most patients. In this study, SUV on NP-59 SPECT helped in the diagnosis of PA and was consistent with the results of AVS in nearly all cases.

Accuracy of two dosimetry software programs for 177Lu radiopharmaceutical therapy using voxel-based patient-specific phantoms

PurposeVirtual dosimetry using voxel-based patient-specific phantoms and Monte Carlo (MC) simulations offer the advantage of having a gold standard against which absorbed doses may be benchmarked to establish the dosimetry accuracy. Furthermore, these reference values assist in investigating the accuracy of the absorbed dose methodologies from different software programs. Therefore, this study aimed to compare the accuracy of the absorbed doses computed using LundADose and OLINDA/EXM 1.0. MethodsThe accuracy was based on 177Lu-DOTATATE distributions of three voxel-based phantoms. SPECT projection images were simulated for 1, 24, 96, and 168 h post-administration and reconstructed with LundADose using 3D OS-EM reconstruction. Mono-exponential curves were fitted to the bio-kinetic data for the kidneys, liver, spleen, and tumours resulting in SPECT time-integrated activity (SPECT-TIA). The SPECT-TIA were used to compute mean absorbed doses using LundADose (LND-DSPECT) and OLINDA (OLINDA-DSPECT) for the organs. Pre-defined true activity images, were used to obtain TRUE-TIA and, together with full MC simulations, computed the true doses (MC-DTrue). The dosimetry accuracy was assessed by comparing LND-DSPECT and OLINDA-DSPECT to MC-DTrue. ResultsOverall, the results presented an overestimation of the mean absorbed dose by LND-DSPECT compared to the MC-DTrue with a dosimetry accuracy ≤6.6%. This was attributed to spill-out activity from the reconstructed LND-DSPECT, resulting in a higher dose contribution than the MC-DTrue. There was a general underestimation (<8.1%) of OLINDA-DSPECT compared to MC-DTrue attributed to the geometrical difference in shape between the voxel-based phantoms and the OLINDA models. Furthermore, OLINDA-DSPECT considers self-doses while MC-DTrue reflects self-doses plus cross-doses. ConclusionThe better than 10% accuracy suggests that the mean dose values obtained with LND-DSPECT and OLINDA-DSPECT approximate the true values. The mean absorbed doses of the two software programs, and the gold standard were comparable. This work shall be of use for optimising 177Lu dosimetry for clinical applications.

Accuracy of Two Dosimetry Software Programs for &lt;sup&gt;177&lt;/sup&gt;Lu Radiopharmaceutical Therapy Using Voxel-Based Patient-Specific Phantoms

PurposeVirtual dosimetry using voxel-based patient-specific phantoms and Monte Carlo (MC) simulations offer the advantage of having a gold standard against which absorbed doses may be benchmarked to establish the dosimetry accuracy. Furthermore, these reference values assist in investigating the accuracy of the absorbed dose methodologies from different software programs. Therefore, this study aimed to compare the accuracy of the absorbed doses computed using LundADose and OLINDA/EXM 1.0.MethodsThe accuracy was based on 177Lu-DOTATATE distributions of three voxel-based phantoms. SPECT projection images were simulated for 1, 24, 96, and 168 hours post-administration and reconstructed with LundADose using 3D OS-EM reconstruction. Mono-exponential curves were fitted to the bio-kinetic data for the kidneys, liver, spleen, and tumours resulting in SPECT time-integrated activity (SPECT-TIA). The SPECT-TIA were used to compute mean absorbed doses using LundADose (LND-D SPECT ) and OLINDA (OLINDA-D SPECT ) for the organs. Pre-defined true activity images, were used to obtain TRUE-TIA and, together with full MC simulations, computed the true doses (MC-D True ). The dosimetry accuracy was assessed by comparing LND-D SPECT and OLINDA-D SPECT to MC-D True .ResultsOverall, the results presented an overestimation of the mean absorbed dose by LND-D SPECT compared to the MC-D True with a dosimetry accuracy ≤ 6.59%. This was attributed to spill-out activity from the reconstructed LND-D SPECT , resulting in a higher dose contribution than the MC-D True . There was a general underestimation (< 8.11%) of OLINDA-D SPECT compared to MC-D True attributed to the geometrical difference in shape between the voxel-based phantoms and the OLINDA models. Furthermore, OLINDA-D SPECT considers self-doses while MC-D True reflects self-doses plus cross-doses.ConclusionThe better than 10% accuracy suggests that the mean dose values obtained with LND-D SPECT and OLINDA-D SPECT approximate the true values. The mean absorbed doses of the two software programs, and the gold standard were comparable. This work shall be of use for optimising 177Lu dosimetry for clinical applications.

Impact of Resolution Recovery in Quantitative 99mTc SPECT/CT Cardiac Phantom Studies

BackgroundThe aim of this study was to validate the quantitative accuracy of single photon emission computed tomography with computed tomography (SPECT/CT) images in cardiac phantom studies. The study was performed by assessing the effect of resolution recovery (RR) when using half-time of imaging acquisition in cardiac phantom. MethodsThe SPECT/CT images of the anthropomorphic phantom with a cardiac insert, liver, lung, and spine were acquired using the GE Discovery (NM/CT 670) SPECT/CT system. Different concentration activity ratios for different organ (10:10:1:0, cardiac:liver:background:lung) regions were acquired by using full- and half-time protocols for 643 and 1283 voxel sizes that were reconstructed using filtered back projection (FBP) method and 3D ordered subset expectation maximization (3D-OSEM). Attenuation correction and scatter correction were applied to both reconstructions, whereas the RR only can be applied for 3D-OSEM. The data were analyzed and reported in terms of absolute recovery coefficient percentage between the cardiac insert and background activity concentration. Another parameter used to assess the quantitative accuracy for defect region was the relative error percentage. ResultsThe result of recovery coefficient percentage shows that the 3D-OSEM reconstruction with the RR gives the highest percentage estimation accuracy of 70% of activity recovery in the cardiac phantom wall compared with FBP (10.6%). The relative error percentage for reconstructed SPECT/CT images using 3D-OSEM reconstruction with RR shows the least error compared with FBP (21% vs. 45.1%) both in the full-and half-time acquisition of images with a larger number of matrix size used. Conclusions3D-OSEM reconstruction with the RR is beneficial in giving better quantitative evaluation with a good resolution myocardial perfusion image. To accomplish this, a larger matrix size is required for 3D-OSEM reconstruction with the RR and it demonstrated an improvement in image resolution and increased quantitative accuracy of the final reconstructed SPECT/CT images.

Quantitative measurement of standardized uptake value of lumbar vertebra by 99Tcm-MDP bone imaging with SPECT/CT

Objective To explore the quantitative determination of standardized uptake value (SUV) of lumbar vertebra by 99Tcm-methylene diphosphonate (MDP) bone imaging with SPECT/CT. Methods A total of 90 patients (16 males, 74 females, age: (60.0±11.2) years) who underwent 99Tcm-MDP bone imaging and local lumbar SPECT/CT imaging from May 2017 to November 2017 were retrospectively analyzed. CT attenuation correction, scatter correction and 3-dimensional ordered subset expectation maximization (3D-OSEM) reconstruction were used. The radioactive counts of lumbar vertebra L1-L4 in patients with high MDP-uptake (including degenerative changes and bone metastases) and patients with non-high MDP-uptake (including osteoporosis and non-osteoporosis) were measured. Basing on that, the radioactivity of unit volume in lumbar and lesion was calculated and converted to SUV. The difference of SUV for lumbar and lesions from different types of patients was analyzed, and the correlation between lumbar SUV and varies of parameters were also analyzed. Data were analyzed by two-sample t test and Pearson correlation analysis. Results The mean SUV (SUVmean) and maximum SUV (SUVmax) of L1-L4 in 54 patients with non-high MDP-uptake were 4.64±1.03 and 7.09±1.61 respectively. They were positively correlated with bone mineral content, CT value and body weight (r values: 0.433-0.700, all P<0.05), and negatively correlated with age (r values: -0.514 and -0.456, both P<0.001). The SUVmean and SUVmax of patients with osteoporosis were 3.81±0.76 and 5.86±1.34 respectively, which were significantly lower than those of non-osteoporosis (5.13±0.84 and 7.82±1.30; t values: -5.892 and -5.240, both P<0.001). The SUVmean and SUVmax of 37 metastases were 16.40±6.07 and 24.25±11.35 respectively in 16 patients, which were significantly higher than those in patients with degenerative changes (20 patients with 27 lesions; 9.30±3.16 and 13.48±5.21; t values: 6.074 and 5.083, both P<0.001). Conclusion Based on the specific reconstruction algorithm and correction method, the SUV of lumbar vertebra on 99Tcm-MDP SPECT/CT bone imaging can be obtained, and the bone mineral metabolism can be reflected. Key words: Lumbar vertebrae; Radionuclide imaging; Technetium Tc 99m medronate; Tomography, emission-computed, single-photon; Tomography, X-ray computed

SUV Harmonization Between Different Hybrid PET/CT Systems.

Current PET/CT lutetium oxyorthosilicate (LSO) or lutetium-yttrium oxyorthosilicate crystal scanners are equipped with sophisticated softwares including point-spread function (PSF) and time-of-flight (TOF) image reconstruction. These softwares are associated with increased SUVs compared with 3D-OSEM reconstructions associated with older BGO PET/CT scanners. The European Association of Nuclear Medicine (EANM) identified the problem of SUV harmonization since 2010 through the EANM Research Ltd European Association of Nuclear Medicine Research Ltd [EARL] FDG PET/CT accreditation program. This required processing 2 reconstructions, one optimized for maximum spatial lesion detection and one for harmonized quantitation. We investigated an alternative single reconstruction method for both qualitative and quantitative analysis optimized to maximize spatial lesion detectability, followed by an intrinsic postreconstruction algorithm for SUV harmonization. Phantom and "in vivo" patient data analysis were acquired and analyzed on (a) a Siemens Biograph mCT system with LSO crystals and PSF and TOF algorithms, and on (b) a General Electric Discovery STE system with BGO crystals, without PSF and TOF. A dedicated algorithm (EQ.filter) was tested to harmonize SUV between the 2 scanners compared with EANM/EARL specifications. NEMA IQ phantom and a Jaszczak cylindrical phantom equipped with small fillable spheres (lesion to background ratios of 8:1 and 4:1) were used. Phantom data were validated on 7 oncologic patients with 39 hyperactive lesions ranging from 3 mm to 26 mm. The main benefit of PSF + TOF LSO PET/CT systems was increased contrast for small active lesions. Recovery coefficients measured according to NEMA standards exceeded those obtained by 3D-OSEM reconstruction. SUVmax discrepancies between the 2 PET/CT systems were as high as 149%, dropping to below 10% when optimized by EQ.filter. A single reconstruction optimized by EQ.filter for maximum spatial lesion detectability is an easy and precise solution to harmonize SUVs between different PET/CT scanners, avoiding a second reconstruction with an additional smoothing filter as requested by EANM/EARL.

Improved quantitative 90 Y bremsstrahlung SPECT/CT reconstruction with Monte Carlo scatter modeling.

In 90 Y microsphere radioembolization (RE), accurate post-therapy imaging-based dosimetry is important for establishing absorbed dose versus outcome relationships for developing future treatment planning strategies. Additionally, accurately assessing microsphere distributions is important because of concerns for unexpected activity deposition outside the liver. Quantitative 90 Y imaging by either SPECT or PET is challenging. In 90 Y SPECT model based methods are necessary for scatter correction because energy window-based methods are not feasible with the continuous bremsstrahlung energy spectrum. The objective of this work was to implement and evaluate a scatter estimation method for accurate 90 Y bremsstrahlung SPECT/CT imaging. Since a fully Monte Carlo (MC) approach to 90 Y SPECT reconstruction is computationally very demanding, in the present study the scatter estimate generated by a MC simulator was combined with an analytical projector in the 3D OS-EM reconstruction model. A single window (105 to 195-keV) was used for both the acquisition and the projector modeling. A liver/lung torso phantom with intrahepatic lesions and low-uptake extrahepatic objects was imaged to evaluate SPECT/CT reconstruction without and with scatter correction. Clinical application was demonstrated by applying the reconstruction approach to five patients treated with RE to determine lesion and normal liver activity concentrations using a (liver) relative calibration. There was convergence of the scatter estimate after just two updates, greatly reducing computational requirements. In the phantom study, compared with reconstruction without scatter correction, with MC scatter modeling there was substantial improvement in activity recovery in intrahepatic lesions (from >55% to >86%), normal liver (from 113% to 104%), and lungs (from 227% to 104%) with only a small degradation in noise (13% vs. 17%). Similarly, with scatter modeling contrast improved substantially both visually and in terms of a detectability index, which was especially relevant for the low uptake extrahepatic objects. The trends observed for the phantom were also seen in the patient studies where lesion activity concentrations and lesion-to-liver concentration ratios were lower for SPECT without scatter correction compared with reconstruction with just two MC scatter updates: in eleven lesions the mean uptake was 4.9 vs. 7.1MBq/mL (P=0.0547), the mean normal liver uptake was 1.6 vs. 1.5MBq/mL (P=0.056) and the mean lesion-to-liver uptake ratio was 2.7 vs. 4.3 (P=0.0402) for reconstruction without and with scatter correction respectively. Quantitative accuracy of 90 Y bremsstrahlung imaging can be substantially improved with MC scatter modeling without significant degradation in image noise or intensive computational requirements.

Impact of a commercial 3D OSEM reconstruction algorithm on the 177Lu activity quantification of SPECT/CT imaging in a Molecular Radiotherapy trial

Impact of a commercial 3D OSEM reconstruction algorithm on the 177Lu activity quantification of SPECT/CT imaging in a Molecular Radiotherapy trial

Optimization of brain PET imaging for a multicentre trial: the French CATI experience.

BackgroundCATI is a French initiative launched in 2010 to handle the neuroimaging of a large cohort of subjects recruited for an Alzheimer’s research program called MEMENTO. This paper presents our test protocol and results obtained for the 22 PET centres (overall 13 different scanners) involved in the MEMENTO cohort. We determined acquisition parameters using phantom experiments prior to patient studies, with the aim of optimizing PET quantitative values to the highest possible per site, while reducing, if possible, variability across centres.MethodsJaszczak’s and 3D-Hoffman’s phantom measurements were used to assess image spatial resolution (ISR), recovery coefficients (RC) in hot and cold spheres, and signal-to-noise ratio (SNR). For each centre, the optimal reconstruction parameters were chosen as those maximizing ISR and RC without a noticeable decrease in SNR. Point-spread-function (PSF) modelling reconstructions were discarded. The three figures of merit extracted from the images reconstructed with optimized parameters and routine schemes were compared, as were volumes of interest ratios extracted from Hoffman acquisitions. The net effect of the 3D-OSEM reconstruction parameter optimization was investigated on a subset of 18 scanners without PSF modelling reconstruction.ResultsCompared to the routine parameters of the 22 PET centres, average RC in the two smallest hot and cold spheres and average ISR remained stable or were improved with the optimized reconstruction, at the expense of slight SNR degradation, while the dispersion of values was reduced.For the subset of scanners without PSF modelling, the mean RC of the smallest hot sphere obtained with the optimized reconstruction was significantly higher than with routine reconstruction. The putamen and caudate-to-white matter ratios measured on 3D-Hoffman acquisitions of all centres were also significantly improved by the optimization, while the variance was reduced.ConclusionsThis study provides guidelines for optimizing quantitative results for multicentric PET neuroimaging trials.

Performance of 3DOSEM and MAP algorithms for reconstructing low count SPECT acquisitions

PurposeLow count single photon emission computed tomography (SPECT) is becoming more important in view of whole body SPECT and reduction of radiation dose. In this study, we investigated the performance of several 3D ordered subset expectation maximization (3DOSEM) and maximum a posteriori (MAP) algorithms for reconstructing low count SPECT images. Materials and MethodsPhantom experiments were conducted using the National Electrical Manufacturers Association (NEMA) NU2 image quality (IQ) phantom. The background compartment of the phantom was filled with varying concentrations of pertechnetate and indiumchloride, simulating various clinical imaging conditions. Images were acquired using a hybrid SPECT/CT scanner and reconstructed with 3DOSEM and MAP reconstruction algorithms implemented in Siemens Syngo MI.SPECT (Flash3D) and Hermes Hybrid Recon Oncology (Hyrid Recon 3DOSEM and MAP). Image analysis was performed by calculating the contrast recovery coefficient (CRC),percentage background variability (N%), and contrast-to-noise ratio (CNR), defined as the ratio between CRC and N%. Furthermore, image distortion is characterized by calculating the aspect ratio (AR) of ellipses fitted to the hot spheres. Additionally, the performance of these algorithms to reconstruct clinical images was investigated. ResultsImages reconstructed with 3DOSEM algorithms demonstrated superior image quality in terms of contrast and resolution recovery when compared to images reconstructed with filtered-back-projection (FBP), OSEM and 2DOSEM. However, occurrence of correlated noise patterns and image distortions significantly deteriorated the quality of 3DOSEM reconstructed images. The mean AR for the 37, 28, 22, and 17mm spheres was 1.3, 1.3, 1.6, and 1.7 respectively. The mean N% increase in high and low count Flash3D and Hybrid Recon 3DOSEM from 5.9% and 4.0% to 11.1% and 9.0%, respectively. Similarly, the mean CNR decreased in high and low count Flash3D and Hybrid Recon 3DOSEM from 8.7 and 8.8 to 3.6 and 4.2, respectively. Regularization with smoothing priors could suppress these noise patterns at the cost of reduced image contrast. The mean N% was 6.4% and 6.8% for low count QSP and MRP MAP reconstructed images. Alternatively, regularization with an anatomical Bowhser prior resulted in sharp images with high contrast, limited image distortion, and low N% of 8.3% in low count images, although some image artifacts did occur. Analysis of clinical images suggested that the same effects occur in clinical imaging. ConclusionImage quality of low count SPECT acquisitions reconstructed with modern 3DOSEM algorithms is deteriorated by the occurrence of correlated noise patterns and image distortions. The artifacts observed in the phantom experiments can also occur in clinical imaging.

Dosimetry for (177)Lu-DKFZ-PSMA-617: a new radiopharmaceutical for the treatment of metastatic prostate cancer.

Dosimetry is critical to achieve the optimal therapeutic effect of radioligand therapy (RLT) with limited side effects. Our aim was to perform image-based absorbed dose calculation for the new PSMA ligand (177)Lu-DKFZ-PSMA-617 in support of its use for the treatment of metastatic prostate cancer. Whole-body planar images and SPECT/CT images of the abdomen were acquired in five patients (mean age 68years) for during two treatment cycles at approximately 1, 24, 48 and 72h after administration of 3.6GBq (range 3.4 to 3.9GBq) (177)Lu-DKFZ-PSMA-617. Quantitative 3D SPECT OSEM reconstruction was performed with corrections for photon scatter, photon attenuation and detector blurring. A camera-specific calibration factor derived from phantom measurements was used for quantitation. Absorbed doses were calculated for various organs from the images using a combination of linear approximation, exponential fit, and target-specific S values, in accordance with the MIRD scheme. Absorbed doses to bone marrow were estimated from planar and SPECT images and with consideration of the blood sampling method according to the EANM guidelines. The average (± SD) absorbed doses per cycle were 2.2 ± 0.6Gy for the kidneys (0.6Gy/GBq), 5.1 ± 1.8Gy for the salivary glands (1.4Gy/GBq), 0.4 ± 0.2Gy for the liver (0.1Gy/GBq), 0.4 ± 0.1Gy for the spleen (0.1Gy/GBq), and 44 ± 19mGy for the bone marrow (0.012Gy/GBq). The organ absorbed doses did not differ significantly between cycles. The critical absorbed dose reported for the kidneys (23Gy) was not reached in any patient. At 24h there was increased uptake in the colon with 50-70% overlap to the kidneys on planar images. Absorbed doses for tumour lesions ranged between 1.2 and 47.5Gy (13.1Gy/GBq) per cycle. The salivary glands and kidneys showed high, but not critical, absorbed doses after RLT with (177)Lu-DKFZ-PSMA-617. We suggest that (177)Lu-DKFZ-PSMA-617 is suitable for radiotherapy, offering tumour-to-kidney ratios comparable to those with RLT agents currently available for the treatment of neuroendocrine tumours. Our dosimetry results suggest that (177)Lu-DKFZ-PSMA-617 treatment with higher activities and more cycles is possible without the risk of damaging the kidneys.

Optimization and evaluation of reconstruction-based compensation methods and reconstruction parameters for Tc-99m MIBI parathyroid SPECT

The value of Tc-99m MIBI parathyroid SPECT for localizing parathyroid hyperplasia in chronic renal failure patients remains inconclusive due to limited image quality. Advanced reconstruction methods to improve image quality have been developed but require optimization and evaluation. The goal of this study was to optimize and evaluate compensation methods and reconstruction parameters for Tc-99m MIBI parathyroid SPECT. A phantom population and projection data that modelled clinically realistic variations found in patients were simulated. The 3D OS-EM reconstruction with compensation for attenuation, detector response and scatter in various combinations were studied. For each compensation, the number of updates for OS-EM and the cutoff frequency of a 3D Butterworth filter were optimized and evaluated using anthropomorphic model observer. With optimal parameters, the method with compensation for attenuation and detector response, with or without the addition of scatter compensation, provided the highest lesion detectability for Tc-99m MIBI parathyroid SPECT.

3D-OSEM and FP-CIT SPECT quantification

Dopamine transporter imaging with single-photon emission computed tomography (SPECT) is a valuable tool for both clinical routine and research studies. Recently, it was found that the image quality could be improved by introduction of the three-dimensional ordered subset expectation maximization (3D-OSEM) reconstruction algorithm, which provides resolution recovery. The aim of this study was to systematically evaluate the potential benefits of 3D-OSEM in comparison with 2D-OSEM under critical imaging conditions, for example, scans with a high radius of rotation. Monte Carlo simulation scans of a digital brain phantom with various disease states and different radii of rotation ranging from 13 to 30 cm were reconstructed with both 2D-OSEM and 3D-OSEM algorithms. Specific striatal binding and putamen-to-caudate ratios were determined and compared with true values in the phantom. The percentage recovery of true striatal binding was similar between both reconstruction algorithms at the minimum rotational radius; however, at the maximum rotational radius, it decreased from 53 to 43% for 3D-OSEM and from 52 to 26% for 2D-OSEM. 3D-OSEM matched the true putamen-to-caudate ratios more closely than did 2D-OSEM in scans with high SPECT rotation radii. 3D-OSEM offers a promising image quality gain. It outperforms 2D-OSEM, particularly in studies with limited resolutions (such as scans acquired with a high radius of rotation) but does not improve the accuracy of the putamen-to-caudate ratios. Whether the benefits of better recovery in studies with higher radii of rotation could potentially increase the diagnostic power of dopamine transporter SPECT in patients with borderline striatal radiotracer binding, however, needs to be further examined.

Imaging Capabilities of the Inveon SPECT System Using Single-and Multipinhole Collimators

The Inveon small-animal SPECT system comes with several types of multipinhole collimator plates. We evaluate here the performance measurements of the Inveon SPECT system using 6 different collimators: 3 dedicated for mouse imaging and 3 for rat imaging. The measured performance parameters include the sensitivity, the spatial resolution using line sources, the ultra-micro Derenzo phantom, the recovery coefficient and the noise measurements using the National Electrical Manufacturers Association NU-4 image quality phantom, obtained with the 2 reconstruction algorithms available with the Inveon Acquisition Workplace, version 1.5-the 3-dimensional ordered-subset expectation maximization (3DOSEM) and the 3-dimensional maximum a posteriori (3DMAP). Further, the overall performance of the system is illustrated by an animal experiment. The results show that the Inveon SPECT scanner offers a spatial resolution, measured at the center of the field of view, ranging from 0.6 to 1 mm with the collimator plates dedicated to mouse imaging and from 1.2 to less than 2 mm with rat collimator plates. The system sensitivity varies from 29 to 404 cps/MBq for mouse collimators and from 53 to 175 cps/MBq for rat collimators. The image quality study showed that 3DMAP allows better noise reduction while preserving the recovery coefficient, compared with other regularization strategies such as the premature termination of the 3DOSEM reconstruction or 3DOSEM followed by gaussian filtering. The acquisition parameters, such as the collimator set and the radius of rotation, offer a wide range of possibilities to apply to a large number of biologic studies. However, special care must be taken because this increase in sensitivity can be offset by image degradation, such as image artifacts caused by projection overlap and statistical noise due to a higher number of iterations required for convergence. 3DMAP allowed better noise reduction while maintaining relatively constant recovery coefficients, as compared with other reconstruction strategies.

Reliability of predicting image signal‐to‐noise ratio using noise equivalent count rate in PET imaging

Several investigators have shown that noise equivalent count rate (NECR) is linearly proportional to the square of image signal-to-noise ratio (SNR) when PET images are reconstructed using filtered back-projection. However, to our knowledge, none have shown a similar relationship in fully 3D ordered-subset expectation maximization (OSEM) reconstruction. This paper has two aims. The first is to investigate the NECR-SNR relationship for 3D-OSEM reconstruction using phantom studies while the second aim is to evaluate the NECR-SNR relationship using patient data. An anthropomorphic phantom was scanned on a GE Discovery-STE (DSTE) PET∕CT scanner in 3D mode with an initial activity concentration of 66.34 kBq∕cc. PET data were acquired over the lower chest∕upper abdomen region in dynamic mode. The experiment was repeated with the same activity concentration on a GE Discovery-RX (DRX) scanner. Care was taken to place the phantom at identical positions in both scanners. PET data were then reconstructed using 3D Reprojection (3D-RP) and 3D-OSEM with different reconstruction parameters and the NECR and SNR for each frame∕image were calculated. SNR(2) was then plotted versus the NECR for each scanner, reconstruction method and parameters. In addition, 40 clinical PET∕CT studies from the two scanners (20 patients∕scanner) were evaluated retrospectively. The patient studies from each scanner were further divided into two subgroups of body mass indices (BMI). Each PET study was acquired in 3D mode and reconstructed using both 3D-OSEM and 3D-RP. The NECR and SNR of the bed position covering the patient liver were calculated for each patient and averaged for each subgroup. Comparisons of the NECR and SNR between scanner types and BMIs were performed using a t-test and a p value less than 0.05 was considered significant. Phantom results showed that SNR(2) versus NECR was linear for 3D-RP reconstruction across all activity concentration on both scanners, as expected. However, when 3D-OSEM was used, this relationship was nonlinear at activity concentrations beyond the peak NECR on both scanners. On the other hand, the plot of SNR(2) versus trues count rate was linear for 3D-OSEM across all activity concentrations on both scanners independent of reconstruction parameters used. In addition, for activity concentrations <30kBq∕cc, phantom results showed a higher SNR (by 12 ± 10%; p < 0.05) and NECR for the DRX scanner compared to DSTE for 3D-RP reconstruction. However, for 3D-OSEM reconstruction, these two scanners had similar SNRs (different by 2% ± 9%; p > 0.05), despite having different NECRs. Patient studies showed a statistically significant difference in NECR as well as the SNR for 3D-RP reconstruction between the two scanners. However, no statistically significant difference was found for 3D-OSEM. A statistically significant difference in both NECR and SNR were found between the different BMI subgroups for both 3D-RP and 3D-OSEM reconstructions. For the scanners and reconstruction algorithm used in this study, our results suggest that the image SNR cannot be predicted by the NEC when using 3D-OSEM reconstruction particularly for those clinical applications requiring high activity concentration. Instead, our results suggest that image SNR varies with activity concentration and is dominated by the 3D-OSEM reconstruction algorithm and its associated parameters, while not being affected by the scanner type for the range of activity concentrations usually found in the clinic.

Image-based point spread function implementation in a fully 3D OSEM reconstruction algorithm for PET

The interest in positron emission tomography (PET) and particularly in hybrid integrated PET/CT systems has significantly increased in the last few years due to the improved quality of the obtained images. Nevertheless, one of the most important limits of the PET imaging technique is still its poor spatial resolution due to several physical factors originating both at the emission (e.g. positron range, photon non-collinearity) and at detection levels (e.g. scatter inside the scintillating crystals, finite dimensions of the crystals and depth of interaction). To improve the spatial resolution of the images, a possible way consists of measuring the point spread function (PSF) of the system and then accounting for it inside the reconstruction algorithm. In this work, the system response of the GE Discovery STE operating in 3D mode has been characterized by acquiring 22Na point sources in different positions of the scanner field of view. An image-based model of the PSF was then obtained by fitting asymmetric two-dimensional Gaussians on the 22Na images reconstructed with small pixel sizes. The PSF was then incorporated, at the image level, in a three-dimensional ordered subset maximum likelihood expectation maximization (OS-MLEM) reconstruction algorithm. A qualitative and quantitative validation of the algorithm accounting for the PSF has been performed on phantom and clinical data, showing improved spatial resolution, higher contrast and lower noise compared with the corresponding images obtained using the standard OS-MLEM algorithm.

Investigation of the possibility of improving spatial resolution in SPECT with the combination of LaBr 3:Ce-based detector and 3D-OSEM reconstruction algorithms

This study investigates the potential of improving spatial resolution in SPECT imaging using a combination of LaBr 3:Ce detectors and 3D-OSEM image reconstruction algorithms. Potential spatial resolution improvement was assessed intrinsically and extrinsically using GATE Monte Carlo simulation. Significantly improved MTF of LaBr 3:Ce detectors suggests better resolution performance at all spatial frequencies. In comparison to conventional NaI(Tl) scintillators, a combination of the LaBr 3:Ce crystal and 3D-OSEM incorporating resolution recovery could significantly improve the extrinsic spatial resolution of SPECT images.

SU‐GG‐J‐132: Estimate Absorbed Dose Using Micro PET Imaging of Rat Brain Tumor with I‐124 Infusion Followed by F‐18 Injection

OBJECTIVE: This study was to estimate absorbed dose using micro PET imaging of the rat brain tumor with I‐124 Infusion followed by F‐18 Injection METHOD: Two tumor‐bearing rats were infused intratumorally with 2.1 uCi I‐124 Gd Tagged Metallofullerenes. A 2‐hour post I‐124 infusion PET scan was performed on each animal to identify the infusion site and possible free I‐124 distribution. Following I‐124 imaging, 32 uCi and 182 uCi of F‐18 FDG were administered respectively by IP injection to highlight brain anatomy at different image contrasts. Each animal was imaged for 5 minutes using a uPET scanner, 350 – 550 keV energy window, without transmission scanning. F‐18 imaging started 5 and 45 minutes subsequently after the IP injection. Image reconstruction algorithms (FBP, 3D OSEM, 3D Re‐Projection with or without Hanning Filter) were used for comparison. Image processing and analysis were conducted on ASIPro VM platform. The infusion site and tumor were measured by FWHM from Gaussian fit of count profiles. Absorbed dose was estimated using MIRD based on RoI counts from the PET images. RESULT: The post infusion PET scan showed high I‐124 concentration in tumor and minimum free iodine in the rest of body. The 3D OSEM reconstruction improved spatial resolution and contrast compared with the FBP and 3DRP algorithms. The infusion site was 5.21 mm (FWHM). Tumor dose of I‐124 (19.8 Gy) was accurate because of high contrast of I‐124 image, and the dose for F‐18 depended on the RoI definition. CONCLUSION: Radiation dosimetry using PET scan depends on image quantification which is affected by the reconstruction algorithm selected and the acquisition or processing protocol. Radiation dosimetry using PET scan depends on image quantification which is affected by the reconstruction algorithm selected and the acquisition or processing protocol.

An advanced fully 3D OSEM reconstruction for positron emission tomography

A fully 3D OSEM reconstruction method for positron emission tomography (PET) based on symmetries and sparse matrix technique is described. Great savings in both storage space and computation time were achieved by exploiting the symmetries of scanner and sparseness of the system matrix. More reduction of storage requirement was obtained by introducing the approximation of system matrix. Iteration-filter was performed to restrict image noise in reconstruction. Performances of simulation data and phantom data got from Micro-PET (Type: Epuls-166) demonstrated that similar image quality was achieved using the approximation of the system matrix.