3D OSEM Research Articles

SU‐F‐J‐224: Impact of 4D PET/CT On PERCIST Classification of Lung and Liver Metastases in NSLC and Colorectal Cancer

Purpose:To quantify the impact of 4D PET/CT on PERCIST metrics in lung and liver tumors in NSCLC and colorectal cancer patients.Methods:32 patients presenting lung or liver tumors of 1–3 cm size affected by respiratory motion were scanned on a GE Discovery 690 PET/CT. The bed position with lesion(s) affected by motion was acquired in a 12 minute PET LIST mode and unlisted into 8 bins with respiratory gating. Three different CT maps were used for attenuation correction: a clinical helical CT (CT_clin), an average CT (CT_ave), and an 8‐phase 4D CINE CT (CT_cine). All reconstructions were 3D OSEM, 2 iterations, 24 subsets, 6.4 Gaussian filtration, 192×192 matrix, non‐TOF, and non‐PSF. Reconstructions using CT_clin and CT_ave used only 3 out of the 12 minutes of the data (clinical protocol); all 12 minutes were used for the CT_cine reconstruction. The percent change of SUVbw_peak and SUVbw_max was calculated between PET_CTclin and PET_CTave. The same percent change was also calculated between PET_CTclin and PET_CTcine in each of the 8 bins and in the average of all bins. A 30% difference from PET_CTclin classified lesions as progressive metabolic disease (PMD) using maximum bin value and the average of eight bin values.Results:30 lesions in 25 patients were evaluated. Using the bin with maximum SUVbw_peak and SUVbw_max difference, 4 and 13 lesions were classified as PMD, respectively. Using the average bin values for SUVbw_peak and SUVbw_max, 3 and 6 lesions were classified as PMD, respectively. Using PET_CTave values for SUVbw_peak and SUVbw_max, 4 and 3 lesions were classified as PMD, respectively.Conclusion:These results suggest that response evaluation in 4D PET/CT is dependent on SUV measurement (SUVpeak vs. SUVmax), number of bins (single or average), and the CT map used for attenuation correction.

Pre-clinical Positron Emission Tomography Reconstruction Algorithm Effect on Cu-64 ATSM Lesion Hypoxia.

Objective:Application of distinct positron emission tomography (PET) scan reconstruction algorithms can lead to statistically significant differences in measuring lesion functional properties. We looked at the influence of two-dimensional filtered back projection (2D FBP), two-dimensional ordered subset expectation maximization (2D OSEM), three-dimensional ordered subset expectation maximization (3D OSEM) without 3D maximum a posteriori and with (3D OSEM MAP) on lesion hypoxia tracer uptake using a pre-clinical PET scanner.Methods:Reconstructed images of a rodent tumor model bearing P22 carcinosarcoma injected with hypoxia tracer Copper-64-Diacetyl-bis (N4-methylthiosemicarbazone) (i.e. Cu-64 ATSM) were analyzed at 10 minute intervals till 60 minute post injection. Lesion maximum standardized uptake values (SUVmax) and SUVmax/background SUVmean (T/B) were recorded and investigated after application of multiple algorithm and reconstruction parameters to assess their influence on Cu-64 ATSM measurements and associated trends over time.Results:SUVmaxSUVmax or T/B between 2D FBP, exhibited convergence for OSEM reconstructions while ANOVA results showed a significant difference in SUVmax or T/B between 2D FBP, 2D OSEM, 3D OSEM and 3D OSEM MAP reconstructions across all time frames. SUVmax and T/B were greatest in magnitude for 2D OSEM followed by 3D OSEM MAP, 3D OSEM and then 2D FBP at all time frames respectively. Similarly SUVmax and T/B standard deviations (SD) were lowest for 2D OSEM in comparison with other algorithms.Conclusion:Significantly higher magnitude lesion SUVmax and T/B combined with lower SD were observed using 2D OSEM reconstruction in comparison with 2D FBP, 3D OSEM and 3D OSEM MAP algorithms at all time frames. Results are SUVmax or T/B between 2D FBP, consistent with other published studies however more specimens are required for full validation.

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.

Task-based evaluation of a 4D MAP-RBI-EM image reconstruction method for gated myocardial perfusion SPECT using a human observer study

We evaluated the performance of a new 4D image reconstruction method for improved 4D gated myocardial perfusion (MP) SPECT using a task-based human observer study. We used a realistic 4D NURBS-based Cardiac-Torso (NCAT) phantom that models cardiac beating motion. Half of the population was normal; the other half had a regional hypokinetic wall motion abnormality. Noise-free and noisy projection data with 16 gates/cardiac cycle were generated using an analytical projector that included the effects of attenuation, collimator-detector response, and scatter (ADS), and were reconstructed using the 3D FBP without and 3D OS-EM with ADS corrections followed by different cut-off frequencies of a 4D linear post-filter. A 4D iterative maximum a posteriori rescaled-block (MAP-RBI)-EM image reconstruction method with ADS corrections was also used to reconstruct the projection data using various values of the weighting factor for its prior. The trade-offs between bias and noise were represented by the normalized mean squared error (NMSE) and averaged normalized standard deviation (NSDav), respectively. They were used to select reasonable ranges of the reconstructed images for use in a human observer study. The observers were trained with the simulated cine images and were instructed to rate their confidence on the absence or presence of a motion defect on a continuous scale. We then applied receiver operating characteristic (ROC) analysis and used the area under the ROC curve (AUC) index. The results showed that significant differences in detection performance among the different NMSE-NSDav combinations were found and the optimal trade-off from optimized reconstruction parameters corresponded to a maximum AUC value. The 4D MAP-RBI-EM with ADS correction, which had the best trade-off among the tested reconstruction methods, also had the highest AUC value, resulting in significantly better human observer detection performance when detecting regional myocardial wall motion abnormality. We concluded that the NMSE-NSDav trade-off was shown to agree with observer performance for the detection task of the regional motion abnormality, and the optimized 4D MAP-RBI-EM method with ADS corrections provides significant improvement compared to 3D FBP and 3D OS-EM with ADS corrections in detecting regional myocardial wall motion abnormali in 4D gated MP SPECT.

Comparison of reconstruction methods and quantitative accuracy in Siemens Inveon PET scanner

PET reconstruction is key to the quantification of PET data. To our knowledge, no comparative study of reconstruction methods has been performed to date. In this study, we compared reconstruction methods with various filters in terms of their spatial resolution, non-uniformities (NU), recovery coefficients (RCs), and spillover ratios (SORs). In addition, the linearity of reconstructed radioactivity between linearity of measured and true concentrations were also assessed.A Siemens Inveon PET scanner was used in this study. Spatial resolution was measured with NEMA standard by using a 1 mm3 sized 18F point source. Image quality was assessed in terms of NU, RC and SOR. To measure the effect of reconstruction algorithms and filters, data was reconstructed using FBP, 3D reprojection algorithm (3DRP), ordered subset expectation maximization 2D (OSEM 2D), and maximum a posteriori (MAP) with various filters or smoothing factors (β). To assess the linearity of reconstructed radioactivity, image quality phantom filled with 18F was used using FBP, OSEM and MAP (β =1.5 & 5 × 10−5).The highest achievable volumetric resolution was 2.31 mm3 and the highest RCs were obtained when OSEM 2D was used. SOR was 4.87% for air and 3.97% for water, obtained OSEM 2D reconstruction was used. The measured radioactivity of reconstruction image was proportional to the injected one for radioactivity below 16 MBq/ml when FBP or OSEM 2D reconstruction methods were used. By contrast, when the MAP reconstruction method was used, activity of reconstruction image increased proportionally, regardless of the amount of injected radioactivity. When OSEM 2D or FBP were used, the measured radioactivity concentration was reduced by 53% compared with true injected radioactivity for radioactivity <16 MBq/ml.The OSEM 2D reconstruction method provides the highest achievable volumetric resolution and highest RC among all the tested methods and yields a linear relation between the measured and true concentrations for radioactivity Our data collectively showed that OSEM 2D reconstruction method provides quantitatively accurate reconstructed PET data results.

The development and initial evaluation of a realistic simulated SPECT dataset with simultaneous respiratory and cardiac motion for gated myocardial perfusion SPECT

We developed a realistic simulation dataset for simultaneous respiratory and cardiac (R&C) gated SPECT/CT using the 4D NURBS-based Cardiac-Torso (NCAT) Phantom and Monte Carlo simulation methods, and evaluated it for a sample application study. The 4D NCAT phantom included realistic respiratory motion and beating heart motion based on respiratory gated CT and cardiac tagged MRI data of normal human subjects. To model the respiratory motion, a set of 24 separate 3D NCAT phantoms excluding the heart was generated over a respiratory cycle. The beating heart motion was modeled separately with 48 frames per cardiac cycle for each of the 24 respiratory phases. The resultant set of 24 × 48 3D NCAT phantoms provides a realistic model of a normal human subject at different phases of combined R&C motions. An almost noise-free SPECT projection dataset for each of the 1152 3D NCAT phantoms was generated using Monte Carlo simulation techniques and the radioactivity uptake distribution of 99mTc sestamibi in different organs. By grouping and summing the separate projection datasets, separate or simultaneous R&C gated acquired data with different gating schemes could be simulated. In the initial evaluation, we combined the projection datasets into ungated, 6 respiratory-gates only, 8 cardiac-gates only, and combined 6 respiratory-gates & 8 cardiac-gates projection datasets. Each dataset was reconstructed using 3D OS-EM without and with attenuation correction using the averaged and respiratory-gated attenuation maps, and the resulting reconstructed images were compared. These results were used to demonstrate the effects of R&C motions and the reduction of image artifact due to R&C motions by gating and attenuation corrections. We concluded that the realistic 4D NCAT phantom and Monte Carlo simulated SPECT projection datasets with R&C motions are powerful tools in the study of the effects of R&C motions, as well as in the development of R&C gating schemes and motion correction methods for improved SPECT/CT imaging.

Multi-modal Ultrasound Imaging for Breast Cancer Detection

This work describes preliminary results of a two-modality imaging system aimed at the early detection of breast cancer. The first technique is based on compounding conventional echographic images taken at regular angular intervals around the imaged breast. The other modality obtains tomographic images of propagation velocity using the same circular geometry. For this study, a low-cost prototype has been built. It is based on a pair of opposed 128-element, 3.2MHz array transducers that are mechanically moved around tissue mimicking phantoms. Compounded images around 360° provide improved resolution, clutter reduction, artifact suppression and reinforce the visualization of internal structures. However, refraction at the skin interface must be corrected for an accurate image compounding process. This is achieved by estimation of the interface geometry followed by computing the internal ray paths. On the other hand, sound velocity tomographic images from time of flight projections have been also obtained. Two reconstruction methods, Filtered Back Projection (FBP) and 2D Ordered Subset Expectation Maximization (2D OSEM), were used as a first attempt towards tomographic reconstruction. These methods yield useable images in short computational times that can be considered as initial estimates in subsequent more complex methods of ultrasound image reconstruction. These images may be effective to differentiate malignant and benign masses and are very promising for breast cancer screening.

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.

Is the standard uptake value (SUV) appropriate for quantification in clinical PET imaging? – Variability induced by different SUV measurements and varying reconstruction methods

IntroductionPET quantification using the standard uptake value (SUV) is very prone to variations by technical factors of the scanner system and patient specific characteristics. Aim of the study was to investigate the reproducibility of SUV values between different measures and different reconstruction algorithms in a PET/CT scanner of the newest generation. MethodsThe time-of-flight PET datasets of 27 consecutive oncological patients were reconstructed with OSEM3D in two different matrix sizes (200×200 and 400×400) as well as in a matrix size of 400×400 and additional point-spread-reconstruction. The standardized uptake values SUVmax, SUVmean and SUVpeak in 60 lesions were compared concerning their variability in the three reconstructions. ResultsThe addition of point-spread-reconstruction causes a significant increase of SUV values in comparison to OSEM 3D. SUVpeak showed the highest reproducibility between the different reconstruction algorithms. The variability of SUVmax and SUVmean increases in small lesions <5ml, while SUVpeak remains more stable. ConclusionSUVmax, SUVmean and SUVpeak can be used for PET quantification in principle. However, quantification of small lesions is difficult. SUVpeak is the most robust method when using varying reconstruction methods, especially in small lesions.

Influences of reconstruction and attenuation correction in brain SPECT images obtained by the hybrid SPECT/CT device: evaluation with a 3-dimensional brain phantom.

The aim of this study was to evaluate the influences of reconstruction and attenuation correction on the differences in the radioactivity distributions in (123)I brain SPECT obtained by the hybrid SPECT/CT device. We used the 3-dimensional (3D) brain phantom, which imitates the precise structure of gray matter, white matter and bone regions. It was filled with (123)I solution (20.1 kBq/mL) in the gray matter region and with K2HPO4 in the bone region. The SPECT/CT data were acquired by the hybrid SPECT/CT device. SPECT images were reconstructed by using filtered back projection with uniform attenuation correction (FBP-uAC), 3D ordered-subsets expectation-maximization with uniform AC (3D-OSEM-uAC) and 3D OSEM with CT-based non-uniform AC (3D-OSEM-CTAC). We evaluated the differences in the radioactivity distributions among these reconstruction methods using a 3D digital phantom, which was developed from CT images of the 3D brain phantom, as a reference. The normalized mean square error (NMSE) and regional radioactivity were calculated to evaluate the similarity of SPECT images to the 3D digital phantom. The NMSE values were 0.0811 in FBP-uAC, 0.0914 in 3D-OSEM-uAC and 0.0766 in 3D-OSEM-CTAC. The regional radioactivity of FBP-uAC was 11.5% lower in the middle cerebral artery territory, and that of 3D-OSEM-uAC was 5.8% higher in the anterior cerebral artery territory, compared with the digital phantom. On the other hand, that of 3D-OSEM-CTAC was 1.8% lower in all brain areas. By using the hybrid SPECT/CT device, the brain SPECT reconstructed by 3D-OSEM with CT attenuation correction can provide an accurate assessment of the distribution of brain radioactivity.

Relative role of motion and PSF compensation in whole‐body oncologic PET‐MR imaging

Respiratory motion and partial-volume effects are the two main sources of image degradation in whole-body PET imaging. Simultaneous PET-MR allows measurement of respiratory motion using MRI while collecting PET events. Improved PET images may be obtained by modeling respiratory motion and point spread function (PSF) within the PET iterative reconstruction process. In this study, the authors assessed the relative impact of PSF modeling and MR-based respiratory motion correction in phantoms and patient studies using a whole-body PET-MR scanner. An asymmetric exponential PSF model accounting for radially varying and axial detector blurring effects was obtained from point source acquisitions performed in the PET-MR scanner. A dedicated MRI acquisition protocol using single-slice steady state free-precession MR acquisitions interleaved with pencil-beam navigator echoes was developed to track respiratory motion during PET-MR studies. An iterative ordinary Poisson fully 3D OSEM PET reconstruction algorithm modeling all the physical effects of the acquisition (attenuation, scatters, random events, detectors efficiencies, PSF), as well as MR-based nonrigid respiratory deformations of tissues (in both emission and attenuation maps) was developed. Phantom and(18)F-FDG PET-MR patient studies were performed to evaluate the proposed quantitative PET-MR methods. The phantom experiment results showed that PSF modeling significantly improved contrast recovery while limiting noise propagation in the reconstruction process. In patients with soft-tissue static lesions, PSF modeling improved lesion contrast by 19.7%-109%, enhancing the detectability and assessment of small tumor foci. In a patient study with small moving hepatic lesions, the proposed reconstruction technique improved lesion contrast by 54.4%-98.1% and reduced apparent lesion size by 21.8%-34.2%. Improvements were particularly important for the smallest lesion undergoing large motion at the lung-liver interface. Heterogeneous tumor structures delineation was substantially improved. Enhancements offered by PSF modeling were more important when correcting for motion at the same time. The results suggest that the proposed quantitative PET-MR methods can significantly enhance the performance of tumor diagnosis and staging as compared to conventional methods. This approach may enable utilization of the full potential of the scanner in oncologic studies of both the lower abdomen, with moving lesions, as well as other parts of the body unaffected by motion.

Positron emission tomography based in-vivo imaging of early phase stem cell retention after intramyocardial delivery in the mouse model

To establish PET as a tool for in-vivo quantification and monitoring of intramyocardially transplanted stem cells after labelling with FDG in mice with induced myocardial infarction. After inducing myocardial infarction in C57BL/6 mice, murine embryonic stem cells were labelled with FDG and transplanted into the border zone of the infarction. Dynamic PET scans were acquired from 25 to 120 min after transplantation, followed by a scan with 20 MBq FDG administered intravenously for anatomical landmarking. All images were reconstructed using the OSEM 3D and MAP reconstruction algorithms. FDG data were corrected for cellular tracer efflux and used as marker for cellular retention. FACS analysis of transplanted cells expressing enhanced green fluorescent protein was performed to validate the PET data. We observed a rapid loss of cells from the site of transplantation, followed by stable retention over 120 min. Amounts of retention were 5.3 ± 1.1 % at 25 min, 5.0 ± 0.9 % at 60 min and 5.7 ± 1.2 % at 120 min. FACS analysis showed a high correlation without significant differences between the groups (P > 0.05). FDG labelling did not have any adverse effects on cell proliferation or differentiation. Up-to-date imaging is a powerful method for tracking and quantifying intramyocardially transplanted stem cells in vivo in the mouse model. This revealed a massive cell loss within minutes, and thereafter a relatively stable amount of about 5 % remaining cells was observed. Our method may become crucial for further optimization of cardiac cell therapy in the widely used mouse model of infarction.

Comparison of integrated whole-body [11C]choline PET/MR with PET/CT in patients with prostate cancer

To evaluate the performance of conventional [(11)C]choline PET/CT in comparison to that of simultaneous whole-body PET/MR. The study population comprised 32 patients with prostate cancer who underwent a single-injection dual-imaging protocol with PET/CT and subsequent PET/MR. PET/CT scans were performed applying standard clinical protocols (5 min after injection of 793 ± 69 MBq [(11)C]choline, 3 min per bed position, intravenous contrast agent). Subsequently (52 ± 15 min after injection) PET/MR was performed (4 min per bed position). PET images were reconstructed iteratively (OSEM 3D), scatter and attenuation correction of emission data and regional allocation of [(11)C]choline foci were performed using CT data for PET/CT and segmented Dixon MR, T1 and T2 sequences for PET/MR. Image quality of the respective PET scans and PET alignment with the respective morphological imaging modality were compared using a four point scale (0-3). Furthermore, number, location and conspicuity of the detected lesions were evaluated. SUVs for suspicious lesions, lung, liver, spleen, vertebral bone and muscle were compared. Overall 80 lesions were scored visually in 29 of the 32 patients. There was no significant difference between the two PET scans concerning number or conspicuity of the detected lesions (p not significant). PET/MR with T1 and T2 sequences performed better than PET/CT in anatomical allocation of lesions (2.87 ± 0.3 vs. 2.72 ± 0.5; p = 0.005). The quality of PET/CT images (2.97 ± 0.2) was better than that of the respective PET scan of the PET/MR (2.69 ± 0.5; p = 0.007). Overall the maximum and mean lesional SUVs exhibited high correlations between PET/CT and PET/MR (ρ = 0.87 and ρ = 0.86, respectively; both p < 0.001). Despite a substantially later imaging time-point, the performance of simultaneous PET/MR was comparable to that of PET/CT in detecting lesions with increased [(11)C]choline uptake in patients with prostate cancer. Anatomical allocation of lesions was better with simultaneous PET/MR than with PET/CT, especially in the bone and pelvis. These promising findings suggest that [(11)C]choline PET/MR might have a diagnostic benefit compared to PET/CT in patients with prostate cancer, and now needs to be further evaluated in prospective trials.

TOF-PET Small-Lesion Image Quality Measured Over a Range of Phantom Sizes

The purpose of this study was to parameterize time-of-flight (TOF) image quality improvement over non-TOF PET using small lesions in a tapering phantom that represents a range of body sizes. A set of 46 1.0-cm spherical inserts was divided into six groups and positioned in a fillable, tapered phantom that represents a range of body dimensions (an oval cross section from 38.5 × 49.5 cm to 6.8 × 17.8 cm with a length of 51.1 cm). These hot spheres were positioned in a uniform, warm background (8:1 radioactivity concentration) and were distributed in six regions of different cross-sectional areas. Two PET/CT studies were acquired on a Discovery 690 PET/CT (GE Healthcare), which has a reported timing resolution of 650 ps. The total exam consisted of six frames (bed positions), with each frame centered on one of the sphere sets. Images were reconstructed with and without TOF information using 3D OSEM with 1-20 iterations of eight subsets, a 50-cm field of view, and a 128 × 128 × 47 image matrix with all corrections modeled in the iterative loop, and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</i> -axis smoothing, but no post-filter was applied. An automated algorithm was used to segment and place regions of interest on the hot spheres in the CT, and then these regions were applied to the PET images to measure contrast recovery and background variability (image noise). As the number of iterations increases for TOF and non-TOF iterative reconstruction, the ratio of contrast for TOF to non-TOF was ~1.0 for small body sizes, but the TOF contrast improvement ranges from 10%-25% for body sizes with diameters >30.0 cm. When comparing the small lesion signal-to-noise ratios between TOF and non-TOF images, the square of the SNR ratios for TOF to non-TOF was a linear function of body size and timing resolution that was parallel to the predicted improvement of TOF. The squares of the SNR ratios for TOF to non-TOF were linearly proportional to <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> /σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> , but with a negative offset. TOF image quality was equal to non-TOF for a diameter of ~17.5 cm.

Optimization of image reconstruction method for SPECT studies performed using [99mTc-EDDA/HYNIC] octreotate in patients with neuroendocrine tumors

Somatostatin receptor scintigraphy (SRS) is a useful tool in the assessment of GEP-NET (gastroenteropancreatic neuroendocrine tumor) patients. The choice of appropriate settings of image reconstruction parameters is crucial in interpretation of these images. The aim of the study was to investigate how the GEP NET lesion signal to noise ratio (TCS/TCB) depends on different reconstruction settings for Flash 3D software (Siemens). SRS results of 76 randomly selected patients with confirmed GEP-NET were analyzed. For SPECT studies the data were acquired using standard clinical settings 3-4 h after the injection of 740 MBq 99mTc-[EDDA/HYNIC] octreotate. To obtain final images the OSEM 3D Flash reconstruction with different settings and FBP reconstruction were used. First, the TCS/TCB ratio in voxels was analyzed for different combinations of the number of subsets and the number of iterations of the OSEM 3D Flash reconstruction. Secondly, the same ratio was analyzed for different parameters of the Gaussian filter (with FWHM = 2-4 times greater from the pixel size). Also the influence of scatter correction on the TCS/TCB ratio was investigated. With increasing number of subsets and iterations, the increase of TCS/TCB ratio was observed. With increasing settings of Gauss [FWHM coefficient] filter, the decrease of TCS/TCB ratio was reported. The use of scatter correction slightly decreases the values of this ratio. OSEM algorithm provides a meaningfully better reconstruction of the SRS SPECT study as compared to the FBP technique. A high number of subsets improves image quality (images are smoother). Increasing number of iterations gives a better contrast and the shapes of lesions and organs are sharper. The choice of reconstruction parameters is a compromise between image qualitative appearance and its quantitative accuracy and should not be modified when comparing multiple studies of the same patient.

MLEM and OSEM Deviate From the Cramer-Rao Bound at Low Counts

Maximum Likelihood (ML) reconstruction estimators are non biased and achieve the lowest variance, called the Cramer-Rao lower bound (CRLB), in the asymptotic regime, which in positron emission tomography (PET) or in single photon emission tomography (SPECT) corresponds to measuring an infinite number of counts. At finite number of counts or iterations, practical reconstruction algorithms are however biased, and nothing guarantees that the minimum variance expressed by the CRLB is achieved. We study the two dimensional Ordered Subsets Expectation Maximization (2D OSEM) algorithm with a finite number of counts and iterations, and investigate the question: given its bias, does this algorithm achieve the minimum variance predicted by the biased Cramer-Rao lower bound? We found a threshold separating two regimes: an asymptotic regime at large counts, where the variance almost equals the biased CRLB, even for a finite number of iterations and in cold regions, and a low counts regime where the variance significatively exceeds the biased CRLB. We extended our analysis to a realistic image by introducing the neighborhood method that evaluates the Cramer-Rao lower bound by inverting a submatrix of the Fisher matrix. We finally showed, both with a simulation and a theoretical toy example, that MLEM shares with OSEM the existence of a threshold in number of counts. A further analysis is needed to investigate the reason of the difference at low counts, which might indicate that there exists an algorithm with a smaller variance than OSEM for the same bias, or that a higher bound could be found.

Quantitative Comparison of Myocardial Perfusion Defects using Different Reconstruction Algorithms

Objective: To investigate the impact of a reconstruction method incorporating corrections for scatter, attenuation, and distance dependent detector response on cardiac single emission computed tomography (SPECT) perfusion studies compared with filtered back projection (FBP). Materials and methods: A total of 20 patients underwent same-day, rest-stress SPECT/CT myocardial perfusion imaging. Images were reconstructed using iterative 3D ordered-subsets expectation-maximization (OSEM 3D) and FBP algorithms. Stress and rest myocardial perfusion defects were quantified using polar maps and normal database comparisons. The Bland-Altman plots were used to assess their degree of agreement. Results were confirmed by coronary angiography. The contrast, contrast to noise ratio and signal to noise ratio were used for the quantitative evaluation of the reconstruction quality. Results: Perfusion defect extent quantification on OSEM 3D reconstructed images agreed and correlated well with defect extent quantification on FBP reconstructed images (bias ± standard deviation, -15% ± 20; r = 0.63) during stress and at rest (-10% ±15; r=0.70). Agreement and correlation were similar for severity scores during stress (-1.02 ± 1.77 SD’s; r=0.62), and at rest (-1.10 ± 1.49 SD’s; r=0.61). There were no statistically significant differences between the methods regarding perfusion defect extent or severity. The overall agreement rate with coronary angiography was similar. OSEM 3D reconstruction algorithm significantly increases the image contrast by 31% (P<0.05). Conclusions: Compensation for detector response, attenuation and scatter improves image contrast compared with FBP. Applying quantitative analysis, OSEM 3D reconstruction produced increased image contrast compared to FBP, but similar results regarding the size and severity of left ventricular perfusion defects.

A Simulation Study of the Effect of Phase-Shift on Dual Gated Myocardial Perfusion ECT.

We evaluated the effect of relative phase-shift of cardiac and respiratory (C&R) motions in myocardial perfusion (MP) ECT. Previously, we generated a set of realistic 3D XCAT (eXtended CArdiac Torso) phantoms that model simultaneous C&R motions for use in the study of new data acquisition methods and corrective image reconstruction techniques for improved gated MP ECT, including PET and SPECT. The respiratory motion (RM) over a respiration cycle was modeled using 24 equally-spaced time frames while the cardiac beating motion (CBM) over a cardiac cycle was divided into 48 equally-spaced time frames for each of the 24 RM phases. Almost noise-free projection datasets were generated separately from the heart, blood pool, lungs, liver, kidneys, stomach, gall bladder and remaining body at each of the 24 × 48 time points using Monte Carlo simulation techniques that include the effect of collimator detector response, photon attenuation and scatter. To demonstrate the effect of relative phase-shift, a typical 99mTc Sestamibi MP SPECT projection dataset were generated. They were then scaled and combined to model different degrees of relative C&R phase shifting and grouped into 6 respiratory-gates with 8 cardiac-gates. Each projection was reconstructed using a 3D OS-EM without and with attenuation correction using an averaged and phase-mismatched gated attenuation maps. The image artifacts of the reconstructed images were compared by visual inspection of the MP polar maps. The results showed significant changes of artifactual non-uniformity in the polar maps for off-phase of RM compared to those of CBM. The changes in the polar maps also demonstrated the effect of phase shifting accordingly. We conclude that the 4D XCAT phantom dataset with simultaneous C&R motions provides a powerful tool in the study of the effects of C&R motions with relative phase shifts, and development of C&R gating schemes and motion correction methods for improved ECT/CT imaging.

Generation and Evaluation of a Simultaneous Cardiac and Respiratory Gated Rb-82 PET Simulation.

The goal is to generate and evaluate a simulated 4D Rb-82 PET dataset that realistically models simultaneous respiratory and cardiac motions for use to study the effects of the motions and their compensation using various gating schemes. Normal cardiac and respiratory (C&R) motions were simulated separately using the realistic 4D XCAT phantoms. The C&R motion cycles were divided into 24 and 48 equally-spaced time points, respectively. The simultaneous dual motions were modeled by 24 × 48 phantoms with different combinations of C&R motion phases. Almost noise-free projections of the heart, blood pool, lungs, liver, stomach, spleen, and the remaining body were simulated separately using the combined SimSET and GATE Monte Carlo simulation program which is 12 times faster than GATE alone. The projections were scaled and combined to simulate a typical Rb-82 myocardial perfusion (MP) PET patient study. The no gating, 6-frame respiratory gating only, 8-frame cardiac gating only, and simultaneous 6-frame respiratory and 8-frame cardiac gating schemes were applied. Each gated projection dataset was reconstructed using a 2D OS-EM without and with attenuation correction (AC) using an averaged and gated attenuation maps. The reconstructed images were evaluated in terms of artifactual non-uniformity in the MP polar map. Significant artifactual non-uniformity was found in the MP polar map over all gating scheme without AC. With AC, the artifactual decreases in both the anterior and inferior regions were reduced with respiratory gating. Cardiac motion alone did not cause significant artifactual non-uniformity. In addition, the combination of dual gating and AC using the gated attenuation map provided the most uniform MP polar map. We demonstrated the flexibility and utility of the 4D XCAT phantom set with simultaneous C&R motions. It is a powerful tool to study motion effects on MP PET studies and to evaluate C&R gating schemes, AC and quantitative 4D PET image reconstruction methods.

Effects of injected dose, BMI and scanner type on NECR and image noise in PET imaging

Noise equivalent count rate (NECR) and image noise are two different but related metrics that have been used to predict and assess image quality, respectively. The aim of this study is to investigate, using patient studies, the relationships between injected dose (ID), body mass index (BMI) and scanner type on NECR and image noise measurements in PET imaging. Two groups of 90 patients each were imaged on a GE DSTE and a DRX PET/CT scanner, respectively. The patients in each group were divided into nine subgroups according to three BMI (20–24.9, 25–29.9, 30–45 kg m−2) and three ID (296–444, 444–555, 555–740 MBq) ranges, resulting in ten patients/subgroup. All PET data were acquired in 3D mode and reconstructed using the VuePoint HD® fully 3D OSEM algorithm (2 iterations, 21(DRX) or 20 (DSTE) subsets). NECR and image noise measurements for bed positions covering the liver were calculated for each patient. NECR was calculated from the trues, randoms and scatter events recorded in the DICOM header of each patient study, while image noise was determined as the standard deviation of 50 non-neighboring voxels in the liver of each patient. A t-test compared the NECR and image noise for different scanners but with the same BMI and ID. An ANOVA test on the other hand was used to compare the results of patients with different BMI but the same ID and scanner type as well as different ID but the same BMI and scanner type. As expected the t-test showed a significant difference in NECR between the two scanners for all BMI and ID subgroups. However, contrary to what is expected no such findings were observed for image noise measurement. The ANOVA results showed a statistically significant difference in both NECR and image noise among the different BMI for each ID and scanner subgroup. However, there was no statistically significant difference in NECR and image noise across different ID for each BMI and scanner subgroup. Although the GE DRX PET/CT scanner has better count rate performance than the GE DSTE PET/CT scanner, this improvement does not translate to a lower image noise when using OSEM reconstruction. Our results show that patients with larger BMI consistently generate poorer image quality. Dose reduction from >555 to 296–444 MBq has minimal impact on image quality independent of the scanner used. A reduction in ID decreases patient and technologist exposure and can potentially reduce the overall cost of the study.