Iterative Ordered Subsets Expectation Maximization Research Articles

Development of Solutions Based on Computer Simulation for Quantitative SPECT/CT

Purpose: In modern nuclear medicine, an ordered subset expectation maximization (OSEM) iterative algorithm is used to obtain images by single photon emission computed tomography (SPECT) method. At the same time, the problem of choosing the stopping iteration number at which the solution will be as close as possible to the exact value has not been solved. The purpose of this work is to study the problem of stopping the OSEM algorithm. Material and methods: The studies were carried out using the method of mathematical modeling based on the software package “Virtual Platform for Simulation Testing of the SPECT/CT Method”. Computer experiments simulated SPECT/CT with 99mTc phosphotech of a patient with metastatic lesion of the posterior segment of the third rib. The stopping criterion of the iterative OSEM algorithm was studied to obtain a solution that is quantitatively closest to the exact value of the accumulated activity at the tumor. Results: In computer modeling experiments, the tumor dependent Pearson chi-square test was shown to provide a solution that should give correct values for the SUVmean score. However, further research is needed on this important problem to determine how strongly the number of iterations depends on the size of the tumor and the amount of activity accumulated in it. Conclusion: Based on the results obtained, we can conclude that in the SPECT method, in order to move to accurate quantitative estimates of the accumulated activity in the tumor, it is necessary to develop a tumor-oriented stopping criterion for iterative algorithm. If there are several tumors, the stopping iteration numbers for them may differ. Further research on this important issue is planned.

Fewer-Angle SPECT/CT Blood Pool Imaging for Infection and Inflammation.

A new protocol for rapid SPECT/CT blood pool imaging consisting of fewer image-angle acquisitions (fewer-angle SPECT/CT, or FASpecT/CT) was evaluated for localization of focal sites of soft-tissue inflammation, infection, and osteomyelitis. Methods: Immediately after dynamic flow and standard planar blood pool imaging with 99mTc-methylene diphosphonate, FASpecT/CT was performed with a dual-head γ-camera consisting of 6 steps over 360°, 12 total images with 30° of separation between angles, and 30 s per image, requiring a total imaging time of approximately 3 min. Images were reconstructed using iterative ordered-subset expectation maximization. Before use in a patient-care setting, various FASpecT/CT acquisition protocols were modeled using a phantom to determine the minimum number of stops and the stop duration required to produce a reliable image. Results: FASpecT/CT images provided excellent 3-dimensional localization of spine osteomyelitis, soft-tissue infection of the foot, and tendonitis of the hand and foot using a 3-min image acquisition time. The FASpecT/CT acquisition protocol required 1.3-3.5 min, including camera movement time. This was a reduction of 72%-90% from the time required for the standard 60-angle, 20-s SPECT/CT acquisition. Conclusion: The ability of FASpecT/CT blood pool images to help localize focal sites of hyperemia and inflammation can increase exam sensitivity and specificity. Additionally, using a FASpecT/CT protocol decreases imaging time by up to 90%.

Impact of the size of the normal database on the performance of the specific binding ratio in dopamine transporter SPECT

BackgroundThis study investigated the impact of the size of the normal database on the classification performance of the specific binding ratio (SBR) in dopamine transporter (DAT) SPECT with [123I]FP-CIT in different settings.MethodsThe first subject sample comprised 645 subjects from the Parkinson’s Progression Marker Initiative (PPMI), 207 healthy controls (HC), and 438 Parkinson’s disease (PD) patients. The second sample comprised 372 patients from clinical routine patient care, 186 with non-neurodegenerative parkinsonian syndrome (PS) and 186 with neurodegenerative PS. Single-photon emission computed tomography (SPECT) images of the clinical sample were reconstructed with two different reconstruction algorithms (filtered backprojection, iterative ordered subsets expectation maximization (OSEM) reconstruction with resolution recovery). The putaminal specific binding ratio (SBR) was computed using an anatomical region of interest (ROI) predefined in standard (MNI) space in the Automated Anatomic Labeling (AAL) atlas or using hottest voxels (HV) analysis in large predefined ROIs. SBR values were transformed to z-scores using mean and standard deviation of the SBR in a normal database of varying sizes (n = 5, 10, 15,…, 50) randomly selected from the HC subjects (PPMI sample) or the patients with non-neurodegenerative PS (clinical sample). Accuracy, sensitivity, and specificity for identifying patients with PD or neurodegenerative PS were determined as performance measures using a predefined fixed cutoff on the z-score. This was repeated for 10,000 randomly selected normal databases, separately for each size of the normal database. Mean and 5th percentile of the performance measures over the 10,000 realizations were computed. Accuracy, sensitivity, and specificity when using the whole set of HC or non-neurodegenerative PS subjects as normal database were used as benchmark.ResultsMean loss of accuracy of the putamen SBR z-score was below 1% when the normal database included at least 15 subjects, independent of subject sample (PPMI or clinical), reconstruction method (filtered backprojection or OSEM), and ROI method (AAL or HV). However, the variability of the accuracy of the putamen SBR z-score decreased monotonically with increasing size of normal database and was still considerable at size 15. In order to achieve less than 5% “maximum” loss of accuracy (defined by the 5th percentile) in all settings required at least 25 to 30 subjects in the normal database. Reduction of mean and “maximum” loss of accuracy of the putamen SBR z-score by further increasing the size of the normal database was very small beyond size 40.ConclusionsThe results of this study suggest that 25 to 30 is the minimum size of the normal database to reliably achieve good performance of semi-quantitative analysis in dopamine transporter (DAT) SPECT, independent of the algorithm used for image reconstruction and the ROI method used to estimate the putaminal SBR.

Evaluation of the effect of reducing administered activity on assessment of function in cardiac gated SPECT

BackgroundWe previously optimized several reconstruction strategies in SPECT myocardial perfusion imaging (MPI) with low dose for perfusion-defect detection. Here we investigate whether reducing the administered activity can also maintain the diagnostic accuracy in evaluating cardiac function. MethodsWe quantified the myocardial motion in cardiac-gated stress 99m-Tc-sestamibi SPECT studies from 163 subjects acquired with full dose (29.8 ± 3.6 mCi), and evaluated the agreement of the obtained motion/thickening and ejection fraction (EF) measures at various reduced dose levels (uniform reduction or personalized dose) with that at full dose. We also quantified the detectability of abnormal motion via a receiver-operating characteristics (ROC) study. For reconstruction we considered both filtered backprojection (FBP) without correction for degradations, and iterative ordered-subsets expectation-maximization (OS-EM) with resolution, attenuation and scatter corrections. ResultsWith dose level lowered to 25% of full dose, the obtained results on motion/thickening, EF and abnormal motion detection were statistically comparable to full dose in both reconstruction strategies, with Pearson’s r > 0.9 for global motion measures between low dose and full dose. ConclusionsThe administered activity could be reduced to 25% of full dose without degrading the function assessment performance. Low dose reconstruction optimized for perfusion-defect detection can be reasonable for function assessment in gated SPECT.

Implementation of patient dosimetry in the clinical practice after targeted radiotherapy using [177Lu-[DOTA0, Tyr3]-octreotate

BackgroundThis study’s aim was to develop our dosimetric methodology using a commercial workstation for the routine evaluation of the organs at risk during peptide receptor radionuclide therapy (PRRT) with 177Lu.MethodsFirst, planar and SPECT sensitivity factors were determined on phantoms. The reconstruction parameters were optimized by SPECT/CT image acquisition using a NEMA IEC phantom containing a 500 ml bottle of 177Lu, to simulate a kidney. The recovery coefficients were determined on various phantoms. For the red marrow, this was calculated using a NEMA IEC phantom that contained a centrally placed bottle of 80 ml of 177Lu (to model the L2-L4 red marrow) flanked by two 200 ml bottles with 177Lu to simulate the kidneys.Then, SPECT/CT images were acquired at 4, 24, 72, and 192 h after injection in 12 patients with neuroendocrine tumors who underwent PRRT with 177Lu-DOTATATE. SPECT data were reconstructed using the iterative ordered subset expectation maximization (OSEM) method, with six iterations and ten subsets, attenuation, scatter, recovery resolution corrections, and a Gaussian post-filter of 0.11 cm. The liver, spleen, kidneys, and red marrow dose per administered activity (AD/A admin) values were calculated with the Medical Internal Radiation Dose (MIRD) formalism and the residence times (Dosimetry toolkit® application) using standard and CT imaging-based organ masses (OLINDA/EXM® V1.0 software).ResultsSensitivity factors of 6.11 ± 0.01 and 5.67 ± 0.08 counts/s/MBq were obtained with planar and SPECT/CT acquisitions, respectively. A recovery coefficient of 0.78 was obtained for the modeled L2–L4 red marrow. The mean AD/A admin values were 0.43 ± 0.13 mGy/MBq [0.27–0.91] for kidneys, 0.54 ± 0.58 mGy/MBq [0.12–2.26] for liver, 0.61 ± 0.13 mGy/MBq [0.42–0.89] for spleen, and 0.04 ± 0.02 mGy/MBq [0.01–0.09] for red marrow. The AD/A admin values varied when calculated using the personalized and standard organ mass, particularly for kidneys (p = 1 × 10−7), spleen (p = 0.0069), and red marrow (p = 0.0027). Intra-patient differences were observed especially in organs close to or including tumor cells or metastases.ConclusionsThe obtained AD/A admin values were in agreement with the literature data. This study shows the technical feasibility of patient dosimetry in clinical practice and the need to obtain patient-specific information.

Impact of muscular uptake and statistical noise on tumor quantification based on simulated FDG-PET studies

PurposeThe aim of this work is to study the effect of physiological muscular uptake variations and statistical noise on tumor quantification in FDG-PET studies. MethodsWe designed a realistic framework based on simulated FDG-PET acquisitions from an anthropomorphic phantom that included different muscular uptake levels and three spherical lung lesions with diameters of 31, 21 and 9mm. A distribution of muscular uptake levels was obtained from 136 patients remitted to our center for whole-body FDG-PET. Simulated FDG-PET acquisitions were obtained by using the Simulation System for Emission Tomography package (SimSET) Monte Carlo package. Simulated data was reconstructed by using an iterative Ordered Subset Expectation Maximization (OSEM) algorithm implemented in the Software for Tomographic Image Reconstruction (STIR) library. Tumor quantification was carried out by using estimations of SUVmax, SUV50 and SUVmean from different noise realizations, lung lesions and multiple muscular uptakes. ResultsOur analysis provided quantification variability values of 17–22% (SUVmax), 11–19% (SUV50) and 8–10% (SUVmean) when muscular uptake variations and statistical noise were included. Meanwhile, quantification variability due only to statistical noise was 7–8% (SUVmax), 3–7% (SUV50) and 1–2% (SUVmean) for large tumors (>20mm) and 13% (SUVmax), 16% (SUV50) and 8% (SUVmean) for small tumors (<10mm), thus showing that the variability in tumor quantification is mainly affected by muscular uptake variations when large enough tumors are considered. In addition, our results showed that quantification variability is strongly dominated by statistical noise when the injected dose decreases below 222MBq. ConclusionsOur study revealed that muscular uptake variations between patients who are totally relaxed should be considered as an uncertainty source of tumor quantification values.

The impact of reconstruction algorithms and time of flight information on PET/CT image quality.

BackgroundThe aim of the study was to explore the influence of various time-of-flight (TOF) and non-TOF reconstruction algorithms on positron emission tomography/computer tomography (PET/CT) image quality.Materials and methods.Measurements were performed with a triple line source phantom, consisting of capillaries with internal diameter of ∼ 1 mm and standard Jaszczak phantom. Each of the data sets was reconstructed using analytical filtered back projection (FBP) algorithm, iterative ordered subsets expectation maximization (OSEM) algorithm (4 iterations, 24 subsets) and iterative True-X algorithm incorporating a specific point spread function (PSF) correction (4 iterations, 21 subsets). Baseline OSEM (2 iterations, 8 subsets) was included for comparison. Procedures were undertaken following the National Electrical Manufacturers Association (NEMA) NU-2-2001 protocol.ResultsMeasurement of spatial resolution in full width at half maximum (FWHM) was 5.2 mm, 4.5 mm and 2.9 mm for FBP, OSEM and True-X; and 5.1 mm, 4.5 mm and 2.9 mm for FBP+TOF, OSEM+TOF and True-X+TOF respectively. Assessment of reconstructed Jaszczak images at different concentration ratios showed that incorporation of TOF information improves cold contrast, while hot contrast only slightly, however the most prominent improvement could be seen in background variability - noise reduction.ConclusionsOn the basis of the results of investigation we concluded, that incorporation of TOF information in reconstruction algorithm mostly affects reduction of the background variability (levels of noise in the image), while the improvement of spatial resolution due to incorporation of TOF information is negligible. Comparison of traditional and modern reconstruction algorithms showed that analytical FBP yields comparable results in some parameter measurements, such as cold contrast and relative count error. Iterative methods show highest levels of hot contrast, when TOF and PSF corrections were applied simultaneously.

SU-C-201-06: Utility of Quantitative 3D SPECT/CT Imaging in Patient Specific Internal Dosimetry of 153-Samarium with GATE Monte Carlo Package

Purpose: Patient-specific 3-dimensional (3D) internal dosimetry in targeted radionuclide therapy is essential for efficient treatment. Two major steps to achieve reliable results are: 1) generating quantitative 3D images of radionuclide distribution and attenuation coefficients and 2) using a reliable method for dose calculation based on activity and attenuation map. In this research, internal dosimetry for 153-Samarium (153-Sm) was done by SPECT-CT images coupled GATE Monte Carlo package for internal dosimetry. Methods: A 50 years old woman with bone metastases from breast cancer was prescribed 153-Sm treatment (Gamma: 103keV and beta: 0.81MeV). A SPECT/CT scan was performed with the Siemens Simbia-T scanner. SPECT and CT images were registered using default registration software. SPECT quantification was achieved by compensating for all image degrading factors including body attenuation, Compton scattering and collimator-detector response (CDR). Triple energy window method was used to estimate and eliminate the scattered photons. Iterative ordered-subsets expectation maximization (OSEM) with correction for attenuation and distance-dependent CDR was used for image reconstruction. Bilinear energy mapping is used to convert Hounsfield units in CT image to attenuation map. Organ borders were defined by the itk-SNAP toolkit segmentation on CT image. GATE was then used for internal dose calculation. The Specific Absorbed Fractionsmore » (SAFs) and S-values were reported as MIRD schema. Results: The results showed that the largest SAFs and S-values are in osseous organs as expected. S-value for lung is the highest after spine that can be important in 153-Sm therapy. Conclusion: We presented the utility of SPECT-CT images and Monte Carlo for patient-specific dosimetry as a reliable and accurate method. It has several advantages over template-based methods or simplified dose estimation methods. With advent of high speed computers, Monte Carlo can be used for treatment planning on a day to day basis.« less

Using adaptive neuro-fuzzy inference system technique for crosstalk correction in simultaneous 99mTc/201Tl SPECT imaging: A Monte Carlo simulation study

This work presents a simulation based study by Monte Carlo which uses two adaptive neuro-fuzzy inference systems (ANFIS) for cross talk compensation of simultaneous 99mTc/201Tl dual-radioisotope SPECT imaging. We have compared two neuro-fuzzy systems based on fuzzy c-means (FCM) and subtractive (SUB) clustering. Our approach incorporates eight energy-windows image acquisition from 28keV to 156keV and two main photo peaks of 201Tl (77±10%keV) and 99mTc (140±10%keV). The Geant4 application in emission tomography (GATE) is used as a Monte Carlo simulator for three cylindrical and a NURBS Based Cardiac Torso (NCAT) phantom study. Three separate acquisitions including two single-isotopes and one dual isotope were performed in this study. Cross talk and scatter corrected projections are reconstructed by an iterative ordered subsets expectation maximization (OSEM) algorithm which models the non-uniform attenuation in the projection/back-projection. ANFIS-FCM/SUB structures are tuned to create three to sixteen fuzzy rules for modeling the photon cross-talk of the two radioisotopes. Applying seven to nine fuzzy rules leads to a total improvement of the contrast and the bias comparatively. It is found that there is an out performance for the ANFIS-FCM due to its acceleration and accurate results.

Development and evaluation of an improved quantitative 90Y bremsstrahlung SPECT method

Yttrium-90 ((90)Y) is one of the most commonly used radionuclides in targeted radionuclide therapy (TRT). Since it decays with essentially no gamma photon emissions, surrogate radionuclides (e.g., (111)In) or imaging agents (e.g., (99m)Tc MAA) are typically used for treatment planning. It would, however, be useful to image (90)Y directly in order to confirm that the distributions measured with these other radionuclides or agents are the same as for the (90)Y labeled agents. As a result, there has been a great deal of interest in quantitative imaging of (90)Y bremsstrahlung photons using single photon emission computed tomography (SPECT) imaging. The continuous and broad energy distribution of bremsstrahlung photons, however, imposes substantial challenges on accurate quantification of the activity distribution. The aim of this work was to develop and evaluate an improved quantitative (90)Y bremsstrahlung SPECT reconstruction method appropriate for these imaging applications. Accurate modeling of image degrading factors such as object attenuation and scatter and the collimator-detector response is essential to obtain quantitatively accurate images. All of the image degrading factors are energy dependent. Thus, the authors separated the modeling of the bremsstrahlung photons into multiple categories and energy ranges. To improve the accuracy, the authors used a bremsstrahlung energy spectrum previously estimated from experimental measurements and incorporated a model of the distance between (90)Y decay location and bremsstrahlung emission location into the SIMIND code used to generate the response functions and kernels used in the model. This improved Monte Carlo bremsstrahlung simulation was validated by comparison to experimentally measured projection data of a (90)Y line source. The authors validated the accuracy of the forward projection model for photons in the various categories and energy ranges using the validated Monte Carlo (MC) simulation method. The forward projection model was incorporated into an iterative ordered subsets-expectation maximization (OS-EM) reconstruction code to allow for quantitative SPECT reconstruction. The resulting code was validated using both a physical phantom experiment with spherical objects in a warm background and a realistic anatomical phantom simulation. In the physical phantom study, the authors evaluated the method in terms of quantitative accuracy of activity estimates in the spheres; in the simulation study, the authors evaluated the accuracy and precision of activity estimates from various organs and compared them to results from a previously proposed method. The authors demonstrated excellent agreement between the experimental measurement and Monte Carlo simulation. In the XCAT phantom simulation, the proposed method achieved much better accuracy in the modeling (error in photon counts was -1.1 %) compared to a previously proposed method (errors were more than 20 %); the quantitative accuracy of activity estimates was excellent for all organs (errors were from -1.6 % to 11.9 %) and comparable to previously published results for (131)I using the same collimator. The proposed (90)Y bremsstrahlung SPECT reconstruction method provided very accurate estimates of organ activities, with accuracies approaching those previously observed for (131)I. The method may be useful in verifying organ doses for targeted radionuclide therapy using (90)Y.

Fully 3D iterative scatter-corrected OSEM for HRRT PET using a GPU

Accurate scatter correction is especially important for high-resolution 3D positron emission tomographies (PETs) such as high-resolution research tomograph (HRRT) due to large scatter fraction in the data. To address this problem, a fully 3D iterative scatter-corrected ordered subset expectation maximization (OSEM) in which a 3D single scatter simulation (SSS) is alternatively performed with a 3D OSEM reconstruction was recently proposed. However, due to the computational complexity of both SSS and OSEM algorithms for a high-resolution 3D PET, it has not been widely used in practice. The main objective of this paper is, therefore, to accelerate the fully 3D iterative scatter-corrected OSEM using a graphics processing unit (GPU) and verify its performance for an HRRT. We show that to exploit the massive thread structures of the GPU, several algorithmic modifications are necessary. For SSS implementation, a sinogram-driven approach is found to be more appropriate compared to a detector-driven approach, as fast linear interpolation can be performed in the sinogram domain through the use of texture memory. Furthermore, a pixel-driven backprojector and a ray-driven projector can be significantly accelerated by assigning threads to voxels and sinograms, respectively. Using Nvidia's GPU and compute unified device architecture (CUDA), the execution time of a SSS is less than 6 s, a single iteration of OSEM with 16 subsets takes 16 s, and a single iteration of the fully 3D scatter-corrected OSEM composed of a SSS and six iterations of OSEM takes under 105 s for the HRRT geometry, which corresponds to acceleration factors of 125× and 141× for OSEM and SSS, respectively. The fully 3D iterative scatter-corrected OSEM algorithm is validated in simulations using Geant4 application for tomographic emission and in actual experiments using an HRRT.

SU‐C‐211‐07: Quantitative Reconstruction of Multiplexed Multi‐Pinhole SPECT with Scatter and Attenuation Correction

Purpose: Small animal microSPECT is an important pre‐clinical imaging modality. However, the quantitative accuracy is limited by photon attenuation in the subject and scatter in the subject and the collimator. In this study we investigate correcting for both scatter and attenuation in the reconstruction of a small animal sized phantom. Methods: A phantom consisting of a cold centre surrounded by a hot shell, surrounded by an outer layer of either water or air, was scanned in a multiplexing multi‐pinhole (MMP) SPECT scanner. Scan parameters were as follows: 9 pinholes/detector, 2.5 mm diameter; 48 projections; helical scan; 3 minutes per projection. Projection data was recorded for two energy windows, one centered at the photopeak (140 keV) and a second window below the photopeak centered at 110 keV. The phantom was also scanned with the built‐in CT scanner (45 keV, cone beam) and reconstructed using filtered backprojection. The SPECT data were reconstructed using an iterative ordered subsets expectation maximization (OSEM) algorithm with no correction, attenuation correction (AC) only, and both scatter (SC) and AC. Attenuation was estimated from the CT and incorporated into the system matrix as part of the reconstruction. Scatter was estimated using the dual energy window method and subtracted from the projections prior to reconstruction. Absolute quantification was derived from a scan of a point source with known activity. Results: With attenuation and scatter correction, the measured activity concentrations in the hot region of the phantom were within 12% of the true value with the external chamber of the phantom both full and empty. Scatter correction in addition to AC improves the accuracy over AC alone in the cold regions. Conclusions: Attenuation correction significantly reduces the subject‐size dependence of the quantitative accuracy in small‐animal MMP SPECT. Scatter correction may provide some additional benefit.Heart and Stroke Foundation of Ontario (NA6374) National Science and Engineering Research Council of Canada (PGS D)

Poster - Thur Eve - 69: Attenuation Correction for 99m Tc-Tetrofosmin Micro SPECT/CT Cardiac Measurements in Rats

Objective: Small animal microSPECT is an important pre‐clinical imaging modality. However, the quantitative accuracy may be somewhat limited by photon loss due to attenuation in the body of the subject. Our goal is investigate the effects of including attenuation correction in the reconstruction of rat SPECT/CT data, and to assess the impact on the quantitative uptake in the myocardium and the uniformity of the distribution of uptake. Method: A rat was injected with ∼2.0 mCi of 99mTc‐tetrofosmin (Myoview) and scanned with a nanoSPECT scanner (BioScan, Washington, DC). Scan parameters were as follows: single, 1.5 mm pinhole, 4 hour scan duration commencing 30 minutes post injection, 48 projections, and circular scan. The rat was also scanned with the built‐in CTscanner (45 keV, cone beam). The CT data were reconstructed using filtered backprojection and rescaled to generate a linear attenuation map for the 140 keV 99mTc photons. The SPECT data were reconstructed using an iterative ordered subsets expectation maximization (OSEM) algorithm. Iterative reconstruction allows photon attenuation to be modelled as part of the reconstruction algorithm, and the reconstruction was done both with and without including attenuation. Results: As expected, inclusion of attenuation correction in the reconstruction increased by about 30% the total observed activity in the heart. However, we found no evidence that ignoring the attenuation correction introduces any significant artefacts in the uniformity of the reconstructed activity map. Conclusions: Including attenuation correction in the reconstruction of 99mTc cardiac images has little impact on the measured distribution of activity in the myocardium.

SU‐FF‐J‐51: The Impact of Source‐To‐Background Ratio, Tumor Size, Scan Duration, and Smoothing Filter On a Two‐Stage PET Tumor Segmentation Method

Purpose: This work systematically evaluates the effect of source‐to‐background ratio, tumor size, scan duration, and smoothing filter on a novel two‐stage PET tumor segmentation method in phantoms. The two‐stage method uses an adaptive region‐growing algorithm and a dual‐front active contour model, resulting in advantages of reproducibility and no need of choosing a threshold. Method and Materials: Six spherical tumor phantoms (0.5 – 20 mL) in a warm cylindrical container were scanned for 120 minutes in a PET/CT scanner. The source‐to‐background (S/B) ratio ranges from 16 to 0.5. For every 2‐min scan, three PET images were reconstructed using the iterative Ordered Subset Expectation Maximization (OSEM) algorithm with 5mm‐FWHM, 2mm‐FWHM post‐reconstruction smoothing filters and without the filter, respectively. At S/B ratio 2, images for longer scan durations were also generated. The two‐stage method was applied to segment the tumor phantoms with optimized parameters. Results: The segmentation accuracy depends mainly on the S/B ratio and tumor size, with a higher accuracy for larger tumor or higher S/B ratio. The 5mm‐FWHM filter reduces more noise than the 2mm‐FWHM filter and the unsmoothed. This leads to better segmentation for images smoothed with the 5mm‐FWHM filter. Lengthening scan duration did not improve the segmentation for tumors ⩾ 6 mL, but made the imperceptible 1 mL tumor detectable. The overall differences were not statistically significant between scan durations or between OSEM‐2mm and unsmoothed, but were statistically significant between OSEM‐5mm and unsmoothed. Conclusions: We concluded that a computerized PET tumor segmentation method should incorporate these factors (S/B ratio, tumor size, scan duration, and smoothing filter) if known or estimable. Secondly, it is very important to use the same PET/CT protocol for consistent quantitative evaluation. Finally, longer scan duration improves small tumor detection in PET.

Discussion generale : quelle technique d’imagerie pour le diagnostic de lesion peritoneale ?

The aim of this work is to study the effect of physiological muscular uptake variations and statistical noise on tumor quantification in FDG-PET studies.We designed a realistic framework based on simulated FDG-PET acquisitions from an anthropomorphic phantom that included different muscular uptake levels and three spherical lung lesions with diameters of 31, 21 and 9 mm. A distribution of muscular uptake levels was obtained from 136 patients remitted to our center for whole-body FDG-PET. Simulated FDG-PET acquisitions were obtained by using the Simulation System for Emission Tomography package (SimSET) Monte Carlo package. Simulated data was reconstructed by using an iterative Ordered Subset Expectation Maximization (OSEM) algorithm implemented in the Software for Tomographic Image Reconstruction (STIR) library. Tumor quantification was carried out by using estimations of SUVmax, SUV50 and SUVmean from different noise realizations, lung lesions and multiple muscular uptakes.Our analysis provided quantification variability values of 17–22% (SUVmax), 11–19% (SUV50) and 8–10% (SUVmean) when muscular uptake variations and statistical noise were included. Meanwhile, quantification variability due only to statistical noise was 7–8% (SUVmax), 3–7% (SUV50) and 1–2% (SUVmean) for large tumors (>20 mm) and 13% (SUVmax), 16% (SUV50) and 8% (SUVmean) for small tumors (<10 mm), thus showing that the variability in tumor quantification is mainly affected by muscular uptake variations when large enough tumors are considered. In addition, our results showed that quantification variability is strongly dominated by statistical noise when the injected dose decreases below 222 MBq.Our study revealed that muscular uptake variations between patients who are totally relaxed should be considered as an uncertainty source of tumor quantification values.

SU‐GG‐I‐112: PET Imaging of Heterogeneous Tumors: An Investigation of the Influence of Motion On Recovered Activity

Purpose: As multi‐day PET studies become part of targeted therapy planning and treatment assessment, it is important to characterize how positioning errors affect observed tumor heterogeneity. We have characterized the uncertainty in recovered‐activity in PET‐imaging of heterogeneous tumors as a function of axial position. Method and Materials: Recovery coefficient (RC) is ratio of maximum activity‐concentration to the true specific activity in an object. The uncertainty in RC was quantified by RCerror: the quotient . We measured RCerrors for a GE Discovery LS PET/CT scanner with an axial detector width of 4.25mm. A Heterogeneous‐tumor phantom, containing three spheres (15mm, 10mm, 5mm diameter), was scanned as the phantom was moved axially in 1mm intervals. RCerror of the spheres were determined in images reconstructed by iterative ordered subset expectation maximization (OSEM) and filtered backprojection (FBP) algorithms. The clinical effect was investigated using head and neck cancer (HN) patients imaged on this scanner. 18FLT‐PET images of patients were thresholded into three levels of relative uptake, and segmented into connected sub‐regions. We simulated multi‐scan studies and calculated the changes in SUV that result from repositioning the patient. Results: OSEM reconstruction provided more accurate recovery of activity in the spheres when compared to FBP. However, uncertainty of these OSEM values was larger, as RCerrors were 27%, 8% and 4% for the 5mm, 10mm and 15mm spheres respectively. In contrast, RCerrors were 10%, 8% and 3% for FBP. The effects of RCerrors were examined in clinical images; a positioning error of 5‐degree tilt caused 23% decrease in correlation within the tumor region of patient 1 and 12% in patient 2. Conclusion: Uncertainty in the RC depends on object diameter, reconstruction algorithm, and axial‐displacement. However, heterogeneities in FBP images of consecutive PET studies would be less susceptible to errors related to the axial positioning of tumors.

Comparison of 180° and 360° acquisition for myocardial perfusion SPECT with compensation for attenuation, detector response, and scatter: Monte Carlo and mathematical observer results

The optimal projection data acquisition strategy for myocardial perfusion (MP) single photon emission computed tomography (SPECT) remains controversial. We compared MP SPECT using 180 degrees and 360 degrees projection data obtained with the same acquisition time, reconstructed either with filtered back projection (FBP) or the iterative ordered-subsets expectation maximization (OS-EM) algorithm with various combinations of attenuation, detector response, and scatter compensation using mathematical observers and a myocardial defect detection task. We used Monte Carlo-simulated projection data from a population of 3-dimensional nurbs-based cardiac-torso (NCAT) phantoms with ranges of variability in patient anatomy, organ uptake, defect location, defect size, and noise level based on clinical data. Projection data from 180 degrees and 360 degrees acquisitions were generated by assuming the same acquisition time. After iterative or FBP reconstruction, standard postprocessing methods were applied. For each acquisition and reconstruction method, we optimized the number of iterations and cut-off frequency of the Butterworth filter using the Channelized Hotelling Observer methodology. The optimum set of parameters was that which gave the maximum area under the curve. For both acquisition protocols, OS-EM with compensations provided better performance than FBP or OS-EM without compensation. For FBP, the optimized 180 degrees acquisition provided a statistically significant increase in AUC as compared with optimized 360 degrees acquisition. For OS-EM, the AUCs for 180 degrees were slightly larger than for 360 degrees acquisitions when comparing images reconstructed with the same compensations. However, the differences were smaller and not statistically significant. With optimized reconstruction and filtering parameters, 180 degrees acquisition provided a statistically significant improvement over 360 degrees acquisition for FBP reconstruction. However, for OS-EM the differences were small and not statistically significant.

Iterative three‐dimensional expectation maximization restoration of single photon emission computed tomography images: Application in striatal imaging

Single photon emission computed tomography imaging suffers from poor spatial resolution and high statistical noise. Consequently, the contrast of small structures is reduced, the visual detection of defects is limited and precise quantification is difficult. To improve the contrast, it is possible to include the spatially variant point spread function of the detection system into the iterative reconstruction algorithm. This kind of method is well known to be effective, but time consuming. We have developed a faster method to account for the spatial resolution loss in three dimensions, based on a postreconstruction restoration method. The method uses two steps. First, a noncorrected iterative ordered subsets expectation maximization (OSEM) reconstruction is performed and, in the second step, a three-dimensional (3D) iterative maximum likelihood expectation maximization (ML-EM) a posteriori spatial restoration of the reconstructed volume is done. In this paper, we compare to the standard OSEM-3D method, in three studies (two in simulation and one from experimental data). In the two first studies, contrast, noise, and visual detection of defects are studied. In the third study, a quantitative analysis is performed from data obtained with an anthropomorphic striatal phantom filled with 123-I. From the simulations, we demonstrate that contrast as a function of noise and lesion detectability are very similar for both OSEM-3D and OSEM-R methods. In the experimental study, we obtained very similar values of activity-quantification ratios for different regions in the brain. The advantage of OSEM-R compared to OSEM-3D is a substantial gain of processing time. This gain depends on several factors. In a typical situation, for a 128 x 128 acquisition of 120 projections, OSEM-R is 13 or 25 times faster than OSEM-3D, depending on the calculation method used in the iterative restoration. In this paper, the OSEM-R method is tested with the approximation of depth independent resolution. For the striatum this approximation is appropriate, but for other clinical situations we will need to include a spatially varying response. Such a response is already included in OSEM-3D.

Assessment of the impact of model-based scatter correction on [18F]-FDG 3D brain PET in healthy subjects using statistical parametric mapping

It is recognized that scatter correction can supply more accurate absolute quantification, and that iterative reconstruction results in better noise properties and significantly reduces streak artefacts; however, it is not entirely clear whether they produce significant changes in [18F]-FDG distribution of reconstructed 3D brain PET images relative to not scatter corrected images and analytic reconstruction procedures. The current study assesses the effect of model-based scatter correction using the single-scatter simulation algorithm and iterative reconstruction in 3D brain PET studies, using statistical parametric mapping (SPM) analysis. The study population consisted of 14 healthy volunteers (6 males, 8 females; age 63–80 years). PET images were reconstructed using an analytic 3DRP reprojection algorithm with (SC) and without explicit scatter correction (NSC), as well as using an iterative ordered subset–expectation maximization (OSEM) algorithm. Calculated attenuation correction was performed assuming uniform attenuation (μ = 0.096 cm−1) for brain tissues when data are precorrected for scatter. The broad-beam attenuation coefficient (μ = 0.06 cm−1) determined from phantom studies was applied to NSC images. The images were coregistered and normalized using the default [15O]-H2O template supplied with SPM99 and an [18F]-FDG template. A t statistic image for the contrast condition effect was then constructed. The contrast comparing SC to NSC images suggest that regional brain metabolic activity decreases significantly in the frontal gyri, in addition to the middle temporal and postcentral gyri. On the other hand, activity increases in the cerebellum, thalamus, insula, brainstem, temporal lobe, and the frontal cortex. No significant changes were detected when comparing images reconstructed using analytic and iterative algorithms. It is concluded that, for some cerebral areas, significant differences in [18F]-FDG distribution arise when images are reconstructed with and without explicit SC. This needs to be considered when interpreting [18F]-FDG 3D brain PET images after applying SC.

Parallel-beam tilted-head analytic SPECT reconstruction: derivation and comparison with OSEM

Parallel-beam tilted-head single photon emission computed tomography (TH-SPECT) was previously implemented on a SPECT system for its potential to image breast lesions and nearby axilla of seated, upright women. All TH-SPECT reconstructions will contain artifacts since the tilted orbit does not satisfy the Orlov sampling criteria. However, it is not clear which reconstruction method, if any, is better suited for TH-SPECT data. Here a geometric derivation of the ramp filter for tilted parallel-beam geometries is presented. A filtered backprojection (FBP) algorithm, using this filter, was then implemented and compared with an iterative ordered subsets expectation maximization (OSEM) algorithm, using TH-SPECT data. A breast scan at various tilt angles was simulated and used to generate a noise versus bias study for both methods. Contrast and signal-to-noise ratio (SNR) values, as well as axial elongation present in all TH-SPECT reconstructions, were characterized using a mini-Defrise disk phantom placed inside a fillable breast phantom and imaged from 0 to 15/spl deg/ head tilt. A fillable breast phantom containing lesions was also imaged with a system dedicated to prone breast SPECT from 0 to 30/spl deg/ to evaluate the effects of incomplete sampling due to greater tilt angles. FBP noise versus bias studies indicated a greater increase in bias with tilt angle compared to OSEM reconstructions. At small tilt angles about the mini-Defrise disk phantom, poorer contrasts were obtained with FBP compared to OSEM at similar noise levels. All reconstructions of the fillable breast phantom indicated axial elongation at greater tilt angles, although FBP reconstructions displayed an increase in stretching distortions of the breast. OSEM SNR and contrast values were higher at all degrees of tilt. In conclusion, measured results indicate OSEM TH-SPECT reconstruction provides better contrast and SNR values and may offer better shape and uniform activity distribution of the breast compared to FBP methods.