The HyPET project proposes a hybrid dedicated time of flight-positron emission tomography for prostate imaging, with pixelated detector blocks in the front layer and monolithic blocks in the back layer. In this work, four detector configurations have been experimentally evaluated for the rear detector layer. The detector configuration consists of LYSO monolithic blocks with the same size (25.4 mm $\times25.4$ mm) but different thicknesses (5, 7.5, 10, and 15 mm) coupled to the same silicon photomultiplier array. Each detector configuration has been experimentally characterized in terms of time, energy, and spatial resolution by scanning the crystal surface using a fan beam in steps of 0.25 mm. Regarding spatial resolution, the interaction position was estimated using a neural network technique. All resolutions except energy, which remains nearly constant at 17% for all cases, show better values for the 5-mm detector thickness. We have achieved spatial resolution values of full width at half of the maximum of 1.02 ± 0.10, 1.19 ± 0.13, 1.53 ± 0.17, and 2.33 ± 0.55 mm, for 5, 7.5, 10, and 15 mm blocks, respectively. The detector time resolution obtained was 275 ± 26, 291 ± 21, 344 ± 48, and 433 ± 45 ps, respectively, using the energy-weighted average method for the time stamps.

Purpose: The goal of this work is to create a user-friendly, personalized remote radiation tracking portable organ dosimetry device (PRRT PODD) that will enable in home radiation monitoring of a patient’s organs at risk (OAR) after receiving Lu-177 DOTATATE treatment. Remote monitoring will allow personalization based upon dose to a patient’s OAR.Methods: The PRRT PODD is an affordable, portable device designed for at home use. It consists of a light-weight frame; a patient specific registration pad; a set of compact radiation detectors; and electronics. We tested a prototype version of the PRRT PODD using an anthropomorphic phantom. The phantom was placed in the PRRT PODD (without detectors) and a CT scan was acquired. This CT scan registered the PODD with the internal "organs" of the phantom.Proprietary software was used to determine where to place up to 16 small radiation detectors on the frame of the PODD to obtain quantitative radiation activity measurements from the OAR in the phantom. Compact, custom electronics have been built for signal processing, data acquisition and data transfer to a laptop computer. For the phantom experiment, washout rates for the OAR, i.e., liver, spleen, right and left kidneys, were set to 68 hr, 72 hr, 45 hr and 48 hr, respectively.Results: Using data from 14 "daily" measurements, the difference between the true and estimated washout rates for the liver and spleen were 5.0% and 6.9%, respectively. Using data for the first 5 measurements, the difference between the true and estimated washout rates for the right and left kidneys were 5.3% and -2.7%, respectively.Conclusions: The first experimental phantom testing of a prototype PRRT PODD device provided excellent results for OAR washout estimation. Accuracy of results can be further improved by better matching PRRT PODD template size to the object being monitored and by tuning the collimation of the detector probes.

The goal of this project is to develop hybrid PET (HyPET) detector technology that will enable moderate cost, high resolution, prostate specific PET imaging. The optimism for an imaging based prostate cancer (PCa) biomarker is being driven by the development of highly targeted PET tracers for PCa cells. Monte Carlo simulations and a mixed data-type weighted least squares (MDT-WLS) image reconstruction algorithm will be used to investigate the imaging performance of different hybrid PET detector systems. HyPET detectors will consist of a front detector optimized for coincidence timing performance (TOF HiRes ) and a rear detector optimized for spatial resolution, depth of interaction (DOI) positioning and detection efficiency. The MDT-WLS software uses the TOF HiRes data to reconstruct an image prior that is then used as a preconditioner for reconstructing the front-back and back-back coincidence events. Detector performance characteristics were modeled based upon experimentally measured performance results. Front detector designs included discrete crystal arrays composed of 3x3x5 mm 3 or 1x1x3(5) mm 3 LYSO crystals coupled to monolithic MPPC arrays. Back layer detector designs included monolithic crystals with 5, 7.5, 10 or 15 mm thick. The monolithic crystal detectors were characterized for spatial resolution, DOI resolution and timing resolution. A 28 cm diameter cylindrical phantom with a prostate-sized (i.e., 4x3 cm 2 ellipse by 2 cm tall) insert was simulated. In addition, a set of six 2 mm diameter spheres and a set of six 3 mm diameter spheres each arranged in a triangular matrix within the prostate phantom was used to assess the image resolution and contrast to noise ratio between the spheres and background prostate uptake. The contrast ratios between sphere and prostate, and prostate and background were 10 and 2, respectively. Initial images were reconstructed. The tools are now in place for optimization of a dedicated prostate PET camera.

56 Objectives: Surprisingly it is unknown what sensitivity gains in noise equivalent counts (NEC) are equivalent to an improvement in time-of-flight (TOF) resolution, or vice-versa. For example, when using a standard clinical protocol, one system may have an NEC of 158 kcps and a TOF resolution of 385 ps, while a second system may have an NEC of 49 kcps and a TOF resolution of 325 ps, and it is unknown a priori how the relative task performance will compare if all other factors are equal. We present initial evaluations with task-based metrics for lesion detection using simulations of a phantom that can potentially be used in practice. Methods: We used SimSET to produce calibrated simulations of a PET/CT scanner (5-ring GE DMI). Calibrated values were the prompt, random, scattered, and true coincidence rates used in NEC calculations. The simulated ACR PET phantom was modified to allow for high uptake regions (i.e. the 9hot rods9 region) to be in a warm background, as has been demonstrated using porous plastics (Wollenweber 2014). We used the same image reconstruction algorithms as used on the scanner but modified to allow for different TOF resolutions. Parameters varied included total NEC (38, 72, 107, 143, 179 Mcts), TOF resolution (193, 386, 550 ps), contrast ratio (1.23, 1.45), and rod diameter (4.8, 6.4, 7.9, 9.5, 11.1, 12.7 mm). Regions of interest (ROIs) were measured using the known true locations for the rods and the background regions. Detectability (d9) was calculated using the approach (described by Wollenweber et al., 2016) and Difillipo (2019), which is essentially a t-test between rod and background ROI values with the noise determined by the replicated rod and background ROIs. The d9 values were converted to the area under the receiver operating curve (AUC) values using the Gaussian approximation approach. Error bars were determined by using separated axial regions for repeated calculations of the AUC values. To determine the relative impact of PET timing resolution versus NEC on image quality, the AUC values were used to generate iso-contour plots for fixed AUC values as a function of timing resolution and NEC. Results: For the 4.8 mm rods the AUC ranged from 0.57 to 0.79 for the variations studied. There was an increase in AUC with increasing counts and also with improved timing resolution. This relative behavior in AUC was consistent and with overall increases in AUC as contrast and road diameter values increased. As the rod diameter exceeded 7.9 mm, the AUC quickly approached values of 1.0. Thus the range where there were meaningful differences in AUC as a function of contrast was relatively narrow. The coefficient of variation for the AUC results was typically 15%. In comparing the impact of PET timing resolution versus NEC on detectability, the 4.8 mm rods showed a roughly linear trade-off in PET timing resolution versus NEC for AUCs ranging from 0.575 to 0.75. Within the linear regime the ratio of NEC /TOF ranged from 0.07 to 0.18 Mcts/ps. As a specific example, for a contrast of 1.45 with 4.8 mm rods, a detection AUC of 0.70 is achieved with an (NEC rate, TOF resolution) performance of either (89 kcps, 200ps) or (121 kcps, 400 ps). In other words a 26% increase in NEC rate was equivalent to a 100% improvement in TOF resolution. This linear behavior became less regular as the AUC approached 1.0, i.e. when rod diameter and/or contrast levels increased. In these regimes, e.g. with AUC > 0.8, the AUC was largely dependent on NEC and independent of TOF resolution. Conclusions: The relative effects of PET timing resolution versus NEC on image quality as determined by detectability are linear for lower AUC values, with NEC sensitivity having a larger impact than TOF resolution. As AUCs approach 1.0 the tradeoffs become non-linear with relatively little effect from TOF resolution. These trade-offs have implications for both PET scanner design and system performance for clinical tasks.

1139 Objectives: Internal radiation therapies are usually personalized by treating to the dose limit of the patient’s main organ at risk (OAR). For Lu-177 DOTATATE, possible OAR are kidneys, bone marrow, liver and spleen, with kidneys the main OAR in a vast majority of patients. We previously introduced the concept of a personalized remote radiation tracking (PRRT) vest to enable customization of Lu-177 DOTATATE therapy for neuroendocrine tumor patients. The goal of the PRRT vest is to enable monitoring of the washout kinetics of Lu-177 DOTATATE from a patient’s OAR without requiring serial visits to a medical center. The monitoring would occur within the comfort of their own home. In this work, we seek to validate our PRRT software tools and personalized detector vest implementation through experimental testing using an in-house built anthropomorphic phantom, where our goal is to estimate washout kinetics for the kidneys within 5%. Methods: We fabricated a 25 cm diameter by 30 cm tall right circular cylinder phantom with anthropomorphic objects representing liver, spleen, right and left kidneys. The phantom inserts were 3D printed and had fill ports to allow additional activity to be added to the different structures without having to disassemble the phantom. A CT scan of the phantom was performed to register the 3D internal organs with a vest-like covering containing a CT visible grid wrapped around the phantom. Simulations were then run to determine the optimal placement of up to 12 small detectors to estimate the washout kinetics for each of the organs/background. A housing to situate 12 detectors around the cylinder was then created based upon the PRRT software results. Each detector was housed in a 14x14x14 mm^3 tungsten alloy box with 3 mm thick sides, an open back to place the sensor and a 4 mm pin-hole in the front plate. Each detector housed a 6x6x3 mm^3 crystal of GAGG coupled to a 6x6 mm^2 MPPC device. The right kidney, left kidney, liver, spleen and background compartments were filled sequentially, with relative activity concentrations of 20, 22, 4.5, 27, 2 for each object, respectively. For these initial experiments, Tc-99m was used in place of Lu-177. Data were collected from all of the sensors after each organ and the background were filled. The sensitivity map between each organ of interest/background and the 12 custom placed detectors was determined from the collected data. The half-lives for the washout kinetics for the right kidney, left kidney, liver, spleen and background were set to 48 hrs, 45 hrs, 65 hrs, 72 hrs and 97 hrs, respectively. Poisson sampling was used for accurate noise modeling of the collected counts for each organ and background. The composite signals for each of the 12 detectors over 21 days were then provided to our PRRT analysis software to estimate the washout kinetics of each individual organ and background from the composite detector signals. Results: Using data from 7, 14 or 21 days of measurements, the estimate of the half-life of the washout from each of the organs and background was always within 4%. Maximum error was 3.8% for the left kidney. Conclusions: Using an in-house built anthropomorphic phantom and our PRRT Vest software tools and experimental protocol, we were able to estimate the washout kinetics for the kidneys, liver, spleen and background to within 4% of the true decay rate for each of the organs of interest validating our simulation and PRRT vest methodologies.

Purpose Physical and digital phantoms play a key role in the development and testing of nuclear medicine instrumentation and processing algorithms for clinical and research applications, including neuroimaging using positron emission tomography (PET). We have developed and tested a digital reference object (DRO) version of the original segmented magnetic resonance imaging (MRI) data used for the three-dimensional (3D) PET brain phantom developed by Hoffman et al., which is used as the basis of a commercially available physical test phantom. Methods The DRO was constructed by subdividing the MRI image planes the original phantom was based on to create equal-thickness slices and re-labeling voxels. The digital data was then embedded in a PET Digital Imaging and Communications in Medicine format and tested for compliance. Results We then tested the DRO by comparing it to computed tomography (CT) images of the physical phantom summed to form composite slices with axial extent similar to the DRO, but with a factor of two better in-slice resolution. For composite slices, 91% of voxels were labeled in full agreement, 5% of the voxels were 50-75% accurate, and the remaining 4% of voxels had 25% or less agreement. Conclusions This DRO can be used as an input for PET scanner simulation studies or for comparing simulations to measured Hoffman phantom images.

We investigated PET image quantification when using a uniform attenuation coefficient (μ) for attenuation correction (AC) of anthropomorphic density phantoms derived from high-resolution breast CT scans. A breast PET system was modeled with perfect data corrections except for AC. Using uniform μ for AC resulted in quantitative errors roughly proportional to the difference between μ used in AC (μ AC) and local μ, yielding approximately ± 5% bias, corresponding to the variation of μ for 511 keV photons in breast tissue. Global bias was lowest when uniform μ AC was equal to the phantom mean μ (μ mean). Local bias in 10-mm spheres increased as the sphere μ deviated from μ mean, but remained only 2-3% when the μ sphere was 6.5% higher than μ mean. Bias varied linearly with and was roughly proportional to local μ mismatch. Minimizing local bias, e.g., in a small sphere, required the use of a uniform μ value between the local μ and the μ mean. Thus, biases from using uniform-μ AC are low when local μ sphere is close to μ mean. As the μ sphere increasingly differs from the phantom μ mean, bias increases, and the optimal uniform μ is less predictable, having a value between μ sphere and the phantom μ mean.

139 Objectives: The overall goal of this work is to develop wearable, personalized remote radiation tracking (PRRT) technology to enable customization of Lu-177 DOTATATE therapy in a cost-effective and patient-friendly manner. Internal radiation therapies are optimized by treating to the dose limit of the patient’s main organ at risk (OAR). For Lu-177 DOTATATE, the OAR are kidneys, bone marrow, liver and spleen, with kidneys the main OAR in a vast majority of patients. We previously introduced the concept of a PRRT vest for at home radiation monitoring of Lu-177 DOTATATE. In this work, we seek to further characterize the vest’s performance investigating the impact radiation detector geometry and repositioning offsets between daily wearing of the vest have on quantitative estimation of Lu-177 DOTATATE washout kinetics. Methods: The PRRT vest utilizes a sparse arrangement of small scintillator-based detectors. Pinhole collimation is used to shape the cone of response of each detector. The detectors are then strategically placed within a vest to measure radioactivity from OAR (e.g., kidneys, liver, spleen), tumors and background. The patient will wear the PRRT vest for ~2 minutes a day for up to three weeks. Using Monte Carlo simulation and digital phantoms, we investigated how the detectors’ pinhole diameter (2, 3 or 4 mm), the collimator geometry (i.e., air gap between pinhole and GAGG detector), the number of measurements (i.e., over many days), and the daily detector repositioning offsets affect estimation performance. Random daily positioning offsets up to 17 mm from the prescribed detector/vest location were simulated. The testing phantom was a 27 cm diameter right circular cylinder with anthropomorphic objects representing liver, spleen, right and left kidneys and two tumors (2.5 cm and 1.5 cm diameter) placed in the liver. Initial activity concentration and washout rates for liver, spleen, right and left kidneys, 2.5 cm and 1.5 cm tumors, and background were 4, 21, 24, 27, 166, 81.5 and 2.5 µCi/cc, and 69, 72, 50, 54, 88, 93 and 90 hrs, respectively. Tests were run for vests with up to 15 sensors. Activity in each object was decayed appropriately. Poisson sampling was used for accurate noise modeling. Decay rates were estimated using 3-21 measurements over 7-21 days. Results: The average error of the estimated washout for all OAR for all testing conditions was less than 6% when there was no repositioning offsets between daily measurements. Using a 4 mm diameter pinhole collimator and acquiring measurements for 21 days mitigated the effects of random daily repositioning and kept the error in washout estimation less than 5% for all OAR and the largest tumor for a maximum daily repositioning offset of ±8.5 mm and less than 5% for the largest tumor and all OAR, except the spleen, for a maximum daily repositioning offset of ±17 mm. Our expectation is that most individuals will be able to reposition the vest within ±8.5 mm. Dosimetry errors for the smallest tumors was larger than 10% for some test cases; however, the average error for the kidneys (the main OAR) remained less than 3% even under the most challenging testing conditions. Conclusions: Taking measurements for 21 days led to the most accurate estimates of radiotracer washout, especially when random repositioning offsets were included in the study. The detectors with the 4 mm pinhole size provided the best overall performance.

It is important to predict and understand how scanner design characteristics will affect image quality relevant to clinical tasks when developing a new system. It is a complex task to understand the interplay among multiple system parameters such as time-of-flight (TOF) resolution, sensitivity, intrinsic spatial resolution, and depth-of-interaction (DOI) resolution. This can be achieved by Monte Carlo (MC) simulation. Here we perform MC simulation using SimSET to simulate brain-dedicated positron emission tomography (PET) scanners with varying system parameters such as DOI resolution, intrinsic spatial resolution and TOF resolution. We report preliminary results from simulating various phantoms. The results show that DOI information greatly improves image resolution particularly in the peripheral regions, reducing parallax errors even without point spread function modeling, that smaller detector elements improve image resolution, and that TOF information lowers image noise. The MC simulation analysis provides useful results for optimizing detector geometry and hardware for high resolution neuro-PET scanner design.

Purpose: Currently, single-photon emission computed tomography (SPECT)/computed tomography (CT) lung phantoms are commonly constructed using polystyrene beads and interstitial radioactive water. However, this approach often results in a phantom with a density (typically -640 HU) that is considerably higher than that of healthy lung (-750 to -850 HU) or diseased lung (-900 to -950 HU). Furthermore, the polystyrene and water phantoms are often quite heterogeneous in both density and activity concentration, especially when reused. This work is devoted to examining methods for creating a more realistic lung phantom for quantitative SPECT/CT using Tc-99m-laced expanding polyurethane foam (EPF). Methods: Numerous aspects of EPF utilization were studied, including stoichiometric mixing to control final foam density and the effect of water during growth. We also tested several ways of molding the foam lung phantoms. The most successful method utilized a three-part silicone mold that allowed for creation of a two-lobe phantom, with a different density and activity concentration in each lobe. Results: The final phantom design allows for a more anatomically accurate geometry as well as customizable density and activity concentration in the different lobes of the lung. We demonstrated final lung phantom densities between -760 and -690 HU in the "healthy" phantom and -930 to -890 HU in the "unhealthy" phantom tissue. On average, we achieved 15% activity concentration nonuniformity and 12% density nonuniformity within a given lobe. Conclusions: Final EPF lung phantoms closely matched the densities of both health and diseased lung tissue and had sufficient uniformities in both density and activity concentration for most nuclear medicine applications. Management of component moisture content is critical for phantom reproducibility. (C) 2019 American Association of Physicists in Medicine

313 Objectives: For patients with metastatic, somatostatin-receptor-2 positive neuroendocrine tumors (NETs), targeted therapy using 177Lu-DOTATATE greatly increases progression-free survival. Now that 177Lu-DOTATATE has received FDA approval it likely will become the standard of care for symptomatic NET patients and those with metastatic spread in the United States. However, FDA package instructions call for patients to receive a standardized protocol of four 7.4 GBq treatments, regardless of size or weight. Traditionally, targeted radionuclide therapies are personalized based upon dose to the main organs at risk (OAR, e.g., kidneys, liver, spleen). Standardized therapy is counter to the ideals of personalized medicine and will lead to non-optimum therapeutic dosing for many patients. Traditional imaging-based methods for organ dosimetry estimation for 177Lu require 3-4 longitudinal imaging sessions spread over 7 days. This is expensive, utilizes a lot of clinic resources and is burdensome to the patient. Objectives: The goal of this project is to enable precise individualized organ dosimetry without requiring serial imaging sessions. We will accomplish this by developing patient friendly, wearable monitoring technology to allow quantitative measurements to be made by the patient at home. Specifically, we propose a multi-detector personalized home dosimetry (MD PHD) vest that will allow patients to track organ specific radiotracer washout without requiring serial return visits to an imaging clinic. The protocol will still require one SPECT/CT taken 24 hours after administration of 177Lu-DOTATATE. This will be followed by at home MD PHD vest measurements taken daily for 7 to 21 days. The SPECT/CT measurement provides a quantitative measure of 177Lu-DOTATATE uptake. The vest measurements provide accurate estimation of the washout from OAR. With this information, physicians can tailor the number of treatments based upon personalized organ dosimetry information. Methods: The MD PHD vest will utilize a sparse set (e.g., 15) of small detectors (e.g., 1.2 cm3) optimally placed within a personalized vest to collect data from OAR over many days. A CT image will be used to register the vest with a patient’s internal organs. Monte Carlo simulations will be used to determine the sensitivity matrix between a 2D array of detectors surrounding the patient and the patient’s OAR. Optimization methods will be used to select the positioning of detectors around the patient. The patient will wear the vest for 2 minutes once a day for 7-21 days. The basic methodology was tested using Monte Carlo simulation. The testing phantom consisted of a 27cm diameter by 70cm long right circular cylinder. Anthropomorphic objects representing the liver, spleen, right and left kidneys and two tumors (i.e., 2.5 cm and 1.5 cm diameter) were placed in the phantom. The ratios of the activity concentration for liver, spleen, right and left kidneys, and the 2.5 cm and 1.5 cm tumors to background were 2, 10, 10, 8, 48, 60, respectively. The washout rate for liver, spleen, right kidney, left kidney, 2.5 cm tumor, 1.5 cm tumor and background were 74, 66, 40, 46, 92, 96 and 95 hrs, respectively. Tests were run for vests with 15 sensors. Simulated acquisition times were 2 minutes. Activity in each object of interest was decayed appropriately. Poisson resampling was used for accurate noise modeling. Decay rates were estimated using 7 and 21 daily vest measurements. Results: The root mean square error (RMSE) of the estimated washout rate for each of the OAR and tumors using the optimized 15 sensor vests are provided in Table 1. Using all 21 measurements the RMSE was less than 3.3% for all objects. Using only 7 measurements, the organ estimates were still less than 4% but RMSE increased for the tumors. Conclusions: Initial simulation studies indicate that a very sparse MD PHD vest can be used to monitor washout from OAR and tumors to enable individualized dosimetry for patients undergoing Lu-177 DOTATATE therapy.

Abstract. The goal for positron emission tomography (PET)/X is measuring changes in radiotracer uptake for early assessment of response to breast cancer therapy. Upper bounds for detecting such changes were investigated using simulation and two image reconstruction algorithms customized to the PET/X rectangular geometry. Analytical reconstruction was used to study spatial resolution, comparing results with the distance of the closest approach (DCA) resolution surrogate that is independent of the reconstruction method. An iterative reconstruction algorithm was used to characterize contrast recovery in small targets. Resolution averaged <2 mm full width at half maximum when using depth-of-interaction (DOI) information. Without DOI, resolution ranged from 2.1±0.13 to 3.1±0.42 mm for scanner crystal thickness between 5 and 15 mm. The DCA resolution surrogate was highly correlated to image-based FWHM. Receiver-operating characteristic analysis showed specificity and sensitivity over 95% for detecting contrast change from 5:1 to 4:1 (area under curve >99%). For PET/X parameters modeled here, the ability to measure contrast changes benefited from higher photon absorption efficiency of thicker crystals while being largely unaffected by degraded resolution obtained with thicker crystals; DOI provided marginal improvements. These results assumed perfect data corrections and other idealizations, and thus represent an upper bound for detecting changes in small lesion radiotracer uptake of clinical interest using the PET/X system.

Abstract With mean survival from glioblastoma multiforme (GBM) barely edging over 15 months, it become imperative to develop novel tools informing on both progression and treatment efficacy. As the aggressive growth of GBM outstrips available resources, regions of hypoxia develop. Hypoxia can be clinically assessed through [18F]-flouromisonidazole (FMISO) PET. However, PET imaging suffers from low resolution and reconstruction artifact. We present the comparative and predictive results of a multi-stage model utilizing the mathematical biology of cancer and the physics of PET imaging. Our mathematical model of GBM characterizes cancer cells into the interacting phenotypes of normoxic, hypoxic, and necrotic cells supported by vasculature. The pharmacokinetic activity of FMISO is induced in the simulated tissue then acquired and reconstructed by an analytic simulator of the PET process. This multi-stage process creates a patient-specific virtual PET images with characteristic of the clinical PET scan. Extending previous work, we model the full three-dimensional dynamics of tumor progression, pharmacokinetic activity, and FMISO PET for the tumor kinetics of six GBM patients. Virtual and clinical FMISO PET images are compared. Overall hypoxic burden and spatial distribution show strong correspondence between virtual and clinical FMISO PET images for all six patients. Simulated FMISO PET dynamics, with tumor kinetics derived from routine clinical MRI scans, provide a unique and evolving tool giving insight into the biological connections between magnetic resonance and molecular imaging. Citation Format: Joshua Jacobs, Andrea Hawkins-Darrud, Robert Harrison, Sandra Johnston, Paul Kinahan, Kristin Swanson. Correlating magnetic resonance and molecular imaging using three dimensional untreated virtual control. [abstract]. In: Proceedings of the AACR Special Conference on Engineering and Physical Sciences in Oncology; 2016 Jun 25-28; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2017;77(2 Suppl):Abstract nr A10.

We examine sources of uncertainty affecting quantitative performance of SPECT/CT imagery in objects with heterogeneous activity and attenuation (i.e. soft tissue and lung). Understanding the cause and amount of uncertainty in SPECT quantitation is a necessary step towards multicenter SPECT studies and for improving the consistency and standards of care. While quantitation has been studied using water filled phantoms, we seek to extend this effort to lung phantoms of varying densities spanning norm lung tissue (i.e., -750 HU) to diseased lung tissue (i.e., -950 HU). A series of images of a prototype lung phantom were acquired with a GE Discovery NM/CT 670. This phantom consists of a repeatable structure of two or three water-filled blocks and two expandable polyurethane foam (UEF) cylinders with activity mixed in the foam and one of the water blocks. Contributions of uncertainty due to VOI selection, intrinsic scanner repeatability, setup repeatability, phantom density, and scatter profile were examined. We found the effects of VOI selection, resolution spill-out, and density-dependent scatter-correction errors all have significant impact (> 5%) on quantitative bias. Scan-to-scan repeatability, even with small (~ 1 cm) setup variation were typically small (<; 2%).

The purpose of this study was twofold: to evaluate the quantitative stability of a SPECT/CT gamma camera over time and to determine if daily flood acquisitions can reliably serve as calibration factors for quantitative SPECT. Using a cylindrical water phantom filled with measured amounts of 99mTc, factors were calculated to convert counts/cc to activity/cps. Measurements were made over an 18-month period. System sensitivity data calculated from 57Co daily quality assurance (DQA) flood acquisitions were then compared to the 99mTc calibration factors to determine the relationship of the factors.

The desire to understand normal and disordered human brain of upright, moving persons in natural environments motivates the development of an ambulatory micro-dose brain PET imager (AMPET) [1]. An ideal system would be light weight and have high sensitivity and spatial resolution. These requirements are often in conflict with each other. Therefore, we performed simulation studies to search for the optimal system configuration and to evaluate the improvement in performance over existing scanners. An intuitive design to achieve high sensitivity is to use a tight geometry that covers the brain. However, a tight geometry also increases parallax error in peripheral lines of response, which may increase the variance in ROI quantification.

1540 Objectives As part of the BRAIN initiative, we are designing a mobile molecular brain imager that can be used on healthy subjects to study the functioning of the human brain during motion. The Ambulatory Micro-Dose, Wearable PET Brain Imager (AMPET) must be as light as possible while still providing adequate sensitativity and resolution. We report here on initial simulations studies to quantify the AMPET design trade-offs. Methods We used the PET simulation system SimSET to model effects of AMPET design. Although SimSET provides fast and accurate simulations, it is limited to cylindrical volumes and surfaces for PET scanners. We added a new processing layer wherein an outer 9virtual target9 cylinder is used to capture all potential annihiliation photon detection events. These events are then projected back to the AMPET detector surface, which is modelled as a truncated hemisphere. After verification we evaluated the relative coicidence efficiency of three designs for AMPET (radius 12 cm, axial extent of truncated hemisphere of 4 6 and 8 cm, no confounding effects, 100,000 decays). For input we used the 3D Hoffman brain digital reference object (DRO). The DRO mimics 18F-FDG uptake in normal brain, but can be modified to represent uptake of other neuro radiotracers. Results The new approach of simulation of non-cylindrical scanners was verified. Using this approach the base efficiency of the AMPET designs were estimated. Doubling the axial extent of truncated hemisphere from 4 to 8 cm would more than triple the coincidence effeciency (3.5% to 11.4%), but would also double the weight, unless detector thickness is reduced. Conclusions Using simulations of non-cylindrical scanners using the 3D Hoffman brain digital reference object (DRO), we are able to evaluate the performance and design trade-offs for AMPET to optimze the trade-offs between mobility, sensitivity, and resolution. Research Support Suporrted by grant R24 MH106057

INTRODUCTION: There is growing interest in using positron emission tomography (PET) standardized uptake values (SUVs) to assess tumor response to therapy. However, many error sources compromise the ability to detect SUV changes. We explore relationships between these errors and overall SUV variability. METHODS: We used simulations in a virtual clinical trial framework to study impacts of error sources from scanning and analysis effects on assessment of SUV changes. We varied tumor diameter, scan duration, pretherapy SUV, magnitude of change in SUV, image reconstruction filter, and SUV metric. Poisson noise was added to the raw data before image reconstruction. Variance from global sources of error, e.g., scanner calibration, was incorporated. Two thousand independent noisy sinograms per scenario were generated and reconstructed. We used SUVs to create receiver operating characteristic (ROC) curves to quantify ability to assess response. Integrating area under the ROC curve summarized ability to detect SUV changes. RESULTS: Scan duration and image reconstruction method had relatively little impact on ability to measure response. SUVMAX is nearly as effective as SUVMEAN, especially with increased image smoothing and despite size-matched region of interest placement. For an effective variability of 15%, we found the Positron Emission Tomography Response Criteria in Solid Tumors criteria for measuring response (±30%) similar to the European Organization for Research and Treatment of Cancer criteria (±25%). CONCLUSIONS: For typical PET variance levels, tumor response must be 30% to 40% to be reliably determined using SUVs. PET scan duration and image reconstruction method had relatively little effect.