Prototype Positron Emission Tomography Research Articles

In-beam PET monitoring of proton therapy: a method for filtering prompt radiation events

Objective. In-beam positron emission tomography (PET) is a promising technology for real-time monitoring of proton therapy. Random coincidences between prompt radiation events and positron annihilation photon pairs can deteriorate imaging quality during beam-on operation. This study aimed to improve the PET image quality by filtering out the prompt radiation events. Approach. We investigated a prompt radiation event filtering method based on the accelerator radio frequency phase and assessed its performance using various prompt gamma energy thresholds. An in-beam PET prototype was used to acquire the data when the 70 MeV proton beam irradiated a water phantom and a mouse. The signal-to-background ratio (SBR) indicator was utilized to evaluate the quality of the PET reconstruction image. Main results. The selection of the prompt gamma energy threshold will affect the quality of the reconstructed image. Using the optimal energy threshold of 580 keV can obtain a SBR of 1.6 times for the water phantom radiation experiment and 2.0 times for the mouse radiation experiment compared to those without background removal, respectively. Significance. Our results show that using this optimal threshold can reduce the prompt radiation events, enhancing the SBR of the reconstructed image. This advancement contributes to more accurate real-time range verification in subsequent steps.

Study on the radiofrequency transparency of partial-ring oval-shaped prototype PET inserts in a 3T clinical MRI system.

The purpose of this study is to evaluate the RF field responses of partial-ring RF-shielded oval-shaped positron emission tomography (PET) inserts that are used in combination with an MRI body RF coil. Partial-ring PET insert is particularly suitable for interventional investigation (e.g., trimodal PET/MRI/ultrasound imaging) and intraoperative (e.g., robotic surgery) PET/MRI studies. In this study, we used electrically floating Faraday RF shield cages to construct different partial-ring configurations of oval and cylindrical PET inserts and performed experiments on the RF field, spin echo and gradient echo images for a homogeneous phantom in a 3T clinical MRI system. For each geometry, partial-ring configurations were studied by removing an opposing pair or a single shield cage from different positions of the PET ring. Compared to the MRI-only case, reduction in mean RF homogeneity, flip angle, and SNR for the detector opening in the first and third quadrants was approximately 13%, 15%, and 43%, respectively, whereas the values were 8%, 23%, and 48%, respectively, for the detector openings in the second and fourth quadrants. The RF field distribution also varied for different partial-ring configurations. It can be concluded that the field penetration was high for the detector openings in the first and third quadrants of both the inserts.

Time and Energy Resolving Time-over-Threshold ASIC for MPPC module in TOF-PET system (ToT-ASIC2)

ToT-ASIC2 is a new custom front-end application specific integrated circuit (ASIC) to readout silicon photomultiplier (SiPM) and multi-pixel photon counter (MPPC) for time-of-flight positron emission tomography (ToF-PET). ToT-ASIC2 is a 64-channel ASIC fabricated using a commercial 0.25 μm CMOS process. Each channel has an input-current buffer that sends signals to the SLOW and FAST paths, which are used for incident radiation energy and timing information, respectively. Time-over-Threshold (ToT) signals from these paths are combined into one ToT signal to be output outside the ASIC. The power consumption is approximately 2.9 mW per channel.This study developed a detection system comprising an MPPC with coupled GAGG pixelated scintillators, a front-end board with ToT-ASIC2 and ToT-DAQ. Consequently, energy and timing measurement experiments were performed using this system to confirm the ToT-ASIC2 energy- and timing-resolving capability. Prototype positron emission tomography experiments with 22Na indicated an energy resolution of 6.7% for 511 keV with the coincidence timing resolution being 215 ps.

Experimental Evaluation of a 3-D CZT Imaging Spectrometer for Potential Use in Compton-Enhanced PET Imaging.

We constructed a prototype positron emission tomography (PET) system and experimentally evaluated large-volume 3-D cadmium zinc telluride (CZT) detectors for potential use in Compton-enhanced PET imaging. The CZT spectrometer offers sub-0.5-mm spatial resolution, an ultrahigh energy resolution (~1% @ 511 keV), and the capability of detecting multiple gamma-ray interactions that simultaneously occurred. The system consists of four CZT detector panels with a detection area of around 4.4 cm × 4.4 cm. The distance between the front surfaces of the two opposite CZT detector panels is ~80 mm. This system allows us to detect coincident annihilation photons and Compton interactions inside the detectors and then, exploit Compton kinematics to predict the first Compton interaction site and reject chance coincidences. We have developed a numerical integration technique to model the near-field Compton response that incorporates Doppler broadening, detector's finite resolutions, and the distance between the first and second interactions. This method was used to effectively reject random and scattered coincidence events. In the preliminary imaging studies, we have used point sources, line sources, a custom-designed resolution phantom, and a commercial image quality (IQ) phantom to demonstrate an imaging resolution of approximately 0.75 mm in PET images, and Compton-based enhancement.

Quantitative assessment of radionuclide production yields in in-beam and offline PET measurements at different proton irradiation facilities

Objective. Reliable radionuclide production yield data are a prerequisite for positron-emission-tomography (PET) based in vivo proton treatment verification. In this context, activation data acquired at two different treatment facilities with different imaging systems were analyzed to provide experimentally determined radionuclide yields in thick targets and were compared with each other to investigate the impact of the respective imaging technique. Approach. Homogeneous thick targets (PMMA, gelatine, and graphite) were irradiated with mono-energetic proton pencil-beams at two distinct energies. Material activation was measured (i) in-beam during and after beam delivery with a double-head prototype PET camera and (ii) offline shortly after beam delivery with a commercial full-ring PET/CT scanner. Integral as well as depth-resolved β +-emitter yields were determined for the dominant positron-emitting radionuclides 11C, 15O, 13N and (in-beam only) 10C. In-beam data were used to investigate the qualitative impact of different monitoring time schemes on activity depth profiles and their quantitative impact on count rates and total activity. Main results. Production yields measured with the in-beam camera were comparable to or higher compared to respective offline results. Depth profiles of radionuclide-specific yields obtained from the double-head camera showed qualitative differences to data acquired with the full-ring camera with a more convex profile shape. Considerable impact of the imaging timing scheme on the activity profile was observed for gelatine only with a range variation of up to 3.5 mm. Evaluation of the coincidence rate and the total number of observed events in the considered workflows confirmed a strongly decreasing rate in targets with a large oxygen fraction. Significance. The observed quantitative and qualitative differences between the datasets underline the importance of a thorough system commissioning. Due to the lack of reliable cross-section data, in-house phantom measurements are still considered a gold standard for careful characterization of the system response and to ensure a reliable beam range verification.

Evaluation of STIR Library Adapted for PET Scanners with Non-Cylindrical Geometry

Software for Tomographic Image Reconstruction (STIR) is an open source C++ library used to reconstruct single photon emission tomography and positron emission tomography (PET) data. STIR has an experimental scanner geometry modelling feature to accurately model detector placement. In this study, we test and improve this new feature using several types of data: Monte Carlo simulations and measured phantom data acquired from a dedicated brain PET prototype scanner. The results show that the new geometry class applied to non-cylindrical PET scanners improved spatial resolution, uniformity, and image contrast. These are directly observed in the reconstructions of small features in the test quality phantom. Overall, we conclude that the revised “BlocksOnCylindrical” class will be a valuable addition to the next STIR software release with adjustments of existing features (Single Scatter Simulation, forward projection, attenuation corrections) to “BlocksOnCylindrical”.

System resolution versus image uncertainty for positron emission tomography scanners.

Purpose: Quantitative measures derived from positron emission tomography (PET) images are subject to statistical uncertainty, depending critically on system parameters, including the spatial resolution of the scanner. Predictions of statistical uncertainty of quantitative measures were compared with measurements. Approach: Measurements were performed on the dual-ring PET prototype setup at the University of Michigan. The setup consisted of multiple detectors that, in combination, span a system resolution ranging between 1 and 5.5mm full-width-at-half-maximum (FWHM). A Micro Jaszczak hot-spot phantom with rod diameters between 1.2 and 4.8mm was imaged and independently reconstructed for different detector combinations. Statistical properties of quantitative measures were evaluated for different reconstructions. Results: Measured signal-to-noise ratios (SNR) of , , and for high (0.92 to 0.98mm FWHM), medium (1.3 to 1.5mm FWHM), and low (5.5mm FWHM) resolution detector configurations and 1million events in general terms followed predications based on the detector resolution. Conclusions: The unique tomograph configuration allowed for experimental comparison of the impact of spatial resolution on statistical properties of reconstructions in the same setup. An SNR advantage in systems with high resolutions was predicted and confirmed even for object features significantly larger than the detector resolution.

Initial results of a mouse brain PET insert with a staggered 3-layer DOI detector

Objective. Small animal positron emission tomography (PET) requires a submillimeter resolution for better quantification of radiopharmaceuticals. On the other hand, depth-of-interaction (DOI) information is essential to preserve the spatial resolution while maintaining the sensitivity. Recently, we developed a staggered 3-layer DOI detector with 1 mm crystal pitch and 15 mm total crystal thickness, but we did not demonstrate the imaging performance of the DOI detector with full ring geometry. In this study we present initial imaging results obtained for a mouse brain PET prototype developed with the staggered 3-layer DOI detector. Approach. The prototype had 53 mm inner diameter and 11 mm axial field-of-view. The PET scanner consisted of 16 DOI detectors each of which had a staggered 3-layer LYSO crystal array (4/4/7 mm) coupled to a 4 × 4 silicon photomultiplier array. The physical performance was evaluated in terms of the NEMA NU 4 2008 protocol. Main Results. The measured spatial resolutions at the center and 15 mm radial offset were 0.67 mm and 1.56 mm for filtered-back-projection, respectively. The peak absolute sensitivity of 0.74% was obtained with an energy window of 400–600 keV. The resolution phantom imaging results show the clear identification of a submillimetric rod pattern with the ordered-subset expectation maximization algorithm. The inter-crystal scatter rejection using a narrow energy window could enhance the resolvability of a 0.75 mm rod significantly. Significance. In an animal imaging experiment, the detailed mouse brain structures such as cortex and thalamus were clearly identified with high contrast. In conclusion, we successfully developed the mouse brain PET insert prototype with a staggered 3-layer DOI detector.

Evaluation of a Hamamatsu TOF-PET Detector Module With 3.2-mm Pitch LFS Scintillators and a 256-Channel SiPM Array

Using time-of-flight (TOF) information in image reconstruction improves image signal-to-noise ratio and quantitative accuracy in positron emission tomography (PET). A new TOF-PET detector module with a 3.2-mm pitch lutetium fine silicate (LFS) scintillator array one-to-one coupled to a 16×16 (256-ch) silicon photomultiplier (SiPM) array was made commercially available (C13500 series, Hamamatsu Photonics K. K.). In this study, as a candidate detector module for the next-generation brain-dedicated PET, we changed the scintillator length of 20 to 10 mm according to our helmet-type PET geometry and investigated the basic performance of the PET detector module, including its energy resolution, sensitivity, coincidence response function (CRF), and coincidence timing resolution (CTR). All performance values were compared with those of the 4.2-mm pitch 144-ch detector module which was selected for our current helmet-type PET prototype. The energy resolution was 12% at 511 keV. The sensitivity was about 10% lower compared with those of the 4.2-mm pitch module. The average full width at half maximum (FWHM) of the CRFs was 1.9 mm. The CTRs had values of 235-241 ps with various energy windows after applying timing calibration. The CTR was not significantly changed (<; 10 ps) at the single count rate of 412 kcps or lower.

Can B Washout Rate be a Biomarker of Tumor Viability in Charged Particle Therapy? A Rat In-beam PET Study

Positron emission tomography (PET) is a practical tool for range verification in charged particle therapy. We aimed to apply in-beam PET imaging for tracking the evolution of irradiated particles in living tissue. The quantitative biological washout rate of the positron emitters has a potential usefulness as a diagnostic index that provides a unique opportunity to probe the status of tumor viability, but the modeling for this has not been established. Therefore, we measured biological washout rates of tumor-bearing nude rat and performed kinetic analysis on dynamic PET data to explore the biological washout mechanism. C6 rat glioma cells were implanted into right shoulder subcutaneous regions of six nude rats. Tumors volume reached to about 3 cm3 on day 14 after implantation. Tumor tissue (right shoulder) and normal tissue (left shoulder) were irradiated by 11C radioactive ion beam. Time activity curves (TACs) of the irradiated field were acquired by our original PET prototype. The biological washout rate was quantified based on the two-washout model assuming medium decay rate (k2m) and slow decay rate (k2s). In this model, k2m is speculated as the component which spread out via interstitial fluid, and k2s is speculated as the trapping component by the stable molecules in the tissue. After irradiation and PET dynamic scan, pathological morphology of tumor tissue (Hematoxylin & Eosin staining) was observed under microscope. The two-washout model fitted the experimental TACs well for five of six nude-rats irradiation studies. The observed k2m of tumor irradiation was 0.31+/-0.04 min-1, which was consistent with the normal tissue irradiation. While k2s of tumor irradiation was faster (0.017 +/-0.002 min-1) than that of normal tissue irradiation (0.006 +/-0.002 min-1). These results suggested that the component, which was trapped in normal tissue, diffused out more smoothly from tumor tissue due to vascular hyperpermeability which characterizes pathological tumor-angiogenesis. For one of six nude rats, only slow washout component (k2s = 0.014 +/-0.003 min-1) was observed in tumor irradiation study. Necrosis in tumor was observed in pathological tissue sample of this nude rat. It is also suggested that the decrease of washout rate was caused by the decrease of blood flow in the necrotic region. We measured the biological washout rate of irradiated 11C ion beam in tumor tissue and normal tissue. The physiological difference of tissue condition reflected the biological washout rates. This effect may be used as a clinical diagnostic index to monitor the dose response of the tumors in individual patients as well as the observation of the functional metabolism of tumor.

Biological washout effect in in-beam PET: animal studies

Positron emission tomography (PET) is a practical tool for range verification of hadron therapy. As well, the quantitative washout of the positron emitters has a potential usefulness as a diagnostic index, but the modelling for this has not been established. In this study, we measured washout rates of rabbit brain and performed kinetic analysis to explore the washout mechanism. Six rabbit brains were irradiated by 11C and 15O ion beams, and dynamic PET scan was performed using our original depth of interest (DOI)-PET prototype. The washout rate was obtained based on the two-compartment model, where efflux from tissue to blood (k2), influx (k3) and efflux (k4) from the first to second compartments in tissue were evaluated. The observed k2, k3 and k4 of 11C were 0.086, 0.137 and 0.007 min−1, and those of 15O were 0.502, 0.360 and 0.007 min−1, respectively. It was suggested permeability of a molecule containing 11C atoms might be regulated by a transporter. The k2 of 15O was comparable with 15O-water. This study provides basic data for modelling of the washout effect.

PET Image Reconstruction Using a Cascading Back-Projection Neural Network

Positron emission tomography (PET) imaging is a noninvasive technique that makes it possible to probe biological metabolic processes in vivo . However, PET image reconstruction is challenging due to the ill-posedness of the inverse problem. Many image reconstruction methods have been proposed over the past few years to improve diagnostic performance. However, most of these methods can compromise the reconstruction of important high-frequency structural details after aggressive denoising. To address this problem, in this work, we propose a novel deep learning method that reconstructs PET images using a cascading back-projection neural network (bpNet). This network consists of a domain translation operation, which acts as prior knowledge, cascaded with a modified encoder-decoder network. The image reconstruction pipeline ranges from the sinogram to the back-projection image and then to the PET image. Quantitative results from simulation data and Derenzo phantom experiments with the small animal PET prototype system developed in our laboratory clearly demonstrate that our proposed method provides favorable reconstructed image quality, especially for low-count PET image reconstruction.

Comparator-less PET data acquisition system using single-ended memory interface input receivers of FPGA

In this study, we propose a linear field-programmable gate array (FPGA)-based charge measurement method by combining a charge-to-time converter (QTC) with a single-ended memory interface (SeMI) input receiver. The QTC automatically converts the input charge into a dual-slope pulse, which has a width proportional to the input charge. Dual-slope pulses are directly digitized by the FPGA input/output (I/O) buffers configured with SeMI input receivers. A proof-of-concept comparator-less QTC/SeMI data acquisition (DAQ) system, consisting of 132 energy and 33 timing channels, was developed and applied to a prototype brain-dedicated positron emission tomography (PET) scanner. The PET scanner consisted of 14 sectors, each containing 2 × 1 block detectors, and each block detector yielded four energy signals and one timing signal. Because a single QTC/SeMI DAQ system can receive signals from up to eight sectors, two QTC/SeMI DAQ systems connected using high-speed gigabit transceivers were used to acquire data from the PET scanner. All crystals in the PET block detectors, consisting of dual-layer stacked lutetium oxyorthosilicate (LSO) scintillation crystal and silicon photomultiplier arrays, were clearly resolved in the flood maps with an excellent energy resolution. The PET images of hot-rod, cylindrical, and two-dimensional Hoffman brain phantoms were also acquired using the prototype PET scanner and two QTC/SeMI DAQ systems.

DPIR-Net: Direct PET Image Reconstruction Based on the Wasserstein Generative Adversarial Network

Positron emission tomography (PET) is an advanced medical imaging technique widely used in various clinical applications, such as tumor detection and neurologic disorders. Reducing the radiotracer dose is desirable in PET imaging because it decreases the patient’s radiation exposure. However, reducing the dose can also increase noise, affecting the image quality. Therefore, an advanced image reconstruction algorithm based on low-dose PET data is needed to improve the quality of the reconstructed image. In this article, we propose the use of a direct PET image reconstruction network (DPIR-Net) using an improved Wasserstein generative adversarial network (WGAN) framework to enhance image quality. This article provides two main findings. First, our network uses sinogram data as input and outputs high-quality PET images direct, resulting in shorter reconstruction times compared with traditional model-based reconstruction networks. Second, we combine perceptual loss, mean square error, and the Wasserstein distance as the loss function, which effectively solves the problem of excessive smoothness and loss of detailed information in traditional network image reconstruction. We performed a comparative study using maximum-likelihood expectation maximization (MLEM) with a post-Gaussian filter, a total variation (TV)-norm regularization, a nonlocal means (NLMs) denoising method, a neural network denoising method, a traditional deep learning PET reconstruction network, and our proposed DPIR-Net method and evaluated the proposed method using both human and mouse data. The mouse data were obtained from a small animal PET prototype system developed by our laboratory. The quantitative and qualitative results show that our proposed method outperformed the conventional methods.

Biological washout modelling for in-beam PET: rabbit brain irradiation by 11C and 15O ion beams

Positron emission tomography (PET) has been used for dose verification in charged particle therapy. The causes of washout of positron emitters by physiological functions should be clarified for accurate dose verification. In this study, we visualized the distribution of irradiated radioactive beams, 11C and 15O beams, in the rabbit whole-body using our original depth-of-interaction (DOI)-PET prototype to add basic data for biological washout effect correction. Time activity curves of the irradiated field and organs were measured immediately after the irradiations. All data were corrected for physical decay before further analysis. We also collected expired gas of the rabbit during beam irradiation and the energy spectrum was measured with a germanium detector. Irradiated radioactive beams into the brain were distributed to the whole body due to the biological washout process, and the implanted 11C and 15O ions were concentrated in the regions which had high blood volume. The 11C-labelled 11CO2 was detected in expired gas under the 11C beam irradiation, while no significant signal was detected under the 15O beam irradiation as a form of C15O2. Results suggested that the implanted 11C ions form molecules that diffuse out to the whole body by undergoing perfusion, then, they are incorporated into the blood-gas exchange in the respiratory system. This study provides basic data for modelling of the biological washout effect.

Gain Invariant Coordinate Reconstruction for SiPM-Based Pixelated Gamma Detectors With Multiplexed Readout

Currently, the prevailing high-resolution Positron Emission Tomography (PET) detector architecture consists of pixelated scintillating crystals coupled to a photosensitive layer, such as position sensitive photomultiplier tubes (PS-PMTs) or silicon photomultiplier (SiPM) arrays. In such detector designs, a light guide layer is commonly used between the finely segmented scintillating crystal array and coarsely segmented photosensitive layer to disperse scintillation light over several elements of the photosensitive layer. Such detectors typically rely on some form of analog signal multiplexing to minimize the number of readout electronics channels and employ Center-of-Gravity (CoG) approach to calculate the positions of gamma-ray interactions. In this article, we report on the Truncated Center of Gravity (TCoG) and Raised-To-the-Power (RTP) algorithms for calculation of gamma photon scintillation position as applied to our newly developed small-scale prototype PET system consisting of a pair of opposing detectors based on $24\times24$ LYSO scintillation crystal matrices and $8\times 8$ SiPM arrays with 64:16 multiplexed readout. The major notable advantage of using TCoG or RTP algorithm in contrast to basic CoG is shown to be their independence of photosensor gain variations. An improvement in the calculation of scintillation positions is demonstrated both for TCoG and RTP algorithms, the Field of View (FOV) distortion inherent to CoG reconstruction is eliminated. On average, the Peak-to-Valley Ratio (PVR) is increased at least by a factor of 2.25. The energy resolution for the considered small-scale PET prototype is ~13.5% when using CoG, TCoG, or RTP reconstruction algorithms.

Prototype Small-Animal PET-CT Imaging System for Image-guided Radiation Therapy.

Molecular imaging is becoming essential for precision targeted radiation therapy, yet progress is hindered from a lack of integrated imaging and treatment systems. We report the development of a prototype positron emission tomography (PET) scanner integrated into a commercial cone beam computed tomography (CBCT) based small animal irradiation system for molecular-image-guided, targeted external beam radiation therapy. The PET component consists of two rotating Hamamatsu time-of-flight PET modules positioned with a bore diameter of 101.6 mm and a radial field-of-view of 53.1 mm. The measured energy resolution after linearity correction at 511 KeV was 12.9% and the timing resolution was 283.6 ps. The measured spatial resolutions at the field-of-view center and 5 mm off the radial center were 2.6 mm × 2.6 mm × 1.6 mm and 2.6 mm × 2.6 mm × 2.7 mm respectively. 18F-Fluorodeoxyglucose-based PET imaging of a NEMA NU 4-2008 phantom resolved cylindrical volumes with diameters as small as 3 mm. To validate the system in-vivo, we performed 64Cu-DOTA-M5A PET and computed tomography (CT) imaging of carcinoembryonic antigen (CEA)-positive colorectal cancer in athymic nude mice and compared the results with a commercially available Siemens Inveon PET/CT system. The prototype PET system performed comparably to the Siemens system for identifying the location, size, and shape of tumors. Regions of heterogeneous 64Cu-DOTA-M5A uptake were observed. Using 64Cu-DOTA-M5A PET and CT images, a Monte Carlo-based radiation treatment plan was created to escalate the dose to the 64Cu-DOTA-M5A-based, highly active, biological target volume while largely sparing the normal tissue. Results demonstrate the feasibility of molecular-image-guided treatment plans using the prototype theranostic system.

Prototype time-of-flight PET utilizing capacitive multiplexing readout method

Positron emission tomography (PET) is an imaging technique that provides the spatial distribution of radiopharmaceuticals labeled with positron emitting radioisotopes by detecting the gamma rays produced from positron–electron annihilation. Recently, a time-of-flight (TOF) PET has drawn an increasing attention because it is capable of reducing the scan time or injected dose with improved the signal-to-noise ratio (SNR) in reconstructed PET images by precisely localizing the emission point along the line-of-response using TOF information.This study presents a multiplexing method that can effectively reduce the number of readout channels of a silicon photomultiplier (SiPM) based TOF PET while achieving excellent timing resolution. A capacitive multiplexing method was employed that could improve the degradation of the timing performance occurring in a conventional resistive multiplexing method. In addition, a high-speed signal processing method is also presented for the TOF PET. A TOF PET prototype was developed to demonstrate the imaging capability of the TOF PET system.A flood histogram of a PET detector module, composed of an 8×8 array of 3.01×3.01×20.00 mm3 lutetium fine silicate (LFS) scintillators and an 8×8 array of 3.16×3.16 mm2 SiPMs, was acquired using the proposed method. All 64 scintillators were successfully resolved in the flood histogram. The average energy resolution and coincidence resolving time (CRT) were 14.2 ± 1.1% and 431 ± 41 ps full width at half maximum (FWHM), respectively. A tomographic image of the hot-rod phantom was successfully acquired using the TOF PET prototype, and rods with a size of 2.4 mm in diameter were clearly resolved in the reconstructed image.

Evaluation of digital methods for energy calculation and timing pick-off in positron emission tomography

Traditionally, pulse processing in Positron Emission Tomography (PET) has been based on analog or discrete circuits forming a decentralized processing system. However, there is a convergence for digital and integrated implementations due to the characteristics of the modern electronic devices which are real-time processing capable, such as Application-Specific Integrated Circuit (ASIC) and Field Programmable Gate Array (FPGA) with fast Analog to Digital Converters (ADC). However, FPGA can provide fast implementation at relatively low cost and also enables the development of sophisticated digital pulse processing algorithms to improve energy, position and time resolutions in PET systems. Our group has developed and evaluated one energy calculation and three timing pick-off methods for implementation onto an FPGA-based system. For a typical PET detector setup, our charge integration method presents energy resolution similar to previously designed PET detectors. The best performance for timing pick-off was achieved by the Initial Rise Interpolation (IRI) method, where a coincidence time resolution of around 440 ps is suitable for Time of Flight (TOF) PET. Future works include embedding the proposed algorithms in a FPGA-based data acquisition system under development by our group which will be employed in a PET prototype.

Integrated optical and nuclear simulation of a monolithic LYSO:Ce based PET detector module

In the recent years new digital photon counter devices (also known as silicon photomultipliers, SiPMs) were designed and manufactured to be used specifically in positron emission tomography (PET) scanners. Finely pixelated SiPM arrays have opened new opportunities in PET detector development, such as the utilization of monolithic scintillator crystals. We worked out a simulation tool (SCOPE2) to assist the optimization and characterization of such PET detector modules. In the present paper we report the first application of SCOPE2 on the performance evaluation of a prototype PET detector module. The PET detector is based on monolithic LYSO:Ce scintillator crystal and a fully digital, silicon photon-counter, SPADnet-I. A new interface has been developed for SCOPE2 to access GATE simulation results. A combination of GATE and SCOPE2 was used to simulate excitation of the prototype PET detector with an electronically collimated γ -beam. Measurement results from the collimated γ-beam experiment were compared with the combined simulation. A good agreement was observed in the tendencies of total count spectrum and point of interaction distribution. We used the performance evaluation to understand and explain the measurement results in detail.