PET Detector Research Articles

In-depth evaluation of TOF-PET detectors based on crystal arrays and the TOFPET2 ASIC

In recent years high efforts have been devoted to enhance spatial and temporal resolutions of PET detectors. However, accurately combining these two main features is, in most of the cases, challenging. Typically, a compromise has to be made between the number of readout channels, scintillator type and size, and photosensors arrangement if aiming for a good system performance, while keeping a moderate cost. In this work, we have studied several detector configurations for PET based on a set of 8×8 Silicon Photomultiplier (SiPMs) of 3×3 mm2 active area, and LYSO crystal arrays with different pixel sizes. An exhaustive evaluation in terms of spatial, energy and timing resolution was made for all detector configurations. In some cases, when using pixel sizes different than SiPM active area, a significant amount of scintillation light may spread among several SiPMs. Therefore, we made use of a calibration method considering the different SiPM timing contributions. Best Detector Time Resolution (DTR) of 156 ps FWHM was measured when using 3×3 mm2 crystal pixels directly coupled to the 3×3 mm2 SiPMs. However, when using 1.5 mm crystal pixels with the same photosensor array, although we could clearly resolve all crystal pixels, an average DTR of 250 ps FWHM was achieved. We also shed light in this work on the timing dependency of the crystal pixel and photosensor alignment.

Characterization and Simulation of an Adaptable Fan-Beam Collimator for Fast Calibration of Radiation Detectors for PET

Monolithic scintillators for positron emission tomography systems perform best when calibrated individually. We present a fan-beam collimator with which a crystal can be calibrated within less than 1 h when suitable positioning algorithms are applied. The collimator is manufactured from lead, features an easily adaptable slit to tune the beam width and can be operated together with a coincidence detector to select a clean sample of 511-keV annihilation photons. We evaluated the performance of the collimator with a Geant4 simulation for slit widths of 0.25 mm, 0.4 mm, and 1mm and validated the shape of the beam profile experimentally by step-wisely moving a detector into the beam. This shows a clear narrow and box-shaped beam profile even if the collimator is operated without the coincidence set-up. In the latter configuration, the fraction of gammas in the beam region on a $50\times 50$ mm2 large detector is between 48% and 79% which is improved significantly to more than 94% by using only coincidence events. Analyzing the energy distribution shows that the fraction of 511-keV photons is increased from less than 50% to more than 96% by selecting coincidences. This demonstrates that our collimator produces a very defined and clean beam and provides optimal conditions for calibration.

Methods to Compensate the Time Walk Errors in Timing Measurements for PET Detectors

We investigate the effects of the signal walk-in timing measurements and the methods to compensate for the error when leading-edge discriminator (LED) triggering was used. Three pairs of crystals (LaBr3 cubes, lutetium-yttrium oxyorthosilicate (LYSO) cubes, and LYSO rectangular prisms) were tested for coincidence timing resolution (CTR) to evaluate four compensation methods: 1) Linear 1-D; 2) Logarithm 1-D; 3) Polynomial 2-D; and 4) artificial neural network (ANN) 2-D. The experimental results show that: 1) the signal walk causes a logarithmic relationship between energy and measured time; 2) the signal walk dramatically affects the timing measurement at a wide energy window. The measured FWHM CTR decreased by 86.2%, from 670 ps to 92.7 ps, when the lower cut of the energy window was used; and 3) the Logarithm 1-D compensation can improve the timing performance. The FWHM CTRs decreased by 69.9% (from 326.8 ps to 101.3 ps) with an energy window of [300 keV–550 keV] and by 12.5% (from 97.2 ps to 85.1 ps) with an energy window of [410 keV–550 keV] for LaBr3 cube crystals. The results show that the Logarithm 1-D, Polynomial 2-D, and ANN 2-D compensations work well, achieving at least a 10% improvement in timing performance.

Malignant Cutaneous Melanoma: Updates in PET Imaging.

Cutaneous malignant melanoma is a neoplasm whose incidence and mortality are dramatically increasing. 18F-FDG PET/CT gained clinical acceptance over the past 2 decades in the evaluation of several glucose-avid neoplasms, including malignant melanoma, particularly for the assessment for distant metastases, recurrence and response to therapy. To describe the advancements of nuclear medicine for imaging melanoma with particular attention to 18F-FDG-PET and its current state-of-the-art technical innovations. A comprehensive search strategy was used based on SCOPUS and PubMed databases. From all studies published in English, we selected the articles that evaluated the technological insights of 18FFDG- PET in the assessment of melanoma. State-of-the-art silicon photomultipliers based detectors ("digital") PET/CT scanners are nowadays more common, showing technical innovations that may have beneficial implications for patients with melanoma. Steady improvements in detectors design and architecture, as well as the implementation of both software and hardware technology (i.e., TOF, point spread function, etc.), resulted in significant improvements in PET image quality while reducing radiotracer dose and scanning time. Recently introduced digital PET detector technology in PET/CT and PET/MRI yields higher intrinsic system sensitivity compared with the latest generation analog technology, enabling the detection of very small lesions with potential impact on disease outcome.

High-Resolution Depth-Encoding PET Detector Module with Prismatoid Light-Guide Array.

Depth-encoding detectors with single-ended readout provide a practical, cost-effective approach for constructing high-resolution and high-sensitivity PET scanners. However, the current iteration of such detectors uses a uniform glass light-guide to achieve depth encoding, resulting in nonuniform performance throughout the detector array due to suboptimal intercrystal light sharing. We introduce Prism-PET, a single-ended-readout PET detector module with a segmented light-guide composed of an array of prismatoids that introduce enhanced, deterministic light sharing. Methods: High-resolution PET detector modules were fabricated with single-ended readout of polished multicrystal lutetium yttrium orthosilicate scintillator arrays directly coupled 4-to-1 and 9-to-1 to arrays of 3 × 3 mm silicon photomultiplier pixels. Each scintillator array was coupled at the nonreadout side to a light-guide (one 4-to-1 module with a uniform glass light-guide, one 4-to-1 Prism-PET module, and one 9-to-1 Prism-PET module) to introduce intercrystal light sharing, which closely mimics the behavior of dual-ended readout, with the additional benefit of improved crystal identification. Flood histogram data were acquired using a 3-MBq 22Na source to characterize crystal identification and energy resolution. Lead collimation was used to acquire data at specific depths to determine depth-of-interaction (DOI) resolution. Results: The flood histogram measurements showed excellent and uniform crystal separation throughout the Prism-PET modules, whereas the uniform glass light-guide module had performance degradation at the edges and corners. A DOI resolution of 5.0 mm full width at half maximum (FWHM) and an energy resolution of 13% FWHM were obtained in the uniform glass light-guide module. By comparison, the 4-to-1 coupled Prism-PET module achieved a DOI resolution of 2.5 mm FWHM and an energy resolution of 9% FWHM. Conclusion: PET scanners based on our Prism-PET modules with segmented prismatoid light-guide arrays can achieve high and uniform spatial resolution (9-to-1 coupling with ∼1-mm crystals), high sensitivity (20-mm-thick detectors and intercrystal Compton scatter recovery), good energy and timing resolutions (using polished crystals and after applying DOI correction), and compact size (depth encoding eliminates parallax error and permits smaller ring-diameter).

Exploring TOF capabilities of PET detector blocks based on large monolithic crystals and analog SiPMs

Monolithic scintillators are more frequently used in PET instrumentation due to their advantages in terms of accurate position estimation of the impinging gamma rays both planar and depth of interaction, their increased efficiency, and expected timing capabilities. Such timing performance has been studied when those blocks are coupled to digital photosensors showing an excellent timing resolution.In this work we study the timing behaviour of detectors composed by monolithic crystals and analog SiPMs read out by an ASIC. The scintillation light spreads across the crystal towards the photosensors, resulting in a high number of SiPMs and ASIC channels fired. This has been studied in relation with the Coincidence Timing Resolution (CTR). We have used LYSO monolithic blocks with dimensions of 50 × 50 × 15 mm3 coupled to SiPM arrays (8 × 8 elements with 6 × 6 mm2 area) which compose detectors suitable for clinical applications.While a CTR as good as 186 ps FWHM was achieved for a pair of 3 × 3 × 5 mm3 LYSO crystals, when using the monolithic block and the SiPM arrays, a raw CTR over 1 ns was observed. An optimal timestamp assignment was studied as well as compensation methods for the time-skew and time-walk errors. This work describes all steps followed to improve the CTR. Eventually, an average detector time resolution of 497 ps FWHM was measured for the whole thick monolithic block. This improves to 380 ps FWHM for a central volume of interest near the photosensors. The timing dependency with the photon depth of interaction and planar position are also included.

Investigations on Physical and Biological Range Uncertainties in Krak\'ow Proton Beam Therapy Centre

Physical and biological range uncertainties limit the clinical potential of Proton Beam Therapy (PBT). In this proceedings, we report on two research projects, which we are conducting in parallel and which both tackle the problem of range uncertainties. One aims at developing software tools and the other at developing detector instrumentation. Regarding the first, we report on our development and pre-clinical application of a GPU-accelerated Monte Carlo (MC) simulation toolkit Fred. Concerning the letter, we report on our investigations of plastic scintillator based PET detectors for particle therapy delivery monitoring. We study the feasibility of Jagiellonian-PET detector technology for proton beam therapy range monitoring by means of MC simulations of the $\beta^+$ activity induced in a phantom by proton beams and present preliminary results of PET image reconstruction. Using a GPU-accelerated Monte Carlo simulation toolkit Fred and plastic scintillator based PET detectors we aim to improve patient treatment quality with protons.

Performance comparison of two signal multiplexing readouts for SiPM-based pet detector

PET scanners using SiPMs as photodetectors could have tens of thousands of SiPMs. To simplify the readout electronics, analog signal multiplexing readouts are always preferred to be used as early as possible. In this paper, two simple analog signal multiplexing readouts, a capacitive charge-division readout, and a resistive charge-division readout were evaluated and compared using dual-ended readout detectors based on 10 × 10 arrays of SensL MicroFJ-30035 SiPMs coupled to both ends of a 20 × 20 LYSO array with a pitch size of 1.5 mm and a length of 20 mm. The performance of the detectors were evaluated at different bias voltages (from 27.0 V to 30.5 V with an interval of 0.5 V) and a temperature of 22.8 °C. The flood histograms show that all the crystals in the LYSO array were clearly identified, whilst better flood histogram was obtained using the resistive charge-division readout. At a bias voltage of 29.5V, the flood histogram quality, energy resolution, DOI resolution, and timing resolution of the detector obtained using the capacitive charge-division readout were 3.28 ± 0.85, 18.9% ± 6.2%, 1.93 ± 0.20 mm, 1.25 ± 0.11 ns respectively, and those obtained using the resistive charge-division readout were 3.57 ± 0.81, 16.9% ± 6.5%, 1.96 ± 0.23 mm and 1.23 ± 0.07 ns, respectively. Overall, the detector with the resistive charge-division readout provided better performance.

Highly multiplexed SiPM signal readout for brain-dedicated TOF-DOI PET detectors

PurposeWe investigated the highly multiplexed readout of SiPM signals that are useful in developing brain-dedicated PET detectors with DOI-capable crystal blocks and large-area SiPM arrays. MethodsThe PET detector module used in this study was equipped with a two-layer relative-offset DOI crystal block and a 2 × 2 array of 16-channel SiPMs. The lower crystal-layer consisted of a 14 × 14 array of 1.78 × 1.78 × 8 mm3 LSO crystals and the upper crystal-layer consisted of a 13 × 13 array of 1.78 × 1.78 × 12 mm3 LSO crystals. The energy and position information was obtained via signals from the 8 × 8 resistive charge division multiplexing circuit. The timing performance was evaluated with varying multiplexing ratios (i.e. 16:1, 32:1, and 64:1) via first-order analog high-pass filtering. ResultsFor three different multiplexing schemes, all LSO crystals with two-layer DOI information were clearly resolved and yielded good energy resolutions of 10.5 ± 1.0% (upper) and 12.1 ± 1.7% (lower). The 16:1 multiplexing yielded an optimal timing performance with average CRT values of 325 ps FWHM (upper) and 342 ps FWHM (lower); however, the timing performances were maintained almost constant even for 64:1 multiplexing with average CRT values of 336 ps FWHM (upper) and 347 ps FWHM (lower). ConclusionsThe highly multiplexed SiPM signal readout via the first-order analog high-pass filtering could be an attractive solution to develop brain-dedicated PET scanners, effectively decreasing the burden of DAQ systems with moderate compromise in terms of TOF and DOI performances.

Feasibility of High-Resolution PET Detector Readout by 2-D Tetra-Lateral Position-Sensitive Silicon Photomultiplier

In this paper, we present the feasibility of high-resolution PET detector readout by a 2-D tetra-lateral position-sensitive Silicon photomultiplier with an intrinsic cap resistive layer for charge division (CRL-SiPM). The scintillation imaging sensor for 511 keV gamma ray detection is realized by coupling a 5 $\times $ 5 array of 0.45 $\times $ 0.45 $ \times $ 6 mm3 LYSO crystals to the CRL-SiPM with an active area of 2.77 $ \times $ 2.77 mm2 and micro APD cell pitch of $ \sim 10~ \mu \text{m}$ . The scintillator arrays are clearly distinguished and good scintillator array imaging with little position distortion has been achieved. An energy resolution of ~13.5% and coincidence time resolution of 555 ps are demonstrated, respectively. Therefore, the potential PET imaging sensor with greatly reduced readout channels has been verified.

The effects of inter-crystal scattering events on the performance of PET detectors

The probability of inter-crystal scattering (ICS) events for 511 keV gamma rays in all current scintillation crystals is high and the ICS events degrade the spatial resolution of PET scanners. In this work, Monte Carlo simulations were performed to study the effects of ICS events on the sensitivity and spatial resolution of PET detectors. LaBr3, LYSO, and PWO that represent scintillation crystals of low, medium and high density, respectively, were used. For a point source placed in the middle of two scintillation detectors of 50 × 50 × 20 mm3 and a lower energy threshold (LET) of 350 keV, the probabilities that at least one gamma ray undergoes ICS are 94%, 84% and 76% for LaBr3, LYSO, and PWO, respectively. The ICS events still provide useful spatial information. The full width at half maximum (FWHM), the full width at tenth maximum (FWTM) and the mean absolute error (MAE) of the curve of the mispositioning of a point source caused by ICS events are 0.45, 3.0 and 0.9 mm if the most popular PET scintillator LYSO is used. The MAE is smaller than the spatial resolution of most current PET scanners. The effect of ICS increases as the detector LET increases, scintillator density decreases, and crystal size decreases. The intrinsic spatial resolutions of a pair of LYSO detectors were calculated using curves of the coincidence counts between one column of the crystals in the two detectors and the sum of the coincidence counts between two opposite crystals of the columns in the two detectors that are in line with the point source changing with the source positions. The latter method removes almost all of ICS events. The FWHM (FWTM) intrinsic spatial resolutions obtained by the two methods are 0.40 (2.0) mm and 0.33 (0.8) mm if the crystal size is 0.5 mm, and are 0.8 (3.0) and 0.68 (1.5) mm if the crystal size is 1.0 mm. ICS events have much bigger contributions to the FWTM rather than the FWHM of the intrinsic spatial resolution of PET detectors. The spatial resolution of a PET scanner can still be improved by decreasing the crystal size to as small as 0.5 mm.

Ensemble of neural networks for 3D position estimation in monolithic PET detectors

We propose an ensemble of multilayer feedforward neural networks to estimate the 3D position of photoelectric interactions in monolithic detectors. The ensemble is trained with data generated from optical Monte Carlo simulations only. The originality of our approach is to exploit simulations to obtain reference data, in combination with a variability reduction that the network ensembles offer, thus, removing the need of extensive per-detector calibration measurements. This procedure delivers an ensemble valid for any detector of the same design. We show the capability of the ensemble to solve the 3D positioning problem through testing four different detector designs with Monte Carlo data, measurements from physical detectors and reconstructed images from the MindView scanner. Network ensembles allow the detector to achieve a 2–2.4 mm FWHM, depending on its design, and the associated reconstructed images present improved SNR, CNR and SSIM when compared to those based on the MindView built-in positioning algorithm.

Development of a Compact PET for integrated PET/CT/RT to Streamline and Enhance Functional/Anatomic Image-Guided Preclinical Radiation Oncology Researches

To present latest progress in developing a compact and high-performance PET that will be inserted inside an existing CT image-guided small animal irradiator to establish an integrated PET/CT/RT for substantially expanded and improved image-guided RT research capabilities. 1) PET will consist of 12 detector panels in a multi-polygon configuration to provide ∼10 cm diameter trans-axial and ∼3.2 cm axial image field-of-view: Each panel consists of 4 detectors that closely tiled in a 2x2 matrix: Each detector consists of 15x15 array of 1x1x20 mm LYSO scintillators, with each end of the scintillator array read out by an 8x8 array of 2x2 mm Silicon Photomultiplier (SiPM) array to provide dual-scintillator-end readout for measuring depth-of-interaction (DOI); 2) PC Board of discrete components based electronics have been developed for reading/processing total 768 detector output channels (after readout channel reduction) with both positive and negative polarity signals from SiPMs’ anodes and cathodes; 3) Standard detector performance evaluation have been conducted, including energy, coincidence timing, DOI, and intrinsic detector spatial resolutions; 4) Imaging performance based on rotating detector panels is under investigation. Two full detector panels have already been assembled and placed opposite to each other on a rotating table, with a radioactive rod phantom between the panels. Coincident data will be acquired by the detector pairs at 6 different rotating angles span over 180 degrees, with 30 degrees between each consecutive detector angles. Data corrections, including attenuation, scatter and random will be applied before the image reconstruction. Expanded dual positive and negative polarity signal readout and processing electronics works well. All 900 crystals on each detector panel can be very well identified; energy, coincidence timing, and DOI are 18%-34%, ∼2.0 ns, and 2.4 – 3.7 mm, respectively. The detector intrinsic spatial resolution is < 1.0 mm. Overall, detectors have achieved expected performance. As anticipated, ongoing phantom imaging study with dual rotating detector panels, potentially along with the performance evaluation study of the assembled PET system, will be reported in the conference. We have developed PET detectors and readout/processing electronics with desired performance, and expect the ongoing system construction and imaging studies will quantify the PET capability for functional image guidance for small animal radiotherapy research.

On light sharing TOF-PET modules with depth of interaction and 157 ps FWHM coincidence time resolution

The performance of a light sharing and recirculation mechanism that allows the extraction of depth of interaction (DOI) are investigated in this paper, with a particular focus on timing. In parallel, a method to optimize the coincidence time resolution (CTR) of PET detectors by use of the DOI information is proposed and tested. For these purposes, a dedicated 64-channels readout setup has been developed with intrinsic timing resolution of 16 ps FWHM. Several PET modules have been produced, based on LYSO:Ce scintillators and commercial silicon photomultiplier (SiPM) arrays, with mm2 individual SiPM size. The results show the possibility to achieve a timing resolution of 157 ps FWHM, combined with the already demonstrated spatial resolution of 1.5 mm FWHM, DOI resolution of 3 mm FWHM, and energy resolution of 9% FWHM at 511 keV, with 15 mm long crystals of section mm2 and mm2. At the same time, the extraction of the DOI coordinate has been demonstrated not to deteriorate the timing performance of the PET module.

An FPGA-Based Fast Linear Discharge Readout Scheme Enabling Simultaneous Time and Energy Measurements for TOF-PET Detectors

The readout electronics in time-of-flight positron emission tomography (TOF-PET) detector modules conventionally contain a separate timing circuit and a separate energy measurement circuit. For channel-by-channel readouts, the front-end electronics will be unacceptably large and must rely on application specific integrated circuit (ASIC) chips. In this paper, we present a novel field programmable gate array (FPGA)-based readout scheme that unifies the time and energy measurements with only one analog amplifier per channel in addition to the FPGA. By constructing a simple and effective fast linear discharge circuit, one time-to-digital converter (TDC) per channel can implement time and energy readouts simultaneously with a short measurement dead time. To evaluate the performance of the new approach, a 16-channel PET detector module consisting of LYSO crystals coupled with a silicon photomultiplier (SiPM) array was constructed connecting to the readout electronics. Using a reference detector with a 164.7-ps time resolution to perform 22Na coincidence measurements, the average energy, and time resolution among 16 channels were measured as 12.44% and 372.6 ps, respectively. The typical measurement dead time is approximately 300 ns. The test results show the simplicity and feasibility of the proposed readout scheme, which is particularly useful for laboratory instrumentation.

Performance Characterization of MPPC Modules for TOF-PET Applications

PET detectors commonly feature large numbers of multipixel photon counters (MPPCs), requiring data acquisition systems with significant signal multiplexing or using application specific integrated circuits (ASICs) for independent MPPC readout. We evaluate here two Hamamatsu C13500-4075LC-12 detector modules designed for time-of-flight (TOF) PET. Each module has a 12 $ {\times }$ 12 lutetium fine silicate crystal array (4.2 $ {\times }$ 4.2 $ {\times }$ 20 mm3) one-to-one coupled to a 12 $ {\times }$ 12 array of 4 mm MPPCs. Each detector has 8 18-channel ASICs utilizing time-over-threshold (ToT) readout. At low event rates, the detectors have full-width at half-maximum (FWHM) energy resolution of 11.8%±0.4%, measured in linearized energy spectra, and 275 ps FWHM coincidence timing resolution (CTR) using energy window of ±1 photopeak FWHM. The 511 keV photopeak amplitude in the ToT spectrum changed 0.24%/°C. There was <10% event loss up to 1450 kcps/detector. From count rates of 35–1450 kcps the 511 keV photopeak position varied by 1.2%, while photopeak FWHM in the ToT spectra increased by 54%, from 5.8% to 9.0%, corresponding to energy resolution changing from 11.8% to approximately 18.0%. Over this count rate range the CTR varied from 285 to 435 ps FWHM. The TOF-PET module performance suggests they are well suited to use in whole-body PET applications.

Compton PET: a layered structure PET detector with high performance

In most high-resolution PET detector designs, there is an inherent trade-off between spatial resolution and detector efficiency. We have developed and tested a new geometry for the detector module which avoids this trade-off. The module uses a layered structure, in which four crystal slabs are stacked in the depth direction and optically separated by enhanced specular reflector (ESR) film. The scintillation light within each layer is measured by 16 SiPMs located on the four sides of the crystal. Analog signals from all SiPMs (4 × 16) on the four sides of the crystal are digitized individually using a 64-channel TOFPET-2 module. The four-sided readout method reduces the problem of light trapping resulting from total internal reflection when reading out the end(s) of traditional scintillation crystal arrays, thus increasing the light collection efficiency. In this work, we demonstrate the readout of a complete layered detector with 4 layers. The high light collection efficiency results in a FWHM energy resolution of 10.3%, and a FWHM timing resolution of 348 ps. The distribution of scintillation light detected by the SiPMs was used to decode the interaction position of each gamma ray using a trained neural network. A FWHM spatial resolution of 1.1 ± 0.1 mm was achieved. This design allows the detection efficiency of the module to be increased by adding additional crystal slabs along the depth direction. Since the position, energy, and timing are measured for each layer independently, increasing the system sensitivity by adding more layers will not affect the spatial/energy/timing resolution. Furthermore, the layered structure allows partial recovery of position information for events that undergo Compton scatter within the detector.

Design and Detector Performance of the PET Component of the TRIMAGE PET/MR/EEG Scanner

The TRIMAGE project aims at developing a brain-dedicated PET/MR/EEG system able to perform simultaneous PET, MR, and EEG acquisitions for application in schizophrenia. Both PET and MR components have been designed in this project. The PET component consists of a full ring with 18 sectors each comprising three square detector modules. The modules are based on dual-layer staggered matrices of LYSO crystals read out by silicon photomultipliers. This paper describes in full detail the final version of the PET detectors and the related electronics. It also reports on the preliminary performance of a pair of sectors in terms of pixel resolvability index (RI), energy resolution, singles count rate capability, and coincidence time resolution (CTR). The procedures used for the optimization and calibration of PET detector are described. Results demonstrate the pixel/layer identification performance with an RI of about 0.2 while the energy resolution resulted in 20% and 22% FWHM for the bottom and top layer, respectively. The maximum singles count rate of a PET detector is about 700 kcps and the CTR of two sectors is 515 ps.

Four-layered DOI-PET detector with quadrisected top layer crystals

Previously, we had developed a four-layered depth-of-interaction (DOI) PET detector based on the light sharing method. Reflectors , which were inserted in every two lines of crystal segments and shifted differently for each layer, projected 3D crystal positions onto a 2D position histogram without any overlapping after applying the Anger-type calculation. The best crystal separation we ever obtained was for the four-layered 32 × 32 array of LYSO crystals sized at 1.45 × 1.45 × 5 mm 3 . However, assembling small crystals tended to cost a lot, and fine tuning of the front-end circuit was required to get fine crystal identification. In this paper, therefore, we proposed a more practical four-layered DOI detector. Its key concept was that the crystals in the top layer, which have the highest detection efficiency, are the biggest contributors to the PET spatial resolution. We applied two new ideas: (1) using quarter size crystals only for the first (top) layer and (2) inserting a thin light guide between the first and the second layers of the crystal array. In the developed prototype detector, we used 24 × 24 LYSO crystals of quarter size (1.4 × 1.4 × 5.0 mm 3 ) in the first layer and the other layers were 12 × 12 arrays of crystals of 2.8 × 2.8 × 5.0 mm 3 . For better crystal identification of small crystals in the first layer, we optimized the optical condition between crystals by using an optical adhesive and air. Also, the thin light guide of 0.5 mm thickness was inserted between the first and the second layers for improvement of crystal identification of the first layer. With the appropriate insertion of the light guide, all crystals of the first layer were identified as well as the crystals in the other layers. Our developed four-layered DOI detector showed good potential for high spatial resolution without a large increase in the number of crystals.

Automated Quantitative Analysis of American College of Radiology PET Phantom Images

Evaluation of PET image quality is central to annual physics surveys, quality assurance, and laboratory accreditation. A common method is to image the American College of Radiology (ACR) PET phantom, which contains hot and cold structures of various sizes in a warm background. Performance evaluation involves qualitative assessment of hot and cold structure visibility and overall image quality. Some criteria are quantitative and rely on manually drawn regions of interest (ROIs) to measure SUV. Fully automated scoring of ACR PET phantom images would improve efficiency, avoid observer-related dependencies, and possibly provide more robust evaluation of image quality. Methods: Software was developed to coregister PET images to a phantom template and to compute ROI measurements of hot vial activity (SUVmax) and background activity (SUVmean) automatically. In addition, 3-dimensional volumes of interest (VOIs) were generated to measure hot vial activity (SUVvial), background activity, and cold rod contrast. Consistency of the ROI-based and VOI-based methods was evaluated using phantom data from a total of 17 annual physics surveys of 3 PET/CT scanners with the same PET detector design. Results: The automated software processed all PET phantom datasets successfully. SUV consistency for hot vials was improved through use of cylindric VOIs and through normalization with respect to assayed activities and dilution volumes used in phantom preparation. Average vial SUV SD improved from 8.0% for standard SUVmax to 3.2% for normalized SUVvial Similarly, the SD for the SUV ratio of 16- to 25-mm vials improved from 5.0% for SUVmax to 3.2% for SUVvial Background SUVmean had a similar consistency between the ROI and VOI methods. Cold rod contrast was highly consistent, offering a potential alternative to qualitative visual assessment of low-contrast performance. Conclusion: Automated quantitative scoring of the ACR PET phantom is feasible and offers the advantages of more efficient, consistent, and thorough performance characterization. Acceptance ranges for SUVs and ratios likely can be tightened if normalized VOI measurements are used. Further testing with phantom data from a variety of PET scanners is necessary to establish suitable quantitative thresholds for acceptable performance.