In positron emission tomography (PET), time-of-flight (TOF) information localizes source positions along lines of response. Cherenkov-radiator-integrated microchannel-plate photomultiplier tubes have achieved 30-ps TOF resolution, demonstrating cross-sectional imaging without reconstruction. Such ultrafast TOF detectors would free PET from conventional ring geometries. Therefore, this study aimed at investigating imaging characteristics of a dual-panel PET with ultrafast TOF detectors using Geant4 simulation. Two detector panels (137 & times; 137 mm(2)), which consisted of 5.0-mm thick bismuth germanate pixelized crystals with a 5.75-mm pitch, were placed face-to-face at a 300-mm distance. Imaging characteristics with various TOF resolutions from 30 to 90 ps were evaluated. Because, degraded efficiency may cancel TOF gain in image quality, detection efficiency was also parameterized by reducing coincidence counts. Data acquisitions for a numerical multirod and uniform phantom (21 MBq) and a modified NEMA NU2 image quality phantom were simulated for 600 s. Results of the maximum likelihood expectation maximization (MLEM) reconstruction were compared with those of a backprojection (i.e., no reconstruction). The dual-panel PET required a 40-ps TOF resolution to have a similar spatial resolution to that of a non-TOF ring PET (300-mm in diameter) for the same detection efficiency. TOF showed benefit in the reconstruction of image quality phantom with 40% efficiency, and the image noise with 20% efficiency at 30-ps TOF was similar to the complete efficiency at 40-ps TOF. MLEM provided better imaging performance than backprojection, even at 30-ps TOF. The feasibility of the proposed dual-panel PET was shown.

Objective.Lutetium oxide (Lu2O3), with its high density (9.4 g cm-3), presents a compelling scintillation host for detecting 511 keV annihilation photons in positron emission tomography (PET). Despite its favorable density, the practical deployment of Lu2O3-based scintillators for PET has faced limitations due to difficulties in crystal growth and inappropriate decay time. Recent progress in ceramic processing has facilitated the development of transparent Lu2O3ceramics, while targeted doping strategies have significantly improved their luminescence performance. This study evaluates the performance of Lu2O3:Yb and a newly developed ceramic scintillator of (Lu,Y)2O3:La, a modified Lu2O3-based compound incorporating yttrium (Y) and doped with lanthanum (La).Approach.Various ceramic disks were fabricated and cut into 3 × 3 × 5 mm3samples. The performance of both Lu2O3:Yb and (Lu,Y)2O3:La ceramic samples in terms of decay time, energy resolution, and coincidence timing resolution (CTR) was assessed. Decay time measurements were conducted using waveform data collected from samples mounted on an H10580 photomultiplier tube (PMT) and irradiated with 511 keV photons from a22Na source. Energy and CTRs were evaluated using both PMT and silicon photomultiplier setups, arranged in coincidence with a reference lutetium-yttrium oxyorthosilicate (LYSO) detector of the same size.Main results.All three (Lu,Y)2O3:La ceramic scintillator samples exhibited a triple exponential decay profile and were dominated by a slow component ranging from 1379.3 to 1515.6 ns. The best energy resolution of 15.4% at 511 keV and the best CTR of 237.9 ps full width at half maximum (FWHM) were observed for the same sample. In contrast, a fast decay time of 1.6 ns was observed for the Lu2O3:Yb samples, which exhibited CTR values ranging from 237.9 ps to 261.4 ps FWHM, while the photopeak at 511 keV was difficult to distinguish. These CTR values were estimated between two identical ceramic samples, derived from coincidence measurements of each ceramic sample against the LYSO reference detector. The (Lu,Y)2O3:La samples achieved CTR values comparable to those of the Lu2O3:Yb samples, as their much higher light yield offsets the disadvantage associated with their slower decay time.Significance.These results highlight the promising potential of the (Lu,Y)2O3:La ceramic scintillators for PET applications, especially for time-of-flight PET.

Precise measurement of radiation has long played a vital role in a wide range of research and industrial fields, from fundamental physics beyond the Standard Model to medical imaging such as time-of-flight positron emission tomography. Developing radiation detectors that achieve high timing precision-on the order of a few tens of picoseconds-and energy measurement capabilities remains indispensable yet challenging. In this study, we developed two types of scintillator-integrated microchannel plate photomultiplier tubes (SCI-IMPs), one incorporating barium fluoride, and the other bismuth germanate, to enable simultaneous high-precision timing and energy measurements. To evaluate their performance over a wide energy range from keV- to GeV-scale, electron-positron annihilation gamma rays and cosmic ray muons were used. For energy measurements, both detectors achieved an energy resolution of approximately 35% at 511 keV. For timing measurements using 511 keV, coincidence time resolutions (CTRs) of approximately 50 ps full width at half maximum (FWHM) were obtained for both detectors. In contrast, for cosmic ray muon experiments where cosmic ray muon energy is typically on the order of GeV, CTRs were measured to be 25.1 and 16.8 ps FWHM for barium fluoride- and bismuth germanate-based detectors, respectively. The versatile scintillator-integration technique established in this study can broaden the applicability of the newly developed SCI-IMPs. In particular, these results demonstrate that the developed detectors push the boundaries of timing performance while retaining energy measurement and hold promise for future applications in fundamental physics experiments and medical imaging.

Objective. Bismuth germanate (BGO) has regained attention as a promising material for hybrid Cherenkov/scintillation time-of-flight positron emission tomography (TOF-PET). While excellent timing performance has been demonstrated in single-crystal studies using prompt Cherenkov photons, practical pixelated detector modules introduce appreciable inter-crystal scattering (InterCS) events that can degrade timing accuracy. The objective of this work was to experimentally investigate the impact ofInterCSon Cherenkov-based timing in pixelated BGO detectors and to identify optimal timestamp selection strategies.Approach. A dual-pixel BGO detector was constructed and coupled to a segmented SiPM readout to enable spatially resolved energy and timing measurements. Events were classified into full-energy deposition (FED); primary crystal 511 keV absorption),InterCS, andpenetrationcategories using energy-weighted positioning. This experimental classification was validated using GATE simulations, which further revealed that intra-crystal scattering (IntraCS) accounted for more than 25% of the events experimentally classified asFED. Multiple timestamp selection strategies were evaluated, and prompt photon statistics were quantified by integrating the first 1 ns of the timing waveform.Main results. ForInterCSevents, selecting the earlier of the two timestamps yielded a coincidence timing resolution of 221 ps FWHM (831 ps FWTM) measured in coincidence with a LYSO:(Ce, Mg) reference detector, compared to 184 ps FWHM (603 ps FWTM) forFEDevents. Energy-based timestamp selection was found to be suboptimal. Prompt photon analysis showed a measurable reduction in early photon yield forInterCSevents, with an average of 4.73 detected photons in the first 1 ns, compared to 5.76 forFEDevents.Significance. these results demonstrate thatInterCSintroduces systematic timing degradation in pixelated BGO Cherenkov TOF-PET detectors through energy redistribution and reduced prompt photon statistics. The findings highlight the necessity of time-aware, per-pixel timestamp selection strategies to preserve optimal timing performance in realistic BGO-based TOF-PET systems operating in the presence of Compton scattering.

Bismuth germanate (BGO) is gaining renewed attention as a viable material for hybrid Cherenkov/scintillation time-of-flight positron emission tomography (TOF-PET) detectors. While single-crystal studies have demonstrated excellent timing resolution by leveraging prompt Cherenkov photons, practical detector modules based on pixelated arrays introduce a high prevalence of inter-crystal scattering (InterCS) events, complicating timing accuracy. In this study, we experimentally investigated the impact of InterCS on BGO Cherenkov timing using a dual-pixel detector coupled to a segmented SiPM readout. Events were classified into full-energy deposition (FED; primary crystal 511 keV absorption), InterCS, and penetration types via energy-weighted positioning and validated using GATE simulations, which also revealed that over 25% of the experimentally identified full-energy events involved intra-crystal scatter (IntraCS). For InterCS events, the optimal timestamp selection was achieved by choosing the earlier of the two timestamps, yielding a coincidence timing resolution of 221 ps FWHM (831 ps FWTM)-markedly worse than the 184 ps (603 ps FWTM) obtained for FED events. Furthermore, prompt photon yield was found to decrease measurably due to energy splitting: InterCS events averaged 4.73 detected photons in the first 1 ns, compared to 5.76 for FED events. These results emphasize the importance of incorporating time-aware, per-pixel timestamping strategies in pixelated BGO TOF-PET systems to maintain optimal timing performance in the presence of scatter.

We propose an FPGA-based Time-to-Digital Converter (TDC) that utilizes a Virtual Bin (VB) approach with opposite-transition (OT) inputs on two Tapped-Delay Lines (TDLs) to obtain less-correlated time bins. The VBs from the proposed OT TDC were obtained by comparing and segmenting the less-correlated bins collected from the two TDLs. The OT TDC was implemented on a 7-series FPGA (Xilinx) to verify performance. A conventional monotonic-transition (MT) TDC, which used identical transition inputs (0-to-1 or 1-to-0 transition) for the two TDLs, was also implemented as a control group. The results were compared with those from the MT TDC and other studies. The proposed method effectively improves time resolution and integral linearity while keeping resource usage low by exploiting these characteristics. The average bin size and RMS value were 5.5 and 4.2 ps, respectively. Moreover, the proposed method exhibits stable performance under temperature variations and implementation location changes. The VB OT TDC, which applies the VB method to the OT TDC, successfully measures detection time differences of two signals from two Cerenkov radiator integrated microchannel plate photomultiplier tubes (CRI-MCP-PMTs) with a high timing precision of sub-100 ps. The VB OT TDC can be used for next-generation applications that require fast-timing measurements.

This study presents a novel field-programmable gate array (FPGA)-based time-to-digital converter (TDC) architecture designed for high-resolution timing applications, incorporating two innovative techniques. First, a cross-detection (CD) method modifies the sampling pattern to minimize bubbles in thermometer codes, which are a primary source of inaccuracy in TDC timing measurements. By eliminating these bubbles, the method reduces the average bin size by half. Second, dual-side monitoring (DSM) simultaneously captures both end-of-propagation (EOP) and start-of-propagation (SOP) thermometer codes. Unlike conventional TDCs that rely solely on SOP, DSM leverages EOP to calibrate SOP, thereby enhancing time resolution and improving stability under temperature and location variations. The proposed DSM scheme requires only two additional CARRY4s to capture EOP, offering a resource-efficient alternative to conventional TDC designs. The proposed CD-DSM TDC was implemented on a Xilinx Virtex-7 (XC7VX485T-2FFG1761C) FPGA, achieving an average bin size of 5.6 ps and a root mean square bin distribution of 6.4 ps. It demonstrates superior linearity, with a differential non-linearity (DNL) range of [-0.9, 2.7] least significant bits (LSB) and an integral non-linearity (INL) range of [-2.1, 8.1] LSB. The ultra-high timing resolution of two Cerenkov radiator integrated microchannel plate photomultiplier tubes was successfully measured using two identical CD-DSM TDCs. The full width at half maximum of the measured coincidence timing histogram was 83 ± 3.5 ps, closely matching the 85 ± 4.8 ps timing resolution obtained using a high-precision oscilloscope. A comparison with state-of-the-art TDCs highlights the superior performance of the CD-DSM TDC.

We present a novel field-programmable gate array (FPGA)-based bismuth germanate (BGO) time-of-flight (TOF) digitizer, implemented on an FPGA (XC7VX485T-2FFG1761C, Xilinx). This digitizer is designed to address the recently highlighted characteristics of BGO, which generates both scintillation and prompt Cerenkov photons when a 511 keV photon interacts with BGO. The developed digitizer independently processes these two types of photons for precise energy and timing measurements. The digitizer incorporates a noise-resistant binary counter that measures energy signals using the time-over-threshold (TOT) method. For timing measurements, we employ an embedded dual-side monitoring time-to-digital converter, which efficiently captures timing information while maintaining low resource usage. We validated the efficacy of our FPGA-based TOF digitizer through extensive experiments, including both electrical testing and coincidence measurements using BGO pixels. Our evaluations of TOT energy and timing performance utilized two 3 × 3 × 20 mm3BGO pixels coupled to CHK-HD MT silicon photomultipliers. The digitizer achieved a coincidence timing resolution (CTR) of 407 ps full width at half maximum (FWHM) for events within the full width at tenth maximum of the photopeak in the measured TOT energy spectrum. Notably, when measured with an oscilloscope, the same detector pair exhibited a CTR of 403 ps FWHM, confirming that the performance of the developed digitizer is comparable to that of an oscilloscope. With its low resource usage, our design offers significant potential for scalability, making it particularly promising for multi-channel BGO-based PET systems.

Positron emission tomography (PET) is the most sensitive biomedical imaging modality for noninvasively detecting and visualizing positron-emitting radiopharmaceuticals within a subject. In PET, measuring the time-of-flight (TOF) information for each pair of 511 keV annihilation photons improves effective sensitivity but requires high timing resolution. Hybrid materials that emit both scintillation and Cherenkov photons, such as bismuth germanate, recently offer the potential for more precise timing information from Cherenkov photons while maintaining adequate energy resolution from scintillation photons. However, a significant challenge in using such hybrid materials for TOF PET applications lies in the event-dependent timing spread caused by the mixed detection of Cherenkov and scintillation photons due to relatively lower production of Cherenkov photons. This study introduces an innovative approach by segmenting silicon photomultiplier (SiPM) pixels coupled to a single crystal, rather than using traditional SiPMs that are as large as or larger than the crystals they read. We demonstrated that multiple timestamps and photon counts obtained from the segmented SiPM can classify events by providing temporal photon density, effectively addressing this challenge. The approach and findings would lead to new opportunities in applications that require precise timing and photon counting.

This study presents a novel field-programmable gate array (FPGA)-based Time-to-Digital Converter (TDC) design suitable for high timing resolution applications, utilizing two new techniques. First, a cross-detection (CD) method is introduced that minimizes the occurrence of bubbles, which cause inaccuracy in the timing measurement of a TDC in thermometer codes, by altering the conventional sampling pattern, thereby yielding an average bin size half of its typical size. The second technique employs dual-side monitoring (DSM) of thermometer codes, including end-of-propagation (EOP) and start-of-propagation (SOP). Distinct from conventional TDCs, which focus solely on SOP thermometer codes, this technique utilizes EOP to calibrate SOP, simultaneously enhancing time resolution and the TDC's stability against changes in temperature and location. The proposed DSM scheme necessitates only an additional CARRY4 for capturing the EOP thermometer code, rendering it a resource-efficient solution. The CD-DSM TDC has been successfully implemented on a Virtex-7 Xilinx FPGA (a 28-nm process), with an average bin size of 6.1 ps and a root mean square of 3.8 ps. Compared to conventional TDCs, the CD-DSM TDC offers superior linearity. The successful measurement of ultra-high coincidence timing resolution (CTR) from two Cerenkov radiator integrated microchannel plate photomultiplier tubes (CRI-MCP-PMTs) was conducted with the CD-DSM TDCs for sub-100 ps timing measurements. A comparison with current-edge TDCs further highlights the superior performance of the CD-DSM TDCs.

This is an explanatory paper on Sun Il Kwon et al., Nat. Photon. 15: 914-918, 2021 and some parts of this manuscript are translated from the paper. Medical imaging modalities such as X-ray computed tomography, Magnetic resonance imaging, positron emission tomography (PET), and single photon emission computed tomography, require image reconstruction processes, consequently constraining them to form cylindrical shapes. However, among them, only PET can use additional information, so called time of flight, on an event-by-event basis. If coincidence time resolution (CTR) of PET detectors improved to 30 ps, which corresponds to spatial resolution of 4.5 mm, directly localizing electron-positron annihilation point is possible, allowing us to circumvent image reconstruction processes and free us from the geometric constraint. We call this concept direct positron emission imaging (dPEI). We have developed ultrafast radiation detectors by focusing on Cherenkov photon detection. Furthermore, the CTR of 32 ps being equivalent to 4.8 mm spatial resolution is achieved by combining deep learning-based signal processing with the detectors. In this article, we explain how we developed the detectors and demonstrated the first dPEI using different types of phantoms, how we will tackle limitations to be addressed to make the dPEI more practical, and how dPEI will emerge as an imaging modality in nuclear medicine.

Objective. Recent SiPM developments and improved front-end electronics have opened new doors in TOF-PET with a focus on prompt photon detection. For instance, the relatively high Cherenkov yield of bismuth-germanate (BGO) upon 511 keV gamma interaction has triggered a lot of interest, especially for its use in total body positron emission tomography (PET) scanners due to the crystal’s relatively low material and production costs. However, the electronic readout and timing optimization of the SiPMs still poses many questions. Lab experiments have shown the prospect of Cherenkov detection, with coincidence time resolutions (CTRs) of 200 ps FWHM achieved with small pixels, but lack system integration due to an unacceptable high power uptake of the used amplifiers. Approach. Following recent studies the most practical circuits with lower power uptake (<30 mW) have been implemented and the CTR performance with BGO of newly developed SiPMs from Fondazione Bruno Kessler tested. These novel SiPMs are optimized for highest single photon time resolution (SPTR). Main results. We achieved a best CTR FWHM of 123 ps for 2 × 2 × 3 mm 3 and 243 ps for 3 × 3 × 20 mm 3 BGO crystals. We further show that with these devices a CTR of 106 ps is possible using commercially available 3 × 3 × 20 mm 3 LYSO:Ce,Mg crystals. To give an insight in the timing properties of these SiPMs, we measured the SPTR with black coated PbF 2 of 2 × 2 × 3 mm 3 size. We confirmed an SPTR of 68 ps FWHM published in literature for standard devices and show that the optimized SiPMs can improve this value to 42 ps. Pushing the SiPM bias and using 1 × 1 mm 2 area devices we measured an SPTR of 28 ps FWHM. Significance. We have shown that advancements in readout electronics and SiPMs can lead to improved CTR with Cherenkov emitting crystals. Enabling time-of-flight with BGO will trigger a high interest for its use in low-cost and total-body PET scanners. Furthermore, owing to the prompt nature of Cherenkov emission, future CTR improvements are conceivable, for which a low-power electronic implementation is indispensable. In an extended discussion we will give a roadmap to best timing with prompt photons.

Time-of-flight (TOF) positron emission tomography (PET) detectors improve the signal-to-noise ratio of PET images by limiting the position of the generation of two 511 keV gamma-rays in space using the arrival time difference between the two photons. Unfortunately, bismuth germanate (BGO), widely used in conventional PET detectors, was limited as a TOF PET scintillator due to the relatively slow decay time of the scintillation photons. However, prompt Cerenkov light in BGO has been identified in addition to scintillation photons. Using Cerenkov photons for timing has significantly improved the coincidence timing resolution (CTR) of BGO. Based on this, further research on improving the CTR for a BGO-based TOF PET system is being actively conducted. Wrapping materials for BGO pixels have primarily employed white reflectors to most efficiently collect scintillation light. White reflectors have customarily been used as reflectors for BGO pixels even after Cerenkov light began to be utilized for timing calculations in pixel-level experiments. However, when the arrival-time differences of the two 511 keV annihilations photons were measured with pure Cerenkov radiators, painting the lateral sides of the radiators black can improve CTR by suppressing the reflection of Cerenkov photons. The use of BGO for TOF PET detectors requires simultaneously minimizing scintillation loss for good energy information and suppressing reflected Cerenkov photons for better timing performance. Thus, reflectors for BGO pixels should be optimized for better timing and energy performance. In this study, colored polytetrafluoroethylene (PTFE) tapes with discontinuous reflectance values at specific wavelengths were applied as a BGO reflector. We hypothesized that CTR could be enhanced by selectively suppressing reflected Cerenkov photons with an optimum colored reflector on the BGO pixel while minimizing scintillation photon loss. CTRs were investigated utilizing white and three colors (yellow, red, and green) PTFE tapes as a reflector. In addition, black-painted PTFE tape and enhanced specular reflector film were investigated as reference reflector materials. When 3 × 3 × 20 mm3BGO pixels were wrapped with the yellow PTFE reflector, the CTR was significantly improved to 365 ± 5 ps from 403 ± 14 ps measured with the conventional white PTFE reflector. Adequate energy information was still obtained with only 4.1% degradation in light collection compared to the white reflector. Colored reflectors show the possibility to further improve CTR for BGO pixels with optimum reflectance design.

Energetic electrons traveling in a dispersive medium can produce Cerenkov radiation. Cerenkov photons’ prompt emission, combined with their predominantly forward emission direction with respect to the parent electron, makes them extremely promising to improve radiation detector timing resolution. Triggering gamma detections based on Cerenkov photons to achieve superior timing resolution is challenging due to the low number of photons produced per interaction. Monte Carlo simulations are fundamental to understanding their behavior and optimizing their pathway to detection. Therefore, accurately modeling the electron propagation and Cerenkov photons emission is crucial for reliable simulation results. In this work, we investigated the physics characteristics of the primary electrons (velocity, energy) and those of all emitted Cerenkov photons (spatial and timing distributions) generated by 511 keV photoelectric interactions in a bismuth germanate crystal using simulations with Geant4/GATE. Geant4 uses a stepwise particle tracking approach, and users can limit the electron velocity change per step. Without limiting it (default Geant4 settings), an electron mean step length of ∼250 μm was obtained, providing only macroscopic modeling of electron transport, with all Cerenkov photons emitted in the forward direction with respect to the incident gamma direction. Limiting the electron velocity change per step reduced the electron mean step length (∼0.200 μm), leading to a microscopic approach to its transport which more accurately modeled the electron physical properties in BGO at 511 keV. The electron and Cerenkov photons rapidly lost directionality, affecting Cerenkov photons’ transport and, ultimately, their detection. Results suggested that a deep understanding of low energy physics is crucial to perform accurate optical Monte Carlo simulations and ultimately use them in TOF PET detectors.

X-ray and gamma-ray photons are widely used for imaging but require a mathematical reconstruction step, known as tomography, to produce cross-sectional images from the measured data. Theoretically, the back-to-back annihilation photons produced by positron–electron annihilation can be directly localized in three-dimensional space using time-of-flight information without tomographic reconstruction; however, this has not yet been demonstrated due to the insufficient timing performance of available radiation detectors. Here we develop techniques based on detecting prompt Cherenkov photons, which, when combined with a convolutional neural network for timing estimation, resulted in an average timing precision of 32 ps, corresponding to a spatial precision of 4.8 mm. We show this is sufficient to produce cross-sectional images of a positron-emitting radionuclide directly from the detected coincident annihilation photons, without using any tomographic reconstruction algorithm. The reconstruction-free imaging demonstrated here directly localizes positron emission and frees the design of an imaging system from the geometric and sampling constraints that are normally present for tomographic reconstruction. Positron emission imaging without tomographic reconstruction is demonstrated. A Cherenkov radiation detector detects gamma rays produced by positron–electron annihilation. The position of a positron source is determined with a precision of 4.8 mm.

Achieving direct imaging of the annihilation position of a positron on an event-by-event basis using an ultrafast detector would have a great impact on the field of nuclear medicine. Cherenkov emission is the most attractive physical phenomenon for realizing such an ultrafast timing performance. Moreover, a microchannel-plate photomultiplier tube (MCP-PMT) is one of the most promising photodetectors for fully exploiting the fast timing properties of Cherenkov emission owing to its excellent single photon time resolution of 25 ps full width at half maximum (FWHM). However, as the MCP structure generally contains a lead compound, the gamma rays frequently and directly interact with the MCP, resulting in the degradation of its timing performance and generation of undesirable side peaks in its coincidence timing histogram. To overcome this problem, we have developed a new MCP-PMT based on an MCP consisting of borosilicate glass, thus drastically reducing the probability of the photoelectric effect occurring in the MCP. To evaluate its insensitivity to gamma rays and its timing performance, a coincidence experiment was performed and showed that the probability of direct interactions was reduced by a factor of 3.4. Moreover, a coincidence time resolution of 35.4 ± 0.4 ps FWHM, which is equivalent to a position resolution of 5.31 mm, was obtained without any pulse height/area cut, improving to 28.7 ± 3.0 ps when selecting on the highest amplitude events by careful optimization of the voltage divider circuit of the new MCP-PMT. The timing performance of this new MCP-PMT presents an important step toward making direct imaging possible.

The availability of large-area, economically produced, microchannel plate (MCP) photodetectors with tens of picosecond timing resolution and millimeter level spatial resolution for single photoelectrons are enabling new techniques where fast timing facilitates critical benefits including: more efficient background rejection and high vertex resolution in large scale high energy and nuclear physics (HEP and NP) experiments, particle track directionality information, and precise track reconstruction, as well as separation of Cherenkov and scintillation light. LAP-PDs are now being produced on a routine pilot production basis, and are available to be employed in high energy and nuclear physics, for commercial applications such as in detectors for mass spectrometers, neutron detection for scientific and homeland security (non-proliferation), and for medical imaging time-of-flight positron emission tomography (TOF-PET). In the following, we provide an update on target performance of routinely produced prototype LAPPDs, including the performance of one specific LAPPD which is being evaluated at UC Davis for potential TOF-PET application. Previously obtained preliminary TOF-PET test results, taken at Incom Inc. with an earlier LAPPD, are also discussed.

PURPOSE:We are investigating the use of promptly emitted Cerenkov photons to improve scintillation detector timing resolution for time-of-flight (TOF) positron emission tomography (PET). Bismuth germanate (BGO) scintillator was used in most commercial PET scanners until the emergence of lutetium oxyorthosilicate, which allowed for TOF PET by triggering on the fast and bright scintillation signal. Yet BGO is also a candidate to generate fast timing triggers based on Cerenkov light produced in the first few picoseconds following a gamma interaction. Triggering on the Cerenkov light produces excellent timing resolution in BGO but is complicated by the very low number of photons produced. A better understanding of the transport and collection of Cerenkov photons is needed to optimize their use for effective triggering of the detectors. METHODS:We simultaneously generated and tracked Cerenkov and scintillation photons with a new model of light transport that we have released in GATE V8.0. This crystal reflectance model was used to study photon detection and timing properties, building realistic waveforms as measured with silicon photomultipliers. RESULTS:We compared the behavior and effect of detecting Cerenkov and scintillation photons at several levels, including detection time stamps, travel time, and coincidence resolving time in 3 × 3 × 20 mm3 BGO crystals. Simulations showed excellent agreement with experimental results and indicated that Cerenkov photons constitute the majority of the signal rising edge. They are therefore critical to provide early triggering and improved the coincidence timing resolution by 50%. POTENTIAL APPLICATIONS:To our knowledge, this is the first complete simulation of the generation, transport, and detection of the combination of Cerenkov and scintillation photons for TOF detectors. This simulation framework will allow for quantitative study of the factors influencing timing resolution, including the photodetector characteristics, and ultimately aid the development of BGO and other Cerenkov-based detectors for TOF PET.