Time-of-flight Positron Emission Tomography Research Articles

Performance evaluation of the FastIC readout ASIC with emphasis on Cherenkov emission in TOF-PET

Objective. The efficient usage of prompt photons like Cherenkov emission is of great interest for the design of the next generation, cost-effective, and ultra-high-sensitivity time-of-flight positron emission tomography (TOF-PET) scanners. With custom, high power consuming, readout electronics and fast digitization the prospect of sub-300 ps FWHM with PET-sized BGO crystals have been shown. However, these results are not scalable to a full system consisting of thousands of detector elements. Approach. To pave the way toward a full TOF-PET scanner, we examine the performance of the FastIC ASIC with Cherenkov-emitting scintillators (BGO), together with one of the most recent SiPM detector developments based on metal trenching from FBK. The FastIC is a highly configurable ASIC with 8 input channels, a power consumption of 12 mW ch−1 and excellent linearity on the energy measurement. To put the timing performance of the FastIC into perspective, comparison measurements with high-power consuming readout electronics are performed. Main results. We achieve a best CTR FWHM of 330 ps for 2 × 2 × 3 mm3 and 490 ps for 2 × 2 × 20 mm3 BGO crystals with the FastIC. In addition, using 20 mm long LSO:Ce:Ca crystals, CTR values of 129 ps FWHM have been measured with the FastIC, only slightly worse to the state-of-the-art of 95 ps obtained with discrete HF electronics. Significance. For the first time, the timing capability of BGO with a scalable ASIC has been evaluated. The findings underscore the potential of the FastIC ASIC in the development of cost-effective TOF-PET scanners with excellent timing characteristics.

Dual threshold input receiver FPGA-only signal digitization method for time-of-flight positron emission tomography

AbstractAs silicon photomultiplier (SiPM)-based time-of-flight (TOF) positron emission tomography (PET) becomes popular, the need for sophisticated PET data acquisition (DAQ) systems is increasing. One promising solution to this challenge is the adoption of a field-programmable gate array (FPGA)-only signal digitization method. In this paper, we propose a new approach to efficiently implement an FPGA-only digitizer. We configured the input/output (IO) port of the FPGA to function as a dual-threshold voltage comparator through the use of simple passive circuitry and heterogeneous IO standards. This configuration overcomes the limitations of existing methods by allowing different threshold voltages for adjacent IO pins, effectively reducing routing complexity and lowering manufacturing costs. An FPGA-only digitizer was implemented by integrating the dual-threshold voltage comparator and FPGA-based time-to-digital converter. By combining the dual-threshold time-over-threshold (TOT) method and curve fitting, precise energy information could be obtained. The performance of the FPGA-only digitizer was assessed using a detector setup comprising a 3 × 3 × 20 mm3 LYSO scintillation crystal and a single pixel SiPM. Using the configured evaluation setup, an energy resolution of 12.5% and a time resolution of 146 ± 9 ps were achieved for a 20 mm scintillation crystal. The dual-threshold TOT implemented using the proposed method showed consistent linearity across an energy range of 100 keV to 600 keV. The proposed method is well-suited for the development of cost-effective DAQ systems in highly integrated TOF PET systems.

Characterisation of a high granularity multi-channel prompt γ-ray detection system prototype for proton range verification based on the PETsys TOFPET2 ASIC

Proton therapy requires range verification in order to exploit its full potential. One of the most promising approaches is to monitor prompt gamma-rays produced by nuclear interactions of the therapeutic particles in the patient tissues. A detector with a wide energy range from 100 keV to 15 MeV and excellent time resolution is required to achieve millimetric precision in proton range. During patient treatment, the detector count rates are usually above 106 s-1 and the fraction of pile-up events is very high for commonly used fast inorganic scintillators. We are investigating a full acceptance approach with increased granularity in order to reduce the size of the scintillators and consequently the count rate per channel. Stacking the scintillators in matrices requires suitable multi-channel photo-multipliers and a fitting acquisition system. Here, we present two geometries of CeBr3 crystals 5×5×20 mm3 and 10×10×30 mm3, together with modern silicon photo-multipliers (SiPM) adapted to work with the PETsys TOFPET2 ASIC. The TOFPET2 ASIC was developed for Time-of-Flight Positron Emission Tomography (TOF-PET) applications. Here we show its potential for higher gamma-ray energies and future hybrid imaging. First results of energy resolution of 6.1%–7.8% are achieved at 3.42 MeV using a 241Am9Be source. The time resolution was found to be below 100 ps and studies of the count rates and the dead time of the full system were performed. Different SiPM models are analysed for their impact on the coincidence time resolution.

Simultaneous activity and attenuation estimation in TOF-PET with TV-constrained nonconvex optimization.

An alternating direction method of multipliers (ADMM) framework is developed for nonsmooth biconvex optimization for inverse problems in imaging. In particular, the simultaneous estimation of activity and attenuation (SAA) problem in time-of-flight positron emission tomography (TOF-PET) has such a structure when maximum likelihood estimation (MLE) is employed. The ADMM framework is applied to MLE for SAA in TOF-PET, resulting in the ADMM-SAA algorithm. This algorithm is extended by imposing total variation (TV) constraints on both the activity and attenuation map, resulting in the ADMM-TVSAA algorithm. The performance of this algorithm is illustrated using the penalized maximum likelihood activity and attenuation estimation (P-MLAA) algorithm as a reference.

The properties of the positronium lifetime image reconstruction based on maximum likelihood estimation

The positronium lifetime imaging (PLI) reconstruction is a technique used in time-of-flight (TOF) positron emission tomography (PET) imaging that involves measuring the lifespan of positronium, which is a metastable electron-positron pair that arises when a PET molecule releases a positron, prior to its annihilation. We have previously developed a maximum likelihood (ML) algorithm for PLI reconstruction and demonstrated that it can generate quantitatively accurate lifetime images for a 570 ps (pico-seconds) TOF PET system. In this study, we conducted further investigations into the statistical properties of the algorithm, including the variability of the reconstruction results, the sensitivity of the algorithm to the number of acquired PLI events and its robustness to hyperparameter choices. Our findings indicate that the proposed ML method produces sufficiently stable lifetime images to enable reliable distinction of regions of interest. Moreover, the number of PLI events required to produce quantitatively accurate lifetime images is computationally plausible. These results demonstrate the potential of our ML algorithm for advancing the capabilities of TOF PET imaging.

16-channel SiPM high-frequency readout with time-over-threshold discrimination for ultrafast time-of-flight applications

BackgroundOver the past five years, ultrafast high-frequency (HF) readout concepts have advanced the timing performance of silicon photomultipliers (SiPMs). The shown impact in time-of-flight (TOF) techniques can further push the limits in light detection and ranging (LiDAR), time-of-flight positron-emission tomography (TOF-PET), time-of-flight computed tomography (TOF-CT) or high-energy physics (HEP). However, upscaling these electronics to a system-applicable, multi-channel readout, has remained a challenging task, posed by the use of discrete components and a high power consumption. To this day, there are no means to exploit the high TOF resolution of these electronics on system scale or to measure the actual timing performance limits of a full detector block.MethodsIn this work, we present a 16-channel HF readout board, including leading-edge discrimination and a linearized time-over-threshold (TOT) method, which is fully compatible with a high-precision time-to-digital converters (TDCs), such as the picoTDC developed at CERN. The discrete implementation allows ideal adaptation of this readout to a broad range of detection tasks. As a first step, the functionality of the circuit has been tested using the TOFPET2 ASIC as back-end electronics to emulate the TDC, also in view of its properties as a highly scalable data acquisition solution.ResultsThe produced board is able to mitigate influences of baseline shifts in the TOFPET2 front end, which has been shown in experiments with a pulsed laser, increasing the achievable intrinsic coincidence timing resolution (CTR) of the TOFPET2 readout electronics from 70 ps (FWHM) to 62 ps (FWHM). Single-channel coincidence experiments including a γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gamma$$\\end{document}-source, 2×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document}2×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document}3 mm3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3}$$\\end{document} LYSO:Ce,Ca crystals and Broadcom NUV-MT SiPMs resulted in a CTR of 118 ps (FWHM). For a 4×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document}4 matrix of 3.88×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document}3.88×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document}19 mm3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3}$$\\end{document} LYSO:Ce,Ca crystals one-to-one coupled to a 4×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document}4 array of Broadcom NUV-MT SiPMs, an average CTR of 223 ps (FWHM) was obtained.ConclusionThe implemented 16-channel HF electronics are fully functionall and have a negligible influence on the timing performance of the back-end electronics used, here the TOFPET2 ASIC. The ongoing integration of the picoTDC with the 16-channel HF board is expected to further set the path toward sub-100 ps TOF-PET and sub-30ps TOF resolution for single-photon detection.

The time resolution improvement of Cherenkov-Radiator-Window Photomultiplier tube

The improvement of time resolution in time-of-flight positron emission tomography (TOF-PET) relies on the development of scintillation materials and photodetectors. With the introduction of Cherenkov TOF-PET, higher requirements have been placed on the intrinsic timing performance of photodetectors. Our team has proposed a Cherenkov Radiator Window Photomultiplier tube (CRW-FPMT) that uses the Cherenkov radiator directly as the light window, which eliminates the boundary between the scintillator and the photodetector for fast time resolution. This paper mainly discusses the factors influencing its time resolution and optimizes it to achieve a single photon time resolution of 27.2 ps.

Prior information-guided reconstruction network for positron emission tomography images.

Deep learning has recently shown great potential in medical image reconstruction tasks. For positron emission tomography (PET) images, the direct reconstruction from raw data to radioactivity images using deep learning without any constraint may lead to the production of nonexistent structures. The aim of this study was to specifically develop and test a flexibly deep learning-based reconstruction network guided by any form of prior knowledge to achieve high quality and high reliability reconstruction. We developed a novel prior information-guided reconstruction network (PIGRN) with a dual-channel generator and a 2-scale discriminator based on a conditional generative adversarial network (cGAN). Besides the raw data channel, an additional channel is provided in the generator for prior information (PI) to guide the training phase. The PI can be reconstructed images obtained via conventional methods, nuclear medical images from other modalities, attenuation correction maps from time-of-flight-PET (TOF-PET) data, or any other physical parameters. For this study, the reconstructed images generated by filtered back projection (FBP) were chosen as the input of the additional channel. To improve the image quality, a 2-scale discriminator was adopted which can focus on both the coarse and fine field of the reconstruction images. Experiments were carried out on both a simulation dataset and a real Sprague Dawley (SD) rat dataset. Two classic deep learning-based reconstruction networks, including U-Net and Deep-PET, were compared in our study. Compared with these two methods, our method could provide much higher quality PET image reconstruction in the study of the simulation dataset. The peak signal-to-noise ratio (PSNR) value reached 31.8498, and the structure similarity index measure (SSIM) value reached 0.9754. The real study on SD rats indicated that the proposed network also has strong generalization ability. The flexible PIGRN based on cGAN for PET images combines both raw data and PI. The results of comparison experiments and a generalization experiment based on simulation and SD rat datasets demonstrated that the proposed PIGRN has the ability to improve image quality and has strong generalization ability.

Hydrothermally grown molybdenum doped ZnO nanorod arrays. The concept of novel ultrafast nanoscintillator

Molybdenum doped ZnO was hydrothermally grown as the arrays of nanorods deposited onto the fused silica glass substrate. The molybdenum doping level varied from 1 to 30 %. The influence of Mo on the electronic and crystalline structure as well as luminescence and defects in the ZnO nanorods is under study of the bundle of experimental techniques complemented with density functional theory calculations. The tendency of Mo to create energy states within the bandgap of ZnO and their influence on the energy levels of native defects as well as excitons were proven by the synergy of experiment and theory. The improvement of the timing characteristics of the exciton- and zinc vacancy-related emission bands upon Mo doping (1–10 %) was observed. This paves the way for the defect engineering strategy in the search of effective and ultrafast scintillator with the improved light yield as well as compared to other materials. The new concept is based on the combination of the exciton and the defect emission. It is expected to have the potential of application in the detection of gamma rays implemented in time-of-flight positron emission tomography (TOFPET). The 20 and 30 % Mo doping levels resulted in the zinc molybdates creation strongly outnumbering ZnO nanorods.

Advances in Design of High‐Performance Heterostructured Scintillators for Time‐of‐Flight Positron Emission Tomography

AbstractCore to advancing time‐of‐flight positron emission tomography (ToF‐PET) toward a less invasive, more flexible procedure with a higher diagnostic power is the development of enhanced radiation detector materials. One promising avenue is the development of heterostructured scintillators where multiple materials work in synergy to exceed the performance of each individual component. Applied to ToF‐PET detectors, one component contributes predominantly to the absorption of gamma rays and the other to the creation of ultra‐fast photons. Whilst other authors have proposed various concepts, heterostructured scintillators are still in their infancy and scientifically guiding their development remains a challenge. Toward this aim and based on simulation and modeling developments, heterostructure properties are directly linked to ToF‐PET performance. This is made possible by redefining the notions of detector photo‐peak efficiency and timing response, as defined for monolithic detectors, in the context of heterostructured scintillators. Their overall potential is then discussed as a function of the materials and design used. This provides a quantitative framework to rapidly and efficiently support the advancement of heterostructured detectors for ToF‐PET technology.

Low-dose TOF-PET based on surface electron production in dielectric laminar MCPs

We present simulations of whole-body low-dose time-of-flight positron emission tomography (TOF-PET) based on the direct surface production (Domurat-Sousa et al., 2023) by 511 keV gamma rays of energetic electrons via the Photo-electric and Compton Effects, eliminating the scintillator and photodetector sub-systems in PET scanners. In Ref. Domurat-Sousa et al. (2023) we described Microchannel Plates (MCP) constructed from thin dielectric laminae containing heavy nuclei such as lead or tungsten (LMCPTM). The laminae surfaces are micro-patterned to form channels, which can then be functionalized to support secondary electron emission in the manner of conventional MCPs. After assembly of the laminae, the channels form the pores of the conversion LMCP. This conversion stage is then followed by a high-gain MCP-based amplification stage, which also can be constructed using the laminar technique, but with pores typical of currently-available large-area MCPs .We have simulated direct conversion using modifications to the TOPAS Geant4-based tool kit. A 20 × 20 × 2.54 cm3 LMCP, composed of 150-micron thick lead-glass laminae, is predicted to have a ≥30% conversion efficiency to a primary electron that penetrates an interior wall of a pore. The subsequent secondary electron shower is largely confined to one pore and can provide high space and time resolutions.In whole-body PET scanners the technique eliminates the scintillator and photodetector subsystems. The consequent absence of a photocathode allows assembly of large arrays at atmospheric pressure and less stringent vacuum requirements, including use of pumped and cycled systems.TOPAS simulations of the Derenzo and XCAT-brain phantoms are presented with dose reductions of factors of 100 and 1000 from a literature benchmark. New applications of PET at a significantly lower radiation dose include routine screening for early detection of pathologies, the use in diagnostics in previously unserved patient populations such as children, and a larger installed facility base in rural and under-served populations, where simpler gamma detectors and lower radiation doses may enable small low-cost portable PET scanners.

Colored reflectors to improve coincidence timing resolution of BGO-based time-of-flight PET detectors

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 mm3 BGO 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.

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.

The value of using heterogeneous detector groups for the development of time-of-flight (TOF) positron emission tomography (PET) systems.

Objective. Much recent attention on positron emission tomography (PET) is the development of time-of-flight (TOF) systems with ever-improving coincidence time resolution (CTR). This is because, when all other factors remain the same, a better CTR leads to images of better statistics and effectively increases the sensitivity of the system. However, detector designs that aggressively improve the CTR often compromise the detection efficiency (DE) and offset the benefit gained. Under this circumstance, in developing a TOF PET system it may be beneficial to employ heterogeneous detector groups to balance the overall CTR and DE of the system. In this study, we examine the potential value of this system design strategy by considering two-dimensional systems that assume several representative ways of mixing two detector groups.Approach. The study is based on computer simulation and specifically considers medium time-resolution (MTR) detectors that have a 528 ps CTR and high time-resolution (HTR) detectors that have a 100 ps CTR and a DE that is 0.7 times that of the MTR detector. We examine contrast recovery, noise, and subjective quality of the resulting images under various ways of mixing the MTR and HTR detectors.Main results. With respect to the traditional configuration that adopts only the HTR detectors, symmetric heterogeneous configurations may offer comparable or better images while using considerably fewer HTRs. On the other hand, asymmetric heterogeneous configurations may allow the use of only a few HTRs for improving image quality locally.Significance. This study demonstrates the value of the proposed system-level design strategy of using heterogeneous detector groups for achieving high effective system sensitivity by factoring into the tradeoff between the CTR and DE of the detector.

Timing advances of commercial divalent-ion co-doped LYSO:Ce and SiPMs in sub-100 ps time-of-flight positron emission tomography

Objective. Together with novel photodetector technologies and emerging electronic front-end designs, scintillator material research is one of the key aspects to obtain ultra-fast timing in time-of-flight positron emission tomography (TOF-PET). In the late 1990s, Cerium-doped lutetium–yttrium oxyorthosilicate (LYSO:Ce) has been established as the state-of-the-art PET scintillator due to its fast decay time, high light yield and high stopping power. It has been shown that co-doping with divalent ions, such as Ca 2+ and Mg 2+, is beneficial for its scintillation characteristics and timing performance. Therefore, this work aims to identify a fast scintillation material to combine it with novel photosensor technologies to push the state of the art in TOF-PET. Approach. This study evaluates commercially available LYSO:Ce,Ca and LYSO:Ce,Mg samples manufactured by Taiwan Applied Crystal Co., LTD regarding their rise and decay times as well as their coincidence time resolution (CTR) with both ultra-fast high-frequency (HF) readout and commercially available readout electronics, i.e. the TOFPET2 ASIC. Main results. The co-doped samples exhibit state-of-the-art rise times of on average 60 ps and effective decay times of on average 35 ns. Using the latest technological improvements made on NUV-MT SiPMs by Fondazione Bruno Kessler and Broadcom Inc., a 3 × 3 × 19 mm3 LYSO:Ce,Ca crystal achieves a CTR of 95 ps (FWHM) with ultra-fast HF readout and 157 ps (FWHM) with the system-applicable TOFPET2 ASIC. Evaluating the timing limits of the scintillation material, we even show a CTR of 56 ps (FWHM) for small 2 × 2 × 3 mm3 pixels. A complete overview of the timing performance obtained with different coatings (Teflon, BaSO4) and different crystal sizes coupled to standard Broadcom AFBR-S4N33C013 SiPMs will be presented and discussed. Significance. This work thoroughly evaluates commercially available co-doped LYSO:Ce crystals and, in combination with novel NUV-MT SiPMs, shows a TOF performance that significantly exceeds the current state of the art.

On the Prospects of BaF2 as a Fast Scintillator for Time-of-Flight Positron Emission Tomography Systems

Future time-of-flight positron emission tomography (TOF-PET) will be in need of ultrafast scintillation materials, with potential seen in cross-luminescence (CL). BaF2, for example, shows a sub-100-ps decay time with 300 photons produced per MeV. However, it poses challenges, such as medium radiation length, low photofraction, moderate light yield, and vacuum ultraviolet (VUV) emission around 200nm. Recent developments in UV-sensitive solid-state photodetectors (SiPMs) have the potential to establish CL for ultrafast TOF in PET. In this work, we aim to study BaF2, read out by these new VUV SiPMs, as a viable alternative to cerium-doped lutetium–yttrium oxyorthosilicate (LYSO:Ce). In order to investigate the prospects of BaF2 in TOF-PET, we compare the CTR of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 2\times $ </tex-math></inline-formula> 20 mm 3 undoped and yttrium-doped BaF2 crystals, readout by Hamamatsu S13370–3050CN SiPMs with a maximum photon detection efficiency (PDE) of 14% at 200nm and air coupling, to LYSO:Ce:Ca crystals of the same size. With high-frequency readout electronics, we reached 233ps with undoped BaF2 and 213ps with BaF2:Y, while the performance of LYSO:Ce:Ca was 125ps (measured with HPK S14160–3050HS, PDE <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 60$ </tex-math></inline-formula> % and glue coupling). Conducting measurements at different depth of interaction positions shows a pronounced impact on the CTR. Furthermore, we investigated the performance for systems of BaF2 with TOFPET2c ASIC measurements and Geant4 simulations for effective sensitivity comparisons to LYSO:Ce:Ca.

Tracking the same fast-LGSO crystals by changing surface treatments for better coincidence timing resolution in PET

Achieving fast coincidence timing resolution (CTR) is an important issue in clinical time-of-flight positron emission tomography (TOF-PET) to improve the reconstructed image quality. One of the major factors affecting the CTR is the crystal surface treatment, which is often parameterized as surface roughness. However, previous studies on the crystal surface treatment optimization had two limitations of crystal-by-crystal variation and worse CTR over 200 ps. Here, we report the effects of the crystal surface treatment on the performance of a 20 mm long fast-LGSO crystal based TOF detector by tracking the same crystals in the sub-180 ps CTR regime. The light collection efficiency (LCE), energy resolution (ER) and CTR of the TOF detector were evaluated with six different crystal surface treatments of chemically polished (C.P), C.P half side roughened (1/2S) treatment, and then the C.P one side roughened (1S) treatment, mechanically polished (M.P) treatment, M.P 1/2S treatment, and M.P 1S treatment. The four lateral surfaces of each crystal were wrapped by using enhanced specular reflector film while the top surface was covered by using Teflon tape. The bottom surface of the crystal was optically coupled to a silicon photomultiplier. The timing and energy signals were extracted by using a custom-made high-frequency readout circuit, and then digitized by using a waveform digitizer. All the experimental conditions were same except the crystal surface treatment. Among the six different crystal surface treatments, the M.P 1S would be the optimal crystal surface treatment which balanced enhancements in the CTR (165 ± 3 ps) and ER (10.5 ± 0.5%). Unlike the M.P 1S, the C.P 1S did not enhance the CTR and ER. Hence, the C.P without roughening would be the second-best optimal crystal surface treatment which balanced the CTR (169 ± 3 ps) and ER (10.5 ± 0.5%).

A novel multi-anode MCP-PMT with Cherenkov radiator window

With the proposal of the road map of Time-of-Flight positron emission tomography (TOF-PET) with a Coincidence Time Resolution (CTR) of 10 ps by Paul Lecoq et al. in 2020, the technology based on TOF puts forward higher requirements on the performance of its core optical devices. In this manuscript, we developed a kind of Multi-Anode micro channel plate photomultiplier tube (MCP-PMT) based on Cherenkov radiator window. By using the Cherenkov radiator as the optical window of Multi-Anode MCP-PMT directly, the optical interface between the scintillator and the PMT window in conventional TOF-PET is eliminated, reducing light loss and further improve the time resolution of TOF-PET. The anode of this MCP-PMT can be split into a finely spaced array of collecting electrodes to achieve good positional sensitivity. In this paper, the single photon detection capability and time resolution of this kind of MCP-PMT are evaluated. The transit time spread (TTS) at single photon mode can reach 42.9 ps, and the TTS at multi photon mode is less than 10 ps. The CTR based on this kind of MCP-PMT is 156.7 ± 10.6 ps FWHM, which is equivalent to a position resolution of 23.5 ± 1.6 mm along a line of response (LOR). The performance uniformity of this kind MCP-PMT has also been calibrated.

HYPR4D kernel method on TOF PET data with validations including image-derived input function

BackgroundPositron emission tomography (PET) images are typically noisy especially in dynamic imaging where the PET data are divided into a number of short temporal frames often with a low number of counts. As a result, image features such as contrast and time–activity curves are highly variable. Noise reduction in PET is thus essential. Typical noise reduction methods tend to not preserve image features/patterns (e.g. contrast and size dependent) accurately. In this work, we report the first application of our HYPR4D kernel method on time-of-flight (TOF) PET data (i.e. PSF-HYPR4D-K-TOFOSEM). The proposed HYPR4D kernel method makes use of the mean 4D high frequency features and inconsistent noise patterns over OSEM subsets as well as the low noise property of the early reconstruction updates to achieve prior-free de-noising. The method was implemented and tested on the GE SIGNA PET/MR and was compared to the TOF reconstructions with PSF resolution modeling available on the system, namely PSF-TOFOSEM with and without standard post filter and PSF-TOFBSREM (TOF Q.Clear) with various beta values (regularization strengths).ResultsResults from experimental contrast phantom and human subject data with various PET tracers showed that the proposed method provides more robust and accurate image features compared to other regularization methods. The preservation of contrast for the PSF-HYPR4D-K-TOFOSEM was observed to be better and less dependent on the contrast and size of the target structures as compared to TOF Q.Clear and PSF-TOFOSEM with filter. At the same contrast level, PSF-HYPR4D-K-TOFOSEM achieved better 4D noise suppression than other methods (e.g. >2 times lower noise than TOF Q.Clear at the highest contrast). We also present a novel voxel search method to obtain an image-derived input function (IDIF) and demonstrate that the obtained IDIF is the most quantitative w.r.t. the measured blood samples when the acquired data are reconstructed with PSF-HYPR4D-K-TOFOSEM.ConclusionsThe overall results support superior performance of the PSF-HYPR4D-K-TOFOSEM for TOF PET data and demonstrate that the proposed method is likely suitable for all imaging tasks including the generation of IDIF without requiring any prior information as well as further improving the effective sensitivity of the imaging system.

Optimization of Adaptive Threshold in Tumor Segmentation Based on I-124 PET/CT NEMA Phantom Study

Numerous methods have been introduced to segment the thyroid tumors in iodine-124 (124I) Positron Emission Tomography/Computer Tomography (PET/CT) imaging. An adaptive threshold-based is preferable to a manual and fixed threshold-based approach for forecasting accurate and precise tumor volume in real-patient studies, as it was reported to be more resistant to noise and resolution. Manual segmentation was reported to be time-consuming, subjective, subject to inter- and intra-observer variability and prone to segmentation errors [1]. This study aimed to determine the optimal adaptive threshold for thyroid tumor segmentation in 124I PET/CT imaging.&#x0D; &#x0D; The objective of this study was firstly to determine the segmented volume using the standardized uptake value threshold method. Secondly, to compare the adaptive threshold and recovery coefficient (RC) value between the time of flight (TOF) and non-time of flight (non-TOF) PET/CT reconstructions in 124I phantom studies. Finally, to determine the accuracy and RC for 124I PET/CT studies. This study was done by placing NEMA 2012/IEC 2008 PET IQ Phantom and filled with 1 kBq/ml and 20 kBq/ml in its background and sphere each to model tumor background ratio (TBR) 20:1 of 124I PET/CT imaging. The phantom was scanned using TOF and non-TOF PET at 3, 4 and 5 minutes per bed position and reconstructed using 600, 1200 and 2000 beta penalization factors. Image registration, contouring and segmentation were performed using MIM Encore Software. A set of different adaptive thresholds (Tadaptive: 10%-50%) was applied to each sphere in the TBR image of the phantom using the tri-dimensional automatic segmentation tool. The optimal Tadaptive for each of the parameter assessed was defined based on the lowest goodness value calculated. The RC was calculated using mean value and known activity. Finally, the RC against the percentage of maximum was plotted to analyse the descending order of beta penalization factors for time per bed position.&#x0D; &#x0D; Figure 1 shows the image of 124I contoured VOIs on the spheres. The results showed that segmented volumes were not affected by the time per bed position in 124I PET/CT imaging. Table 1 shows the descending order beta penalization factors for time per bed position. Beta penalization factors of 600 to 2000 resulted in either underestimation or overestimation of the measured volume. Comparing TOF to non-TOF PET/CT 124I imaging, consistent and reproducible data were observed for the TOF PET for the three beta penalization factors. Tadaptive of 28.0% consistently gave the lowest goodness value shown by the closest R2 value to one, thus can produce lowest error if it is being used to reconstruct images.&#x0D; As a conclusion, 28% Tadaptive is suggested as the optimal threshold for 124I TOF PET/CT imaging due to consistent data obtained for all beta penalization factors assessed in this study. The inconsistent data obtained by non-TOF PET thus requires further optimization.