Toward 100 ps Coincidence Time Resolution Using Multiple Timestamps in Depth-Encoding PET Modules: A Monte Carlo Simulation Study

Coincidence time resolution (CTR) in modern time-of-flight (TOF) PET scanners is limited by properties of the detector system, namely scintillator size and material, as well as single-photon time resolution (SPTR) and photon detection efficiency of the readout pixels. Recent studies have demonstrated the effectiveness of incorporating depth of interaction (DOI) information into TOF readout to improve CTR. Having multiple timestamps per event has also been shown to improve CTR through leading edge slope estimation. We propose using convolutional neural networks (CNNs) to improve CTR in PET modules with excellent DOI resolution and access to multiple timestamps per event. Monte Carlo simulations were used to generate PET-like data on 4-to-1 coupled single-ended readout depth-encoding modules with 1.5 ×1.5 ×20 mm³ LYSO crystals and prismatoid light guide arrays. We trained our CNN to perform TOF estimation between the two simulated modules with and without DOI information, as well as with single and multiple timestamps. The results show using DOI and multiple timestamps provides more than 50% improvement in CTR over standard single timestamp acquisition when sufficient SPTR (<; 100 ps) is available. Our CNN demonstrates the importance of having multiple timestamps and excellent DOI resolution for improving CTR in practical depth-encoding PET modules.

I. Introduction

Time-of-flight (TOF) PET has become increasingly prevalent in clinical systems as well as research studies due to technological advancements over the past decade, including the mass production of silicon photomultiplier (SiPM) arrays with excellent single-photon time resolution (SPTR), making TOF PET the new industry standard [1]–[3]. Coincidence time resolution (CTR) in TOF PET is limited by multiple factors related to the detection system, including scintillator size, scintillation response to gamma ray absorption, photon detection efficiency (PDE, which is related to light transfer efficiency of the scintillator and detection efficiency of the photodetector), and the timing of the readout system, all of which contribute to temporal blur along the leading edge of the energy deposition curve [4]–[6]. Previous studies have demonstrated the benefits of TOF PET, including improved effective sensitivity which may result in reduced patient dose (through shorter scan times and/or lower administered dose) and/or improved signal-to-noise ratio (SNR) [7], [8]. The unofficial benchmark set by the TOF PET research field in recent years is to achieve 100 ps CTR, which would enable a fivefold improvement in SNR and possibly enable new imaging capabilities for PET in low signal applications [8]–[10], while achieving 10 ps CTR would enable direct image acquisition with precise event positioning based on timing alone [11], [12]. While many studies have demonstrated that sub-100 ps timing is achievable with modern PET detector technology, these studies rely on using thin scintillator crystals (≤5 mm), which are not practical to use in clinical systems due to the large degradation in gamma ray absorption efficiency [4], [5], [9], [10], [13], [14]. Other studies have explored the use of reading out from the sides of thick ( mm³) scintillator crystals to reduce the impact of scintillation photon transport on timing, but this configuration is not practical to use in large systems due to its bulky design [8], [15].