The challenge to reach 10 ps coincidence time resolution (CTR) in time-of-flight positron emission tomography (TOF-PET) is triggering major efforts worldwide, but timing improvements of scintillation detectors will remain elusive without depth-of-interaction (DOI) correction in long crystals. Nonetheless, this momentum opportunely brings up the prospect of a fully time-based DOI estimation since fast timing signals intrinsically carry DOI information, even with a traditional single-ended readout. Consequently, extracting features of the detected signal time distribution could uncover the spatial origin of the interaction and in return, provide enhancement on the timing precision of detectors. We demonstrate the validity of a time-based DOI estimation concept in two steps. First, experimental measurements were carried out with current LSO:Ce:Ca crystals coupled to FBK NUV-HD SiPMs read out by fast high-frequency electronics to provide new evidence of a distinct DOI effect on CTR not observable before with slower electronics. Using this detector, a DOI discrimination using a double-threshold scheme on the analog timing signal together with the signal intensity information was also developed without any complex readout or detector modification. As a second step, we explored by simulation the anticipated performance requirements of future detectors to efficiently estimate the DOI and we proposed four estimators that exploit either more generic or more precise features of the DOI-dependent timestamp distribution. A simple estimator using the time difference between two timestamps provided enhanced CTR. Additional improvements were achieved with estimators using multiple timestamps (e.g. kernel density estimation and neural network) converging to the Cramr–Rao lower bound developed in this work for a time-based DOI estimation. This two-step study provides insights on current and future possibilities in exploiting the timing signal features for DOI estimation aiming at ultra-fast CTR while maintaining detection efficiency for TOF PET.

在飞行时间正电子发射断层扫描 (TOF-PET) 中达到 10 ps 重合时间分辨率 (CTR) 的挑战正在引发全球范围内的重大努力，但在没有交互深度 (DOI) 的情况下闪烁探测器的时间改进仍然难以捉摸校正长晶体。尽管如此，这种势头恰逢其时地带来了完全基于时间的 DOI 估计的前景，因为快速定时信号本质上携带 DOI 信息，即使使用传统的单端读数也是如此。因此，提取检测到的信号时间分布的特征可以揭示相互作用的空间起源，并反过来提高检测器的计时精度。我们分两步证明了基于时间的 DOI 估计概念的有效性。首先，使用当前的 LSO:Ce 进行实验测量：与 FBK NUV-HD SiPM 耦合的 Ca 晶体由快速高频电子设备读出，以提供新的证据，证明 DOI 对 CTR 的明显影响以前使用较慢的电子设备无法观察到。使用该检测器，还开发了在模拟定时信号上使用双阈值方案以及信号强度信息的 DOI 鉴别，无需任何复杂的读出或检测器修改。作为第二步，我们通过模拟探索未来检测器的预期性能要求，以有效地估计 DOI，我们提出了四个估计器，它们利用 DOI 相关时间戳分布的更通用或更精确的特征。使用两个时间戳之间的时间差的简单估计器提供了增强的 CTR。使用多个时间戳的估计器实现了额外的改进（例如 核密度估计和神经网络）收敛到在这项工作中开发的 Cramr-Rao 下界，用于基于时间的 DOI 估计。这项两步研究提供了关于利用时序信号特征进行 DOI 估计的当前和未来可能性的见解，旨在实现超快 CTR，同时保持 TOF PET 的检测效率。