DOI estimation through signal arrival time distribution: a theoretical description including proof of concept measurements

Abstract

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 singleended 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 Cramér-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.

Image Reconstruction Analysis for Positron Emission Tomography With Heterostructured Scintillators

“…Our findings are in agreement with past studies [32], [52]. Also, several groups have proposed the time-walk correction or other methods [25], [53]- [55] to improve the shape of the distribution or account for it with bulk (pixelated or monolithic) crystals. However, heterostructures add another layer of complexity to this.…”

Image Reconstruction Analysis for Positron Emission Tomography with Heterostructured Scintillators

“…Our findings are in agreement with past studies [32], [50]. Also, several groups have proposed the time-walk correction or other methods [45], [51]- [53] to improve the shape of the distribution or account for it with bulk (pixelated or monolithic) crystals. However, heterostructures add another layer of complexity to this.…”

Low power implementation of high frequency SiPM readout for Cherenkov and scintillation detectors in TOF-PET

“…Low noise and high frequency front-end electronic readout of silicon photomultipliers (SiPMs) ( Cates et al 2018 , Gundacker et al 2019 ) is a primary method for demonstrating the latest experimental limits in achievable CTR with fast scintillation detectors and detection media that derive 511 keV photon interaction time estimation from small populations of promptly emitted optical photon signatures ( Gundacker et al 2020 , 2021 , Kratochwil et al 2021 , Loignon-Houle et al 2021 ). By minimizing the influence of electronic noise jitter on signal processing, this readout topology can provide single photon time resolution (SPTR) for large area SiPMs that is near the limit dictated by the device architecture ( Cates et al 2018 ), and it also provides excellent SNR for leading edge time estimators.…”