Timing perspective with dual-ended high-frequency SiPM readout for TOF-PET

With recent advancement in SiPM technology and fast readout electronics, one limiting factor on achievable timing performance of gamma-ray detectors for time-of-flight positron emission tomography (TOF-PET) is light transport in the crystal. The effects of light transport manifest as depth-of-interaction (DOI)-dependent detection time jitter. For Cherenkov photons, the increase of optical path mitigates the intrinsic advantage of this prompt signature. For high aspect-ratio crystals with a single-ended readout, the information of about half the photons cannot be fully harvested as they initially travel in the direction opposed to the photodetector. Low-noise high-frequency dual-ended SiPM readout can contribute to mitigate the aforementioned challenges and, at the same time, has the potential to combine ultra-fast timing with highest gamma-ray detection efficiency. We have studied the timing properties of LYSO:Ce and BGO in a symmetric dual-ended SiPM readout configuration. Coupling $3 \times 3 \times 20 \mathrm{~mm}^{3}$ polished BGO crystals to Broadcom AFBR-S4N44P014M SiPMs a CTR of 455 ps FWHM was measured for all photopeak events using the average of the front and back timestamp. Three optimization steps were used in the analysis: baseline correction, a novel adaptive timestamp weighting, and rise time correction. CTR values improved to 371 ps, 259 ps, and 232 ps, respectively, when all photopeak events were included. Additionally, for one third of the events per channel, the CTR could further be improved to 172 ps. Using polished 3x3x20 mm³ LYSO:Ce crystals, we obtained CTR values of 112 ps after baseline correction, and upon applying a time-based DOI correction, sub-100 ps for all photopeak events. This comprehensive study employs various light-emitting materials and geometries and provides detailed insights on the role of light transport and the efficient usage of information, toward the fundamental physical timing limits in TOF-PET.