Investigating the Timing Behavior of Compton Scattering in BGO for Time-of-Flight PET

TL;DR

This study examines how inter-crystal scattering affects timing resolution in BGO-based TOF-PET detectors, highlighting the importance of timing strategies to mitigate scatter effects for improved performance.

Contribution

It provides experimental insights into the impact of inter-crystal scattering on BGO Cherenkov timing and proposes optimal timestamping methods for pixelated detectors.

Findings

InterCS events have worse timing resolution than FED events.

Optimal timing achieved by selecting the earlier timestamp in InterCS events.

Prompt photon yield decreases due to energy splitting in InterCS events.

Abstract

Bismuth germanate (BGO) is gaining renewed attention as a viable material for hybrid Cherenkov/scintillation time-of-flight positron emission tomography (TOF-PET) detectors. While single-crystal studies have demonstrated excellent timing resolution by leveraging prompt Cherenkov photons, practical detector modules based on pixelated arrays introduce a high prevalence of inter-crystal scattering (InterCS) events, complicating timing accuracy. In this study, we experimentally investigated the impact of InterCS on BGO Cherenkov timing using a dual-pixel detector coupled to a segmented SiPM readout. Events were classified into full-energy deposition (FED; primary crystal 511 keV absorption), InterCS, and penetration types via energy-weighted positioning and validated using GATE simulations, which also revealed that over 25% of the experimentally identified full-energy events involved intra-crystal scatter (IntraCS). For InterCS events, the optimal timestamp selection was achieved by choosing the earlier of the two timestamps, yielding a coincidence timing resolution of 221 ps FWHM (831 ps FWTM)-markedly worse than the 184 ps (603 ps FWTM) obtained for FED events. Furthermore, prompt photon yield was found to decrease measurably due to energy splitting: InterCS events averaged 4.73 detected photons in the first 1 ns, compared to 5.76 for FED events. These results emphasize the importance of incorporating time-aware, per-pixel timestamping strategies in pixelated BGO TOF-PET systems to maintain optimal timing performance in the presence of scatter.