# 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 accuracy in BGO-based time-of-flight PET detectors and proposes strategies to mitigate its impact.

## Contribution

The work introduces a method to experimentally assess and mitigate timing degradation caused by inter-crystal scattering in pixelated BGO detectors.

## Key findings

- InterCS events showed a 221 ps FWHM timing resolution, worse than FED events at 184 ps FWHM.
- InterCS events had a 4.73 average prompt photon yield in the first 1 ns, compared to 5.76 for FED events.
- Energy-based timestamp selection was found to be suboptimal for maintaining timing accuracy.

## Abstract

Objective. Bismuth germanate (BGO) has regained attention as a promising material for hybrid Cherenkov/scintillation time-of-flight positron emission tomography (TOF-PET). While excellent timing performance has been demonstrated in single-crystal studies using prompt Cherenkov photons, practical pixelated detector modules introduce appreciable inter-crystal scattering (InterCS) events that can degrade timing accuracy. The objective of this work was to experimentally investigate the impact of InterCS on Cherenkov-based timing in pixelated BGO detectors and to identify optimal timestamp selection strategies. Approach. A dual-pixel BGO detector was constructed and coupled to a segmented SiPM readout to enable spatially resolved energy and timing measurements. Events were classified into full-energy deposition (FED); primary crystal 511 keV absorption), InterCS, and penetration categories using energy-weighted positioning. This experimental classification was validated using GATE simulations, which further revealed that intra-crystal scattering (IntraCS) accounted for more than 25% of the events experimentally classified as FED. Multiple timestamp selection strategies were evaluated, and prompt photon statistics were quantified by integrating the first 1 ns of the timing waveform. Main results. For InterCS events, selecting the earlier of the two timestamps yielded a coincidence timing resolution of 221 ps FWHM (831 ps FWTM) measured in coincidence with a LYSO:(Ce, Mg) reference detector, compared to 184 ps FWHM (603 ps FWTM) for FED events. Energy-based timestamp selection was found to be suboptimal. Prompt photon analysis showed a measurable reduction in early photon yield for InterCS events, with an average of 4.73 detected photons in the first 1 ns, compared to 5.76 for FED events. Significance. these results demonstrate that InterCS introduces systematic timing degradation in pixelated BGO Cherenkov TOF-PET detectors through energy redistribution and reduced prompt photon statistics. The findings highlight the necessity of time-aware, per-pixel timestamp selection strategies to preserve optimal timing performance in realistic BGO-based TOF-PET systems operating in the presence of Compton scattering.