# Exploring spatiotemporal information in a Cherenkov and scintillation photon counting BGO TOF-PET semi-monolithic detector concept

## TL;DR

Researchers tested a new BGO detector for PET scans that uses timing and spatial data from light signals to improve imaging accuracy.

## Contribution

A semi-monolithic BGO TOF-PET detector concept using spatiotemporal photon data for improved timing and positioning.

## Key findings

- Approximately 77% of events were uniquely timestamped with the first photon using 1.8 ns signal shaping.
- A coincidence time resolution of 172 ps FWHM was achieved using FPDD-based corrections.
- Cherenkov detection probability correlated consistently with time resolution parameters.

## Abstract

Objective. Cherenkov signatures from a bismuth germanate (BGO) crystal open the possibility of establishing BGO as a promising material for time-of-flight positron emission tomography (TOF-PET) detectors, particularly if the first Cherenkov photons can be uniquely timestamped. To maximize the utility of Cherenkov signatures, we employed an optical photon counting detector concept based on a thick, semi-monolithic BGO crystal coupled to a silicon photomultiplier (SiPM) array that provides digital photon timestamps from each SiPM channel. We characterized a prototype detector to demonstrate this concept and explored the use of rich spatiotemporal information of photon transport kinetics. Approach. The detector was built using a 42.68 × 2 × 20 mm3 BGO crystal and a 16 × 1 array of 2 × 2 mm3 SiPMs with a 2.68 mm pitch. A 16-channel low-noise high-frequency signal processing chain with fast comparators generated digital photon signals, which were recorded using waveform digitizers. Three-dimensional (3D) position calibration and first photon delay distribution (FPDD) construction provided the basis for data-driven methods to improve time resolution and estimate the probability of Cherenkov detection for each event. Main results. With a sufficient number of SiPM channels and 1.8 ns signal shaping, approximately 77% of events were uniquely timestamped with the first photon. FPDD clearly captured photon arrival properties, parameterized with the Cherenkov and scintillation contributions. A coincidence time resolution with a reference detector of 172 ps full width at half maximum was achieved by FPDD-based correction of 3D position dependence. Parameters investigated for Cherenkov detection probability estimation showed consistent correlation with time resolution. Significance. The results demonstrated the feasibility of a photon counting BGO detector for TOF-PET with both promising timing and positioning performance. The abundance of photon information provides a strong basis for further performance gains through data-driven Cherenkov identification and advanced event-by-event corrections.