Simulation study of wedge-shaped light guide for compensating depth-of-interaction induced timing uncertainty in Cherenkov radiator.

BACKGROUND: Coincidence timing resolution (CTR) is a key performance factor in PET systems. Cherenkov radiation, due to its ultrafast emission on the femtosecond scale, offers potential for achieving improved timing resolution compared to traditional scintillation-based detectors. However, the inherently low photon yield in pure Cherenkov radiators makes depth-of-interaction (DOI) estimation and its associated timing uncertainty a significant challenge, especially when trying to achieve sub-30 ps CTR. PURPOSE: This study aims to reduce DOI-induced timing uncertainty in Cherenkov-based PET detectors by proposing a novel wedge-shaped light guide design. The goal is to improve the CTR performance by compensating for optical path differences resulting from varying interaction depths. METHODS: A Monte Carlo simulation framework was developed to model gamma-ray interactions within a Cherenkov radiator, incorporating electron scattering and Cherenkov photon generation. Two detector geometries were compared: one with a conventional planar light guide and another with the proposed wedge-shaped light guide. The wedge design was tailored to delay early-arriving photons and synchronize photon transit times, thereby reducing time spread. The simulation tracked photon paths through the wedge and evaluated performance using metrics such as spatial resolution, timing spread, and sensitivity. RESULTS: Simulations demonstrated that the wedge-shaped light guide significantly reduced DOI-induced timing uncertainty-by 26.3% in a 3 mm-thick Cherenkov radiator-compared to the conventional configuration. The wedge geometry also enabled improved spatial resolution in multi-layer detector configurations. However, a trade-off with reduced sensitivity was observed. To address this, strategies such as utilizing discarded photons via refractive index manipulation and cascade detector structures were proposed. CONCLUSIONS: The wedge-shaped light guide effectively compensates for DOI-related path length differences and enhances CTR in Cherenkov-based PET detectors. This approach opens avenues for more accurate event localization without increasing photon yield, and could serve as a building block for advanced PET systems requiring ultrafast timing performance.