Analytic timing calculations and timing limits with prompt photons, high-aspect-ratio crystals, and complex TOF-kernels in TOF-PET

Abstract

Modeling the timing performance of light-based radiation detectors accurately is essential for optimizing time-of-flight positron emission tomography (TOF-PET). We present an analytic framework that combines existing models to predict the timing behavior of high-aspect ratio crystals, including contributions from prompt photons such as Cherenkov radiation. This framework is built on a closed-form solution for optical light transport, convolved with the photodetector response and photon production characteristics. Using conditional and joint probability distributions, we compute the first-photon arrival time distribution for hybrid detectors with scintillation and Cherenkov light. The detection time distribution is then self-convolved to derive the time delay spectra and three timing metrics are used to characterize complex TOF kernels. Additionally, we perform Cram\'er-Rao Lower Bound calculations with and without depth-of-interaction bias to evaluate the theoretical timing limits. Our analytic predictions align well with Monte Carlo simulations for BGO detectors under varying crystal thicknesses and single photon time resolution considering a digital photodetector. We show that the TOF shape is significantly affected by prompt photon statistics, crystal thickness, scintillation yield, and photodetector properties resulting in distinct metric-dependent timing performance. The proposed model enables rapid timing predictions for polished crystals, with the calculation time of a detector configuration in under a second, allowing for comprehensive parametric studies. This makes it a powerful tool for guiding detector development in fast-timing applications.