The minimum achievable timing resolution with high-aspect-ratio scintillation detectors for time-of-flight PET

Abstract

Clinically translatable scintillation detectors employ relatively long crystals (20-30 mm) for increased system sensitivity in whole-body PET. In these high-aspect ratio (HAR) crystals (long and narrow), the degradation in timing performance due to variance in the optical path length of scintillation light is non-negligible. In this work, an accurate, analytical probability density function (p.d.f.) for photon transit time was incorporated into a calculation of the Cramr-Rao Lower Bound (CRLB) on timing resolution to investigate the minimum achievable timing resolution for crystals with HAR geometries. The analytical p.d.f. was compared against light transport simulations and direct measurement of photon arrival time statistics using time-correlated single photon counting techniques (TCSPC), demonstrating consistency between the observed and theoretical distributions (χ2 p-value > 0.95). A coincidence timing resolution for two 3×3×20 mm3 LYSO:Ce scintillators coupled to MPPCs was measured to be 162 ps FWHM, approaching the statistically calculated limit of 152 ps within 6.5%. Parametric studies, where crystal and photosensor properties were varied, revealed that the increased optical transport spread in HAR crystals does not preclude an achievable coincidence timing resolution of 100 ps FWHM. The CRLB for various HAR scintillation detectors is presented, and instrumentation performance improvements necessary for achieving 100 ps FWHM coincidence timing resolution with HAR scintillators are discussed.