Towards 50 ps TOF-PET for brain imaging

Purpose: Time-of-flight (TOF) been successfully implemented in whole body PET, significantly improving clinical performance. However, for dedicated brain PET systems, TOF has not been a priority due the relatively small size of the human head, where coincidence timing resolution (CTR) below 200 ps is necessary to arrive at substantial performance improvements. The Brain PET (BET) consortium is developing a dual-ended PET detector block concept with ultrafast CTR, high sensitivity and high spatial resolution (X, Y, depth-of-interaction, DOI) that provides a pathway to significantly improved brain PET. Methods: We have implemented analytical and Monte Carlo models of scintillation photons transport in scintillator segments with arbitrary trans-axial cross-section dimensions. Results: Timing performance is independent of trans-axial cross-section as long as there is a gap between the scintillator and reflector wrapping. Intimate contact between the wrapping with the scintillator decreases the percentage of total internally reflected photons, degrading CTR performance. Excellent CTR performance can be achieved using simple fixed voltage thresholding techniques to determine the arrival times at the top and bottom SiPM. The average of the top and bottom arrival time corresponds to the time of gamma ray absorption, while the difference in arrival time corresponds to DOI. A simple algorithm to use the difference in arrival time to compensate for gamma ray transit time and optical photon transit achieves performance within 20% of the Cramer-Rao lower bound. We established that the advanced silicon photomultiplier designs with high single photon detection efficiency (QE=80%) and high single photon timing resolution (SPTR) ∼50 ps are critical for achieving ultrafast TOF-PET performance with CTR ∼50 ps and ∼4 mm DOI resolution.