Comparison of count rate sensitivity performance for a LSO-TOF system with a Cherenkov radiation based PbF2-TOF system

The use of Cherenkov light production and timing for TOF PET has been suggested, and there are papers indicating the possibility to improve timing in this way. The present paper, however, focuses only on count rate sensitivity issues by comparing a LSO-LSO system against a potential PbF₂ system. This has been accomplished by using a simple two detector set up first looking at LSO-LSO coincidences and then exchanging the LSO crystal on one of the channels with PbF₂ and then extracting the coincidence sensitivity for a pure PbF₂ system. A simple comparison between the resulting LSO-LSO coincidence count rate and the PbF₂-PbF₂ coincidence count rate shows more than a 10 to 1 difference. In addition we have evaluated the time resolution achievable with two different photo sensor combinations, PMT-PMT and PMT-SiPM. Even if a time resolution of 50 ps may be possible to achieve in a PbF₂ system, this requires very special photo sensors. Our results with standard photo sensors indicate time resolutions of the order of ~250 ps. If a 50 ps time resolution can be achieved, this may be equivalent to a 500 ps LSO system with 10 times higher sensitivity. One must consider, however, the minimum number of counts needed in a 50 ps system to achieve reasonable image quality. In addition to use very fast, photo sensors, these have to have high quantum efficiency from 250 nm up to 600 nm. Possible photo sensors for Cherenkov PET may be MCP PMTs.