Exploring Cherenkov Emission of BGO for TOF-PET

Bismuth germanate (BGO) was the preferred crystal for positron emission tomography (PET) scanners, but was substituted with the emergence of faster crystals. Improvements in silicon photomultipliers (SiPMs) and the use of fast high frequency readout make it possible to use the prompt Cherenkov emission in BGO in order to boost the achievable coincidence time resolution (CTR) significantly. The large fluctuations in the detected Cherenkov photon yield are causing time or amplitude walk effects in the leading edge time discrimination, which are corrected by measuring the initial signal rise time via a double threshold system. Further a classification of “fast” and “slow” timing events is shown to make best use of all the information and upgrades the CTR for most of the 511-keV events. In order to assess the practicability of this novel approach various crystal geometries and state-of-the-art SiPMs from HPK, Ketek, Broadcom, and FBK have been evaluated with the focus on the applicability in total body PET systems. For typical PET sized crystals (3 × 3 × 20 mm³) coupled to area matching 3 × 3-mm² Broadcom SiPMs a time resolution of 261 ± 8 ps full width at half maximum (FWHM) was measured when applying time walk corrections, while the CTR of individual types of events with different Cherenkov yield range from 205 to 302-ps FWHM. A further thorough discussion and prospects of TOF-PET with BGO are given, especially in view of timing event classification of all detected 511-keV events, corresponding to various time of flight kernels ranging from high to low time resolution.