Evaluation of a clinical TOF-PET detector design that achieves ≤100 ps coincidence time resolution

Commercially available clinical positron emission tomography (PET) detectors employ scintillation crystals that are long (>= 20 mm length) and narrow (4-5 mm width) optically coupled on their narrow end to a photosensor. The aspect ratio of this traditional crystal rod configuration and 511 keV photon attenuation properties yield significant variances in scintillation light collection efficiency and transit time to the photodetector, due to variations in the 511 keV photon interaction depth in the crystal. These variances contribute significant to coincidence time resolution degradation. If instead, crystals are coupled to a photosensor on their long side, near-complete light collection efficiency can be achieved, and scintillation photon transit time jitter is reduced. In this work, we compare the achievable coincidence time resolution (CTR) of LGSO:Ce(0.025 mol%) crystals 3-20 mm in length when optically coupled to silicon photomultipliers (SiPMs) on either their short end or long side face. In this 'side readout' configuration, a CTR of 102 +/- 2 ps FWHM was measured with 2.9 x .2.9 x 20 mm(3) crystals coupled to rows of 3 x 3 mm(2) SensL-J SiPMs using leading edge time pickoff and a single timing channel. This is in contrast to a CTR of 137 +/- 3 ps FWHM when the same crystals were coupled to single 3x3 mm(2) SiPMs on their narrow ends. We further study the statistical limit on CTR using side readout via the Cramer-Rao lower bound (CRLB), with consideration given to ongoing work to further improve photosensor technologies and exploit fast phenomena to ultimately achieve 10 ps FWHM CTR. Potential design aspects of scalable front-end signal processing readout electronics using this side readout configuration are discussed. Altogether, we demonstrate that the side readout configuration offers an immediate solution for 100 ps CTR clinical PET detectors and mitigates factors prohibiting future efforts to achieve 10 ps FWHM CTR.