Improving the Time Resolution of TOF-PET Detectors by Double-Sided Readout

State-of-the-art scintillation detectors for time-offlight positron emission tomography (TOF-PET) typically employ scintillation crystals with a high aspect ratio (e. g. mm) read out on one of the small crystal surfaces. This single-sided readout (SSR) geometry may be unfavorable in terms of the coincidence resolving time (CRT) that can be achieved because of its effect on the light collection efficiency; the spread in the scintillation photon propagation times; and depth-of-interaction (DOI) related, variable detection delays. In this work it is investigated to which extent these effects can be mitigated by applying a fast photosensor on each of the small crystal surfaces. Such double-sided readout (DSR) has been introduced previously to counter the issue of DOI-related parallax errors. For the present purpose, we used Hamamatsu MPPC-S10362-33-050 C silicon photomultipliers (SiPMs) optically coupled to LSO: Ce, Ca scintillators. For polished mm crystals with SSR, CRT values of ps FWHM and ps FWHM were determined for irradiation head-on and from the side, respectively. In this case, DSR improved the CRT measured under side irradiation to ps FWHM. A much more substantial improvement was observed for equally sized crystals having etched side surfaces. For these crystals the CRT changed from ps FWHM (head-on irradiation) and FWHM (side irradiation) with SSR to ps FWHM with DSR (side irradiation). These values are compared with the time resolution of detectors employing LSO: Ce, Ca crystals with a size of mm (ps FWHM) and mm (ps FWHM and ps for polished and etched crystals, respectively).