Improving the time resolution of TOF-PET detectors by double-sided readout

Abstract

State-of-the-art scintillation detectors for time-of-flight positron emission tomography (TOF-PET) typically employ scintillation crystals with a high aspect ratio (e.g. 4 × 4 × 22 mm3) 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 3 × 3 × 20 mm3 crystals with SSR, CRT values of 184 ± 6 ps FWHM and 215 ± 6 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 174 ± 6 ps FWHM. A much more substantial improvement was observed for equally sized crystals having etched side surfaces. For these crystals the CRT changed from 358 ± 5 ps FWHM (head-on irradiation) and 343 ± 6 FWHM (side irradiation) with SSR to 180 ± 5 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 3 × 3 × 5 mm3 (CRT = 121 ± 2 ps FWHM) and 3 × 3 × 10 mm3 (CRT = 162 ± 9 ps FWHM and CRT = 183 ± 3 ps for polished and etched crystals, respectively).