Scalable, Time-of-Flight and Depth-of-Interaction Detector Units for High-Resolution PET Systems

Resolving the depth of interaction (DOI) of a $\gamma $ -photon in the scintillator is necessary to correct for parallax errors in organ-dedicated and large-scale time-of-flight positron emission tomography (TOF-PET) scanners or enable the precise recovery of Compton-scattered $\gamma $ -photons. Doubling the number of readout channels and moving toward more complex detector designs are methods to encode DOI, often associated with high costs. We propose a DOI-capable TOF-PET detector unit concept confining light-sharing to two detector channels, where the high benefit lies in scalability and the prospect of Compton recovery between adjacent units. We evaluate these scalable, DOI-capable unit concepts, realizing DOI encoding between two LYSO:Ce,Ca crystals ( $3 \times 3 \times 20$ mm3; Taiwan Applied Crystals) one-to-one coupled to two Broadcom AFBR-S4N33C013 silicon-photomultipliers (SiPMs) read out with the TOFPET2 ASIC. The best-performing unit employing a triangular reflector sheet and optical glue between the two crystals and mounted on two FBK NUV-MT SiPMs results in a DOI resolution of about 3 mm (RMSE) based on the energy ratio digitized by the two channels while maintaining a coincidence time resolution (CTR) of 226 ps (FWHM) with TOFPET2 ASIC readout, applying a linear DOI correction. Using HF readout, the CTR of the proposed detector unit was improved to 141 ps (FWHM).