Towards 1mm PET resolution using DOI modules based on dual-sided SiPM readout

Parallax error in PET modules can be reduced by measuring the annihilation photon depth of interaction (DOI) in the scintillation crystal on an event-by-event basis. Following implementations described in prior literature, we selected a dual-sided readout PET module design in which SiPMs are placed at both ends of a scintillation array and the ratio of the signal from one photodetector (A) divided by the signal sum of both detectors (A+B), as well as the plot of Signal A vs. Signal B are used to measure DOI. Our experimental apparatus consisted of a 12×12 scintillation array with 1×1×10mm³ pixels [from Proteus] with 50μ Toray Lumirror septa for DOI optimization. Both polished ends of the scintillation array were optically coupled to a 12×12×2mm³ anti-reflection coated UV fused silica light spreader window [from Edmund Optics]. The reverse side of each window was optically coupled to a low profile (~1mm thick) 4×4 element SiPM with 3 × 3mm² active area pixels [SPMArray2 from sensL]. Flexible Printed Circuit (FPC) cables were used to interface the DOI PET module to custom 16 channel differential pre-amplifiers connected to evaluation/power supply boards [SPMArray2-A0 and SPMArray2-A1, respectively, from sensL]. Average DOI resolution of 1.5±0.1mm FWHM and energy resolution of 20% FWHM was obtained. We conclude that for the limited instances of parallax error expected in realistic PET systems combined with ~1mm spatial resolution in the scintillation plane, the obtained result for DOI spatial resolution indicates that ~1mm spatial reconstruction resolution PET imaging is possible with the selected technical approach. Applications for such a compact high DOI resolution PET module include a prostate PET probe working in conjunction with a standard clinical PET imager or a dedicated prostate PET imager aiding in prostate cancer diagnosis and biopsy guidance, as well as- - a carotid artery PET probe imaging vulnerable plaque or a surgical breast imaging probe.