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 12x12 scintillation array with 1x1x10mm(3) pixels [from Proteus] with 50 mu Toray Lumirror septa for DOI optimization. Both polished ends of the scintillation array were optically coupled to a 12x12x2mm(3) 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 (similar to 1mm thick) 4x4 element SiPM with 3 x 3mm(2) 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 similar to 1mm spatial resolution in the scintillation plane, the obtained result for DOI spatial resolution indicates that similar to 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.