Impact of DOI Capability on Detector Performance: A Comparative Study of DOI and Non-DOI Detectors for High-Resolution and Sensitivity Organ-Specific PET Inserts

Abstract

High-sensitivity organ-specific positron emission tomography (PET) inserts employing long scintillation crystals in combination with small ring diameters require depth of interaction (DOI) encoding to ensure uniform spatial resolution across the field of view. In this study, we have developed high-resolution, DOI-capable detectors based on the Hamamatsu C13500 TOF-PET modules, using $1.6\times 1.6$ mm2 LSO crystals coupled to $4\times 4$ mm2 silicon photomultiplier. Detector configurations were designed either for a brain PET/MR insert (26 mm long crystals with a top light guide (TLG) for DOI encoding via light spread) or for a breast PET/MR insert (15 mm long with a TLG; 15 mm long with a stacked dual-layer configuration). Corresponding reference configurations without DOI capability were included to assess the impact of DOI implementation on detector performance. The 26 mm long brain detector configuration with TLG achieved an energy resolution of 12.1% and a coincidence time resolution (CTR) of 684 ps, with a degradation of 148 ps compared to the non-DOI variant. A newly developed DOI parameter calculation method improved DOI resolution from 9.1 ± 1.7 to 7.1 ± 1.8 mm. The 15 mm long single layer breast detector configuration with TLG yielded an energy resolution of 10.8%, a CTR of 460 ps, and a DOI resolution of 6.3 ± 1.8 mm. The 15 mm dual layer detector configuration provided comparable performance (11.5% energy resolution, 605 ps CTR) while eliminating the need for DOI calibration. For both the 15 and 26 mm single layer crystal block configurations, introducing a TLG preserved crystal identification and energy resolution. Overall, the DOI-capable designs demonstrated only minor performance tradeoffs, supporting the feasibility of both approaches for integration into organ-specific PET/MR inserts.