An optically coupled hybrid semi-monolithic PET detector for simplified DOI and position decoding.

OBJECTIVE: PET detectors that encode depth-of-interaction (DOI) while achieving high timing and energy resolution are essential for long axial field-of-view (FOV) clinical PET systems. In this work, we propose an optically coupled hybrid semi-monolithic detector design, in which a pixelated scintillator array is coupled to a monolithic scintillator slab using optical adhesive. This design enables DOI capability while simplifying positioning calibration and preserving high timing and energy performance. APPROACH: Based on this concept, a prototype detector composed of a single-layer hybrid structure was fabricated and comprehensively evaluated. The prototype consists of a 1 × 8 array of pixelated lutetium-yttrium oxyorthosilicate (LYSO) crystals, each measuring 3.1 × 2.1 × 20 mm3 with a pitch of a 3.2 mm, coupling via a high-refractive-index adhesive (n=1.7) to a monolithic LYSO slab of 25.5 × 1 × 20 mm3. The interface surfaces between the pixelated array and the monolithic slab were unpolished. The detector was read out using a TOFPET2 application-specific integrated circuit (ASIC). MAIN RESULTS: Experimental results show clearly separated pseudo-pixel event clusters in the flood image, enabling planar event positioning using a conventional energy-weighted average method. Using a 400-600 keV energy window, the detector achieved an average DOI resolution of 4.0 ± 0.5 mm, an average energy resolution of 11.7% ± 0.9%, an intrinsic spatial resolution of 1.7 mm along the layer-stacking direction and 2.0± 0.1 mm along the pixelated array direction, and a coincidence timing resolution (CTR) of 281 ps. SIGNIFICANCE: These results demonstrate that the proposed optically coupled hybrid semi-monolithic detector balances detector performance (including DOI, timing and energy resolution) with calibration complexity. The design offers a promising detector solution for long axial FOV clinical PET systems.