Wen He's latest paper Design and characterizatiion of a hybrid  PET detector with DOI capability was published in Medical Physics. In this study, we present an innovative hybrid detector design for positron emission tomography (PET) scanners that combines pixelated and semi-monolithic elements to enhance depth-of-interaction (DOI) capability, while simplifying the calibration process required to determine gamma photon interaction positions.

As shown in Figure 1, each hybrid layer was fabricated by cutting a 1.5-mm-thick monolithic slab halfway through and creating multiple slots to form a 1 × 16 row of lutetium–yttrium oxyorthosilicate (LYSO) crystals with dimensions of 1.53 × 1.0 × 15 mm³ (1.61 mm pitch), along with a monolithic slab measuring 25.8 × 0.5 × 15 mm³. The detector comprises eight hybrid layers of LYSO, with overall dimensions of 25.8 × 12.9 × 15 mm³. To estimate the x-position (pixelated direction) and y-position (non-pixelated direction), energy-weighted and energy-squared weighted averages were utilized. Pseudo-pixels were defined as discrete regions on the flood image based on a crystal look-up table (LUT), facilitating straightforward position determination. Spatial resolutions were measured in both pixelated and non-pixelated directions. DOI positions were estimated using the maximum-to-sum ratio of signals from the multipixel photon counter (MPPC). Coincidence timing resolution (CTR) was assessed through both the average and energy-weighted average of the earliest n time stamps, with two energy windows of 250–700 keV and 400–600 keV applied.

Results indicated that the flood image pattern yielded clear event clusters, demonstrating that straightforward calibration is sufficient for identifying x- and y-positions. Using a 400–600 keV energy window, the intrinsic spatial resolutions averaged 1.15 mm and 1.34 mm in the pixelated and non-pixelated directions, respectively. The second row of pseudo-pixels achieved an average DOI resolution of 5.1 mm (FWHM). The optimal CTR of 350 ps was attained with the energy-weighted average of the earliest four time stamps. Extending the energy window to 250–700 keV slightly reduced the DOI resolution but maintained the intrinsic spatial resolution, with the best CTR deteriorating to 410 ps.

In conclusion, the proposed hybrid detector concept was validated, and the prototype demonstrated high 3D positioning and timing performance. This novel detector design holds significant potential for enhancing DOI capability in both preclinical and clinical PET applications.

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Design and characterization of a hybrid PET detector with DOI capability