Monolithic PET Detector Calibration Using Uncollimated Source and Gamma Interaction Position Distribution Constraint

Abstract

Monolithic PET detector design is promising for low cost, high packing fraction as well as intrinsic DOI capability compared to the conventional pixelated detector design. The key to gamma interaction position estimation of monolithic detector is to calibrate the light response function (LRF). Conventionally LRF is obtained with a tedious and time-consuming process by acquiring training datasets of gamma events with known interaction positions using collimated sources attached to robotic stages. In this work, we propose a calibration method to obtain LRFs for monolithic detector using uncollimated gamma source. The basic idea of the calibration method is to find the appropriate LRF to match the IPD calculated from estimated LRF and the true IPD. LRF was iteratively estimated by minimizing the discrepancy between the two IPDs with a gradient descent algorithm. The proposed calibration method was performed on a monolithic detector consisting of a continuous LYSO crystal (size: $25 \times 25 \times 14 \mathrm {mm}^{3}$) and a $6 \times 6$ SiPM array. Detector positioning performance was evaluated with the proposed calibration method and compared with that obtained with conventional calibration method. In x/y direction, the average positioning resolution and mean absolute positioning bias were respectively $\sim 2.70\,\mathrm{mm}$ and $\sim 0.5\,\mathrm{mm}$ with the proposed calibration method, and $\sim 2.57\,\mathrm{mm}$ and $\sim 0.25\,\mathrm{mm}$ mm with conventional calibration method. In z direction, the average misclassification rate (events positioned to the wrong DOI layer) were 39% with the proposed calibration method and 33% with conventional calibration method. On the whole, the detector positioning performance with the proposed calibration method is slightly worse but comparable with that obtained with conventional calibration method. Based on the experimental validation results, we conclude that the proposed calibration method is accurate and feasible for monolithic detector design and could greatly simplify the calibration process of monolithic detector and potentially facilitate its application in real PET systems.