Calibration of DOI-capable PET detector panels using uncollimated front-face irradiation

OBJECTIVE: Gold-standard depth-of-interaction (DOI) calibration using collimated gamma-ray irradiation is time-consuming and requires complex experimental setups, making it impractical for system-level calibration of detector arrays. This work investigates an efficient method for DOI and energy calibration of detector panels using uncollimated irradiation, in which gamma rays are incident parallel, or nearly parallel, to the crystal depth direction. Approach. The 511-keV photopeak location on a dual-ended readout PET detector block as a function of crystal depth was first evaluated using both collimated and uncollimated 22Na source irradiation to determine whether the latter yields comparable estimates when DOI calibration parameters are known. A 4 × 4 dual-ended readout PET detector panel was then assembled, and three detector blocks were calibrated using the gold-standard method. Two DOI calibration approaches based on uncollimated irradiation-a physics-informed model and a multilayer perceptron (MLP)-were compared against the gold-standard calibration. Finally, the detector panel was calibrated for DOI and energy using the MLP-based approach. Main Results. The median relative root mean squared error (RMSE) between second-order polynomial fits to the depth-dependent photopeak location obtained using collimated and uncollimated irradiation was 1%, demonstrating that uncollimated irradiation provides reliable estimates when accurate DOI calibration parameters are available. The RMSE between the gold-standard DOI calibration and the physics-informed and MLP-based approaches ranged from 0.38-0.58 mm and 0.36-0.61 mm, respectively, with no statistically significant difference. However, DOI resolution estimates from the MLP-based approach outperformed those of the physics-informed model, favouring its use for full detector panel calibration. After saturation correction, the detector panel achieved a mean energy resolution of 15.6% and a DOI resolution of 2.0 mm. Significance. The proposed MLP-based calibration requires only a single uniform 511-keV source irradiation, making it simple to implement and sufficiently robust for in situ calibration of DOI-capable PET detector arrays.