Development of a prompt gamma imaging system with multi-slit collimator and BGO + SiPM detector for proton therapy monitoring

Objectives: Real-time monitoring of beam delivery accuracy with imaging techniques attracts increasing interest in precise proton therapy routines. The aim of this work is to develop a prompt gamma imaging system with multi-slit collimator and BGO + SiPM detector. Methods: The detector consists of 12 x 10 detector blocks. Each block is composed of a 12 x 12 BGO array with a crystal pixel size of 3.5 mm x 3.5 mm x 30 mm, coupled to an 8 x 8 SiPM array. We intentionally chose the low-light-yield BGO scintillator to avoid SiPM saturation in 2~8 MeV photon detection. For the same reason, the 5.4 mm spacing between SiPM pixels was experimentally optimized to achieve balanced spatial and energy performance. The output signals of SiPM array were fed into a self-developed 64-channel ASIC and converted to analog E, X and Y signals with built-in Anger logic circuits and digital T signal with leading edge discrimination. The analog E, X and Y signals were then digitized by an 80 MHz 12-bit ADC on a digital processing board (DPB). Crystal index identification and energy window discrimination were performed inside FPGA on DPB. Timing synchronization was performed with an open source Ethernet-based White Rabbit switch. The multi-slit collimator consists of 27 slits with 4 mm width and 13.3° opening angle. Each slit has different length and oblique angles. The collimator parameters were determined from an optimization study considering FOV coverage, spatial resolution, sensitivity, projection overlapping and slit-edge penetration. The system response was modeled with a hybrid analytical calculation / Monte Carlo simulation approach. ML-EM reconstruction algorithm was used to reconstruct the prompt gamma image. Detector's intrinsic spatial resolution and energy performance were experimentally measured. Spatial resolution, sensitivity and Bragg peak positioning accuracy of the system design were evaluated from Monte Carlo simulation. Initial imaging studies were performed to test the collimator design and image reconstruction process. Results: Clear crystal identification map were measured with 88Y(0.898 MeV and 1.836MeV), 22Na(1.275 MeV) and 232Th(2.6 MeV). Measured energy resolution was 16.31% ± 3.91% @ 2.6 MeV. Excellent energy response linearity was observed at least up to 2.6 MeV. In the 2D FOV of 200 mm × 150 mm, the FWHM spatial resolution was 1.88 mm ~ 2.73 mm and the average sensitivity was 0.2%. The bias and root mean square error of Bragg peak positioning were 0