Experimental evaluation of depth-encoding absorber designs for prompt-gamma Compton imaging in proton therapy

Prompt-gamma Compton imaging (PGCI) has been presented as a promising in-vivo method for proton range verification. An accurate estimation of the Bragg-peak position can potentially be achieved by imaging high-energy prompt-gamma rays (in the range of several MeV). Therefore, scintillation detectors with thick and high Z crystals are mostly used as Compton absorbers for high-energy gamma-ray detection. However, an absorber using thick crystals degrades the angular resolution unless the absorber can provide continuous depth-of-interaction (DOI) measurement. The study investigated various detector configurations using thick crystals for developing a Compton absorber with DOI resolving capability without compromising energy performance. Two groups of lutetium yttrium oxyorthosilicate (LYSO) arrays with dimensions of 1.8 × 1.8 × 50 mm³ and 1.8 × 1.8 × 20 mm³, and each with four different surface treatments (combining crystal surface finishing and type of reflector coverage), were constructed for the study. The DOI detector utilized the dual-ended readout of pixelated scintillator arrays for depth encoding. The results revealed that the influences of type of the reflector coverage and crystal surface roughness on the performance of the DOI detectors for the 50- and 20-mm-thick LYSO differed greatly. Moreover, the combined use of the proposed partial inter-crystal reflector coverage and the unpolished surface finishing on the 50-mm-thick crystals were shown to improve both the DOI resolution and the energy performance but degraded the flood map quality. These results provide useful guidance for selecting an applicable Compton DOI absorber design for developing a high-performance PGCI system.