Development and validation of a measurement-driven inter crystal scatter recovery algorithm with in-system calibration

In PET a high percentage of gamma photons being detected undergo Compton scattering in the scintillator. Scintillator blocks are often built from optically isolated crystals. Depending on the angle of incidence and the scintillator geometry this might lead to inter crystal scatter (ICS) events, where energy is deposited in two or more crystals in the detector, which common positioning and reconstruction algorithms cannot resolve. Therefore, ICS events worsen the spatial resolution and the signal-to-noise ratio in the reconstructed image. We want to address this challenge by recovering individual crystals from ICS events with their corresponding energy deposits. This information could ultimately be fed into an image reconstruction framework. In this work, we established an algorithm based on a detector that couples a readout channel to each crystal (one-to-one coupling), which combines a measurement-driven calibration and a fitting routine to achieve the recovery of crystal interactions from measured light patterns. Using Geant4 simulations, we validated and optimized this approach by comparing the recovered events to the simulation ground truth. We showed that, with the best performing algorithm versions, all correct crystals could be identified for 95-97% of the simulated events and the crystal energies as well as the event energy sum could be recovered adequately. For the event energy sum a deviation of less than 5% could be achieved for 96% of all events. Overall, the developed ICS recovery algorithm was successfully validated for one-to-one coupled detector. Future application for other detector configurations should be possible and will be investigated. Additionally, using the new crystal interaction information to determine the most likely first interaction crystal is being examined to improve efficiency and signal-to-noise ratio in the PET reconstruction.