Characterization and Modeling of Knife-Edge Slit Collimator Response for MeV Prompt Gamma Photons in Proton Therapy Monitoring

Gamma camera imaging process can be well characterized and modeled by the response of the imaging system and collimator response is one of the most important parts in system response. In previous research, many models were proposed to formulate analytically the pinhole or parallel-hole collimator response for conventional gamma cameras. However, few studies take the photons of MeV energy range into consideration. In proton therapy monitoring, the prompt gammas emitted during proton therapy delivery have high energy with a range from 2 MeV to 10 MeV, which makes the collimator response dramatically different from that of conventional gamma cameras. Accurate analytical modelling of collimator response for high energy photons is of vital importance to generate the system matrix within a reasonable time and improve the accuracy of the reconstructed images in proton therapy monitoring. The knife-edge slit collimator based imaging system is one of the most promising systems designs for proton therapy monitoring. However, few models were proposed for knife-edge slit collimator response in the existing literatures, especially for MeV photons. We performed MC simulation using Gate and generated the projections of primary, penetrated and scattered photons. By analyzing the detection efficiency and projections of the three kinds of photons, we characterized the knife-edge slit collimator response for prompt gammas dependent on the point source position. Meanwhile, fitting based modelling of primary, penetrated and scattered projection profiles were developed and verified. Preliminary results show good fitting of the simulated projections in limited cases and model based analytical system matrix can be generated in a reasonably short time. More investigations will be done to analyze the influences of various factors including photon energy and collimator design on the projections quantitatively and extend the model to multi-slit collimator. We aim at proposing a general applicable semi-empirical equations or index look-up tables to calculate the collimator response for high energy photons.