In-beam PET monitoring of proton therapy: a method for filtering prompt radiation events

In-beam Positron Emission Tomography (PET) is a promising technology for real-time monitoring of proton therapy. Random coincidences between prompt radiation events and positron annihilation photon pairs can deteriorate imaging quality during beam-on operation. This study aimed to improve the PET image quality by filtering out the prompt radiation events. We investigated a prompt radiation event filtering method based on the accelerator radio frequency (RF) phase and assessed its performance using various prompt gamma energy thresholds. An in-beam PET prototype was used to acquire the data when the 70 MeV proton beam irradiated a water phantom and a mouse. The signal-to-background ratio indicator was utilized to evaluate the quality of the PET reconstruction image. The selection of the prompt gamma energy threshold will affect the quality of the reconstructed image. Using the optimal energy threshold of 580 keV can obtain a signal-to-background ratio of 1.6 times for the water phantom radiation experiment and 2.0 times for the mouse radiation experiment compared to those without background removal, respectively. Our results show that using this optimal threshold can reduce the prompt radiation events, enhancing the signal-to-background ratio of the reconstructed image. This advancement contributes to more accurate real-time range verification in subsequent steps.