SU‐E‐T‐350: Calibration of a Prototype Proton CT Scanner

Abstract

Purpose: To calibrate a novel proton CT scanner using polystyrene degraders of accurately known thickness and relative stopping power.Methods: A prototype proton CT scanner was constructed and installed on a horizontal proton research beam line for further study of this novel, potentially low‐dose tomographic imaging modality. The proton CT scanner registers individual protons up to a maximum rate of about 10̂5 protons/sec, recording their entry and exit coordinates and directions with high‐resolution silicon strip detectors, as well as their residual energy by stopping them in an augmented 18‐crystal CsI calorimeter. To avoid uncertainties in the mean excitation energy (I) of water when converting energy loss to water equivalent path length (WEPL) via the Bethe‐Bloch equation, the response of the calorimeter was calibrated using polystyrene plates of known water‐equivalent thickness (WET). Stacks of polystyrene plates of variable thickness were placed between the proton CT detectors and about 3 × 10̂5 protons were recorded for each thickness. For each proton event, the calorimeter response was derived from the weighted sum of the response of the 18 crystals, taking into account variations in the sensitivity of each crystal. A Gaussian distribution function was fitted to the response data at each WET, and the mean of each distribution was used to establish the WEPL calibration curve. Results: For protons of 100 MeV and 200 MeV, the mean response of the calorimeter was obtained for 7–8 different WET values (including zero) and the WEPL vs. response calibration curve was fitted with a quadratic polynomial with an excellent goodness of fit. A WEPL measurement accuracy of 1 mm or better can be achieved by measuring the calorimeter response of about 150 protons Conclusions: This work is an important step towards a thorough investigation of proton computed tomography for therapeutic and imaging applications.Funded in part by DOD Contract W81XWH‐08‐1‐0205 and the Department of Radiation Medicine, Loma Linda University Medical Center