Electronic portal imaging using Cherenkov radiation

Abstract

Most electronic portal imaging devices (EPIDs) developed so far use a Cu plate/phosphor screen to absorb x rays. The main problem with this approach is that the Cu plate/phosphor screen must be thin (~ 2 mm) in order to obtain a high spatial resolution, resulting in a low quantum efficiency (QE) for megavoltage (MV) × rays (typically 2-4%). In addition, the phosphor screen contains high atomic number (high-Z) materials, resulting in an over-response of the detector to low energy x rays in dosimetric verification. Our overall goal is to develop a new high QE MV x-ray detector made of a low-Z material for both geometric and dosimetric verification in radiotherapy. Our approach is based on radiation-induced light (Cherenkov radiation) in optical fibers to convert x-ray energy into light. With our approach, a thick (~ 10-30 cm) fiber-optic taper (FOT) consisting of a matrix of optical fibers aligned with the incident x rays is used to replace the thin Cu plate/phosphor screen to dramatically improve the QE. In this work, we demonstrated that the predominant light source in optical fibers under irradiation of a MV beam is indeed Cherenkov radiation, and thus validated the feasibility of using Cherenkov radiation as the primary light source in our proposed Cherenkov detector. A prototype Cherenkov detector array was also built and images were obtained.