Electronic portal imaging based on Cerenkov radiation: A new approach and its feasibility

Most electronic portal imaging devices (EPIDs) developed so far use a Cu plate/phosphor screen to absorb x rays and convert their energies into light, and the light image is then read out. The main problem with this approach is that the Cu plate/phosphor screen must be thin ( ∼ 2 mm thick) in order to obtain a high spatial resolution, resulting in a low x‐ray absorption or low quantum efficiency for megavoltage x 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. In this paper, we propose a new approach that uses Cerenkov radiation to convert x‐ray energy absorbed by the detector into light for portal imaging applications. With our approach, a thick ( ∼ 10 – 30 cm ) energy conversion layer made of a low‐ Z dielectric medium, such as a large‐area, thick fiber‐optic taper consisting of a matrix of optical fibers aligned with the incident x rays, is used to replace the thin Cu plate/phosphor screen. The feasibility of this approach has been investigated using a single optical fiber embedded in a solid material. The spatial resolution expressed by the modulation transfer function (MTF) and the sensitivity of the detector at low doses ( ∼ one Linac pulse) have been measured. It is predicted that, using this approach, a detective quantum efficiency of an order of magnitude higher at zero frequency can be obtained while maintaining a reasonable MTF, as compared to current EPIDs.