Preliminary study of a metal/ portal detector

Abstract

A feasibility study has been performed on metal/amorphous selenium detectors for megavoltage portal imaging. The metal plates of the detectors were positioned facing the incident 6 MV and Co-60 photon spectra. The detectors consist of various thicknesses (0.15 mm, 0.30 mm, and 0.50 mm) of amorphous selenium (a-Se) deposited on metal plates of varying thicknesses: aluminum (2.0 mm), copper (1.0 mm and 1.5 mm), stainless steel (0.9 mm), or glass (1.1 mm). The detectors were charged prior to irradiation by corona methods, and the portal images were subsequently digitized after irradiation with a noncontact electrostatic probe. The sensitivity of the detectors to dose, electric field across the a-Se layer, metal plate type and a-Se thickness, was studied. The electrostatic voltage remaining on the a-Se layer was found, both theoretically and experimentally, to exhibit a cubic relationship with respect to dose. An increase in electric field increases the sensitivity (gradient of the a-Se surface voltage vs dose curve) and dynamic range of the resultant image. An increase in a-Se thickness, however, although also increasing the sensitivity, decreases the dynamic range. The metal plate types and thicknesses within the range studied do not have a significant effect on detector sensitivity. Image quality and contrast resolution of the detector were evaluated with a contrast-detail phantom and compared to commercially available film based and electronic portal imaging devices. Image quality of the metal/a-Se detector as a function of dose was studied by discharging the a-Se to various fractions of its initial charge, and as expected, increases with dose due to a decrease in quantum noise. Contrast-detail images obtained by metal/a-Se detectors are superior to those obtained at higher dose levels by other commercial systems.