The effect of copper conversion plates on low-Z target image quality

Purpose: Common electronic portal imaging devices (EPIDs) contain a 1.0 mm copper conversion plate to increase detection efficiency of a therapeutic megavoltage spectrum. When used in imaging with a photon beam generated with a low atomic number (Z) target, the conversion plate attenuates a substantial proportion of photons in the diagnostic range, thereby reducing the achievable image quality. In this work, we measure directly dependence on low-Z target image quality as a function of copper plate thickness, for both planar imaging and cone beam computed tomography (CBCT). Methods: Monte Carlo modeling was used to quantify changes to the diagnostic spectrum and detector response for low-Z target beams generated with either 2.35 or 7.00 MeV electrons incident on a carbon target. Planar contrast-to-noise ratio (CNR) and spatial resolution measurements were made as a function of copper thickness. CNR measurements were made for CBCT imaging as a function of dose both with and without the copper plate present in the EPID. Results: The presence of copper in the EPID decreased the diagnostic photon population by up to 20 and suppressed the peak detector response (dose deposited in the scintillator) at 60 keV by a factor of 6.4. Planar CNR was increased by a factor ranging from 1.4 to 4.0, depending on the material imaged, with no copper present compared to a standard 1.0 mm thickness. Planar spatial resolution was only slightly degraded with increasing copper thickness. Increases in CBCT image CNR ranged from a factor of 1.3-2.1 with the copper plate removed. Conclusions: It is possible to increase the proportion of photons in the diagnostic energy range (25 keV-150 keV) reaching the phosphor screen by as much as 20 when removing the copper conversion plate. This results in significant increases of planar and CBCT image CNR. Consequently, we suggest that the copper conversion plate be removed from the EPID when used for low-Z target planar or CBCT imaging.