AbstractID: 6578 Title: Investigation into the front plate thickness of thick portal
detectors
Metal front plates have been traditionally used for portal film detectors, both to
increase the quantum efficiency and reduce lower-energy patient scatter. Typically a
thickness equivalent to about 1-1.5 mm copper (or equivalent mass thickness of a denser
metal) is used. Modern megavoltage detectors, however, are typically thicker than film.
This study investigates whether such thick detectors should still use the same front plate
thickness which has been optimized for film. The effect of the front plate on the detective
quantum efficiency (DQE), contrast, and differential signal-to-noise ratio (DSNR) is
explored. The DQE is calculated using a cascade analysis formalism, which is evaluated
with Monte Carlo techniques. The contrast and DSNR are affected by the scatter rejection
properties of the front plate, which is calculated using Monte Carlo. Graphs are presented
which indicate the dependence of each parameter on the front plate for various detector
thicknesses. We focus on metal/phosphor and metal/a-Se detectors since these are under
investigation for use in active matrix flat-panel imagers. The results for a 6 MV beam
indicate that although 1-1.5 mm of copper is adequate for sensitive layers less than about
300 µm, a smaller front plate of about 0.4 mm Cu may be optimal for larger thicknesses
of the sensitive layer.