Investigation into the physical characteristics of active matrix flat panel imagers for radiotherapy

The effect of physical characteristics on active matrix flat panel imagers (AMFPIs) at megavoltage energies is studied. The detective quantum efficiency (DQE) of both direct and indirect AMFPIs is modeled using a modified cascade analysis combined with Monte Carlo simulations. It is found that for a given thickness of the sensitive layer less than about 1 mm, there should be no significant difference between the detection techniques, but for larger mass thicknesses (> or = 1 mm) there should be an advantage to using direct detection if such thick layers can be manufactured. The effect of the front plate on both direct and indirect techniques is also explored in terms of both the DQE and scatter rejection. It is found that for small sensitive layer thicknesses (< or =0.3 mm) a front plate thickness of about 1 mm Cu is optimal, whereas for larger mass thicknesses about 0.4 mm Cu should lead to better image quality.