Detective quantum efficiency of a direct-detection active matrix flat panel imager at megavoltage energies

The use of an amorphous selenium (a-Se) based direct-detection active matrix flat-panel imager (AMFPI) is studied for megavoltage imaging. The detector consists of a 1.2 mm copper front plate and 200 microm a-Se layer, and has a 85 microm pixel pitch. The Modulation Transfer Function (MTF), Noise Power Spectrum (NPS), and Detective Quantum Efficiency (DQE) are measured for 6 and 15 MV photon beams. A theoretical expression for the DQE is derived using a recently developed formalism for nonelementary cascade stages. A comparison of theory with experiment is good for the 6 and 15 MV beams. The model is used to explore the DQE for more typical pixel sizes. The results indicate that with proper modifications, such as a larger a-Se thickness, a direct flat-panel AMFPI is a very promising detector for megavoltage imaging.