Performance of a 41X41-cm2 amorphous silicon flat panel x-ray detector for radiographic imaging applications

We report the performance of a 41 X 41-cm2 amorphous silicon-based flat panel detector designed for radiographic imaging applications. The detector consists of an array of photodiodes and thin film transistor switches on a 0.2-mm pitch with an overlying thallium-doped cesium iodide scintillator. The performance of the detector was evaluated through measurement of the frequency-dependent detective quantum efficiency [DQE(f)]. Measurements of the characteristic curve and modulation transfer function (MTF) are also reported. All measurements were made in a radiographic imaging mode with a readout time of 125 ms. We evaluated a total of 15 detectors. One detector was characterized at a range of exposures and at three different electronic gain settings. Measurements of DQE(f) and MTF were also performed as a function of position on one detector. The measured DQE at an exposure of about 1 mR was 0.66 at zero spatial frequency and fell smoothly with frequency to a value of 0.24 at the Nyquist frequency, 2.5 cycles/mm. The DQE is independent of exposure for exposures in the upper 80% of each gain range, but is reduced somewhat at lower exposures because of the influence of additive system noise. The reduction can be controlled by adjusting the electronic gain. For a gain that allows a maximum exposure of 5 mR, the DQE at 0.056 mR was 0.64 at zero frequency and 0.19 at 2.5 cycles/mm. The standard deviation in DQE among measurements on different detectors was less than 0.02 at any frequency. The presampling MTF was 0.26 at 2.5 cycles/mm. The standard deviation in MTF among measurements on different detectors was less than 0.01 at any frequency. Both MTF and DQE were substantially independent of position on the detector.