Modulation transfer function for a large-area amorphous silicon image receptor

The modulation transfer function (MTF) of an amorphous silicon (aSi) sensor array was measured using proper sampling techniques to determine the edge spread function (ESF). The detector under study was a 10 cm2 area detector (EG&G Heimann, RTM128) consisting of 128 x 128 aSi photodiodes arranged in a square array. Two independent methods for calculating the presampling MTF were implemented, based on finely sampling the ESF measurements produced using 40 kV x-rays from a Faxitron microfocal spot x-ray tube. The two calculations of the detector's presampling MTF are in excellent agreement, and are within 20% at the Nyquist frequency when compared with the ideal MTF based only on the size of the detector elements. ESF measurements were also made at 6 MV on a Siemens MD-2 linear accelerator. A calculation of the system presampling MTF was performed which included effects from the linear accelerator source, the lead block used to create the high contrast edge, and the aSi detector response.