The dream ofan all-solid state large area x-ray image sensor with digital readout and full dynamic performance will most probably find a first realisation in two-dimensional thin-film amorphous silicon arrays. In this paper we address in particular the evaluation of the limits of the signal/noise ratio in this concept. Using small prototype detectors measurements of MTF and noise power spectra have been made as a function of x-ray dose. The results are given in terms of the detective quantum efficiency (DQE) as a function of dose and spatial frequency. We further present an analysis of the different noise so rces and their dependence on the detector parameters, and we provide estimates on the maximum signals that may be achieved per unit dose. The intrinsic lag of the amorphous silicon photodiodes causes a second problem area with this type ofx-ray detectors. Especially in radiography/fluoroscopy mixed applications, memory effects may not be negligible.

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