Amorphous silicon flat panel x-ray detectors (a-Si FXD) are expected eventually to replace traditional x-ray image intensifier systems (XRII) in medical radiography in the long term. The advantages of FXD's are their large detection area, no distortion, no sensitivity to magnetic fields, low weight and compactness. However, they do not provide the high sensitivity of specific optimized systems based on image intensifiers, which approach the sensitivity of single x-ray photon counting in an appropriate configuration whereas the noise equivalent number ofphotons for an a-Si imager is typically several photons at medical energies. That is, the detective quantum efficiency (DQE) of an XMl at low dose is expected to be higher.The consequences of this difference in sensitivity on the performance of both types of detectors are studied for typical conditions in medical fluoroscopy. Detector performance is analyzed in terms of DQE, noise power spectrum (NPS) and modulation transfer function (MTF). Both detectors show similar performance w.r.t. MTF and DQE down to 5 nGy. The comparison is extended to radiographic imaging at 1 tGy and a pixel size of 155 tm. A fmal comparison shows that the inherent spatial resolution of state of the art XMl's exceeds 10002 pixels. An increase in DQE of a factor 3 at 1.6 lp/mm is measured if a camera having 20002 pixels replaces a 1 0002-pixel camera.
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