Large area X-ray detectors based on amorphous silicon technology

The replacement of the radiographic film in medical imaging is a major issue. It requires an similar to 40 cm wide detector to cover all examinations, an equivalent noise level of 1-5 X-ray quanta per pixel, and spatial resolution in the range of a few hundred microns. The need for entirely electronic imaging equipment has fostered the development of many X-ray detectors, most of them based on an array of amorphous silicon pixels, which is the only technology able to achieve such large areas. Essentially two concepts have been implemented: intermediate conversion of X-rays to light by a scintillator, detected by an array of light sensitive pixels, comprising a photodiode and a switching device, either a TFT or a diode conversion into electron hole pairs in a photoconductor, collected by an array of electrodes and switches. In both cases, charge amplifiers read the generated charges line by line. Scintillator and photoconductor based systems are now close to production. They achieve better image quality than the classic filmscreen combination, at lower X-ray doses and with a much broader dynamic range. Dynamic imaging up to 30 frames/s has been demonstrated. The technical challenges at the level of the a-Si array are the number of acceptable defects, the on/off ratio of the switches, the quantum efficiency of the photodiodes, the memory effects associated with traps in a-Si. Of course, long-term reliability is a major concern for medical components. (C) 1999 Elsevier Science S.A. All rights reserved.