Recent developments in X-ray imaging detectors

The replacement of the radiographic film in medical imaging has been the driving force in X-ray imaging developments. It requires a 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 100-150 mu m. The need for entirely electronic imaging equipments 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 capable 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 film-screen combination, at lower X-ray dose 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) 2000 Elsevier Science B.V. All rights reserved.