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Real-time radiologic imaging necessitates at least the conversion of X-ray quanta into visible photons and that, seemingly, without delay. At any moment, as judged by a human observer, the visible image displayed corresponds to the X-ray intensity distribution in the central shadow projection of the irradiated objects. The X-ray luminescent screen may serve as an example of such images. Most X-ray imaging applications require that the energy content of the displayed image be larger than that of the input radiation. As energy amplification cannot directly be applied to X-ray quanta, intermediate conversion into electrically charged particles is necessary. They can be utilized then, either directly or indirectly, to eventually generate the display image at the required energy level. This implies that there are various avenues leading toward image intensification, exemplifications of which will be given. In order to communicate intelligently about the achievements and limitations of various X-ray imagers, one has to refer to characteristics like modulation transfer function, noise power factor, detector quantum efficiency, or noise-equivalent spatial bandwidth. Although such notions are frequently used, their respective quantitative values may not be comparable due to lack of standardization on definitions and boundary conditions regarding the applicability of these concepts.
detection, image intensification, gain, resolution, noise, image intensifiers
N. V. Philips' Gloeilampenfabrieken, Eindhoven,