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Most radiologic imaging utilizes a phosphor as the primary X-ray absorber. The absorption of the X-ray, subsequent X-ray fluorescence and Auger relaxation, and the diffusion and scattering of secondary electrons influence the effective X-ray absorption and the picture noise. Scattering, diffusion, and absorption of the light within the phosphor layer and at its surfaces, and the optical coupling to the photoemitter, photoconductor, or photographic film, influence the sensitivity, noise level, contrast, and resolution of the imaging system. The best X-ray phosphors convert about 20 to 25 percent of the energy of an X-ray into useful radiant energy. The “speed” of phosphor layers has been increased about four times in the past decade. Our understanding of the limitations on their efficiency is incomplete but it is unlikely that materials having efficiencies much inexcess of 25 percent can be found. The quantum absorption in X-ray phosphor screens is clearly important to reducing the statistical noise, or the X-ray dose. While the need for good quantum efficiency is generally well understood, it is less well known that the variability of the amount of light detected between separate X-ray events can seriously reduce the signal-to-statistical noise ratio, even when the quantum absorption is nearly unity. A variation from pulse to pulse causes an additional statistical fluctuation so that the X-ray receptor behaves as though it were absorbing a smaller percentage of the X-ray quanta. The “noise-equivalent absorption” is the most meaningful measure of the degree to which an X-ray receptor preserves statistical information present in the X-ray signal incident on the device. The X-ray absorption is maximized by choosing a phosphor host which has elements with X-ray absorption edges in the most useful energy range, the greatest density, and the greatest thickness possible. Since the modulation transfer function and the limiting resolution decrease monotonically with layer thickness, a compromise between X-ray absorption and resolution must be made. Techniques are now being developed for fabricating screens containing isolated phosphor columns which can “pipe” the light through a thick layer without losing resolution.
X-rays, phosphors, screens, fluorescence, quantum detection efficiency
Manager, Optoelectronics Branch, General Electric Corporate Research and Development, Schenectady, N. Y.