We measured modulation transfer function (MTF) to evaluate the resolution property of a CRT imaging camera which is used as a hard-copy device in the digital imaging system. We examined the effects of various factors on MTFs, including the adjustment of CRT setting (contrast and brightness), the variation of division number on film, the variation of positions on CRT, and raster-erase technique. The hypothetical "slit" images for two orthogonal directions relative to the electron-beam scanning of the CRT were produced by use of digital subtraction angiography system. Then the MTFs were calculated from the Fourier transformation of the line spread function (LSF) derived from the hypothetical "slit" images. The MTF measured in the perpendicular direction to the electron-bea m scanning was greater than that obtained in the parallel direction. The MTFs determined in different positions on CRT indicated the variations of the values; these are mainly attributable to the changes of shape and size of the electron-beam spot. The MTF results measured by different conditions on CRT adjustment indicated that the higher contrast setting and the lower brightness setting showed the higher MTF values than another settings. We also indicated that the MTF was slightly decreased by rastererase technique. Two different methods, the neutral-density (ND) filter and brightness-meter methods, to measure the characteristic curie of a single-emulsion film for linearization were also examined, and showed in good agreement with each other. This means that the both methods are effective for sensitometry of the film.

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Modulation transfer functions (MTFs) for the defocused objective lens system of the microdensitometer used for analysis of radiographic image qualities, are investigated for two objective lenses. Medical X-ray films have a unique structure characterized by the use of double-coated emu lsion. However, When MTFs are measured using an objective lens with a focus shallower than the thickness of the film in a screen-film system, large measurement errors do not occur. For an objective lens with a focal depth of 0.10mm, the measured MTFs errors are approximately 5% at 4 cycles/mm.
The Wiener spectrum values measured using lenses with focal depths of 0.1 0 m m and 0.24mm almost agree. However, when measured with the 0.24mm objective lens, the Wiener spectr a show significantly higher values in the low frequencies due to the effect of the stage glass.

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We described a simple method, i. e, sequential-dependence method, for quantitative evaluation of computed radiographic (CR) images. This method was applied to investigate the effects of image processing which is generally employed in CR systems. Mammograms of breast with microcalcification and chest radiographs processed by gradient enhancement and spatial frequency enhancement techniques, respectively, were used as samples for analysis. Our calculated results indicated that; the higher the gradient used for gradient enhancement the stronger the sequential dependence is, and the lower the spatial frequency used for frequency enhancement the stronger the sequential dependence is. These calculated results are in good agreement with visual results. The preliminary results suggest that the sequential dependence approach is a useful tool for evaluation of overall performance of digital images.

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