Journal of The Society of Instrument and Control Engineers

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In the introduction, basic physical properties of imaging systems and their components in radiography are briefly described. The modulation transfer function (MTF) and the Wiener spectrum, which evaluate the objective imaging properties of sharpness and granularity, respectively, are defined. The analysis of imaging systems is compared with the analysis of electronic communication systems, and the analogy between MTF and temporal frequency response is explained. A direct scanning apparatus performing analogue Fourier transformation of X-ray images is described. By means of this apparatus, the MTF of X-ray fluorescent screens and of the focal spot of X-ray tubes can be measured photoelectrically. MTF's of X-ray fluorescent screens are approximated best by exponential distributions. MTF's of the focal spot of X-ray tubes correspond well to Siemens' star radiographs which show spurious resolution. From a consideration on the MTF's of the X-ray tube focal spot and of the imaging system, a general equation is derived which gives the optimum magnification for magnification radiography. The optimum magnification is related to the maximum value of the overall MTF. The equation is applied to two hypothetical cases, one in which both component MTF's are exponential and another in which both are Gaussian. It is shown that no optimum magnification exists where the exponential MTF's are identical (optimum-free magnification system). The Wiener spectra of radiographic mottle and its components are measured separately. By comparing measured and calculated Wiener spectral distributions, the contribution to radiographic mottle is obtained with each of the three known causes: the quantum mottle, the structure mottle and the film graininess. The dominant source of radiographic mottle is proved to be quantum mottle caused by the statistical fluctuations of absorbed X-ray quanta.

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