Some Characteristics of the Line Spread-Function and Modulation Transfer Function of Medical Radiographic Films and Screen-Film Systems

Abstract

The line spread-function (LSF) and modulation transfer function (MTF) of a screen-film system convey important information regarding the light diffusion in the system, which is one of the parameters influencing overall image quality. The LSF of a screen-film system is the relative illuminance distribution in the image of a narrow slit. Thus, the LSF is a direct measure of light diffusion in screens and film. The MTF is a measure of the ability of the systems to image, frequency for frequency, a radiation pattern in which intensity varies sinusoidally with distance in one dimension in the object plane. By definition, the LSF and MTF of screen—film systems are parameters that can be used as a measure of the image deterioration due to light diffusion only. The effect on image quality of other system characteristics, such as film contrast and absorption of x-ray quanta in the screens resulting in quantum mottle, is not measured by the LSF and MTF. Thus, the LSF and MTF do not, in themselves, provide a measure of the overall image quality attainable with a screen—film system (1). The LSF and MTF are essentially different means of expressing the effect of light diffusion in the screen—film system, and they are mathematically related (Fourier transform pair) so that when one has been measured, the other can be calculated. In this paper, both functions are shown for the systems under discussion, since either function may be useful, depending on the problem to be investigated. A method for measuring the LSF and MTF of screen—film systems has been described previously (2, 3). A similar method has been employed by Morgan, Bates, Gopala Rao, and Marinaro (4). Briefly, it consists of exposing the screen—film system2 to x-rays through a 10-micron (µ) slit formed by platinum jaws, 2 mm thick, as shown in Figure 1, and scanning the resulting developed line image with a microdensitometer. From this scan, the LSF is calculated on a digital computer by means of the characteristic curve of the film and the calibration curve of the microdensitometer. The corresponding MTF is also calculated on the computer. By this method, some interesting characteristics of the LSF and MTF of a number of radiographic screen—film systems have been found. The feasibility of using the same method to measure the LSF and MTF of radiographic films exposed without screens has also been investigated. Dependence on System Speed Sometimes it is tacitly assumed that screen—film systems of identical exposure speeds but consisting of different screens and the same film will have identical optical properties. To test this assumption, Kodak Royal Blue Medical X-ray Film was exposed with a large number of screens using radiation of the same energy. The relative speeds of the screen—film systems ranged approximately from 20 to 200. The LSF3 and MTF4 of the sharpest and least sharp systems in this speed range are shown in Figure 2. Three pairs of screen—film systems having approximate relative speeds of 50, 100, and 200, respectively, were selected. The corresponding LSF and MTF are shown in Figures 3 to 5. Each illustrates the variation of light diffusion for a given system speed.