Measurement of Resolving Power of Intensifying Screens

Abstract

One of the more important characteristics which determine the interpretative value of a roentgenogram is the ability of the film or film-screen combination to record detail. A quantitative measure of this characteristic, therefore, is needed in order to evaluate accurately the various types of screens and films available to the radiologist, as well as to assist in determining the optimum conditions of technic. Because of difficulties to be mentioned later, satisfactory methods of measuring this ability to record detail have not previously been developed. In this article such a method is described and the results obtained with a number of commercial screens are tabulated. In the case of photographic materials, the procedure has been fairly well standardized. It consists of photographing on the material to be tested, either by contact or projection, a test object such as that shown in Figure 1, containing a series of alternate lines and spaces of different width. The number of lines per millimeter which can be distinguished on the processed material, then, is a measure of the ability to record detail and is called resolving power. If the test object is photographed, the resulting figure for resolving power will be a measure of the resolving power of the lens-film combination and will not be greater than that of either component alone. The same applies to printing by projection. If the test object is printed by contact, the result will be the resolving power of the emulsion alone. In general, therefore, the resolving power recorded by any emulsion is limited by that factor of the optical-photographic system having the lowest resolving power. It has been shown (1) that the resolving power obtained by these methods depends upon a large number of factors other than the photosensitive material itself, namely, (a) the line-space ratio of the test object, (b) test object contrast, (c) level of illumination, (d) development time, (e) wave length of light, (f) composition of the developer, (g) developer concentration, (h) developer temperature, (i) reduction and intensification, (j) dyes. Obviously, therefore, any determination of resolving power must be made under carefully standardized conditions if the results are to be comparable and reproducible. The first of these factors, the line-space ratio, has been generally fixed at 1; that is, the lines and spaces are of equal width. The other factors are easily controlled or are dictated by the technic and therefore determined by the conditions of test. Thus, in x-ray procedures, the wave length of the radiation is determined by kilovoltage, character of structure being radiographed, and type of intensifying screen used; dyes are encountered only as they are incorporated in different types of films. The difficulties in determining the resolving power of x-ray materials—film alone, film-screen combinations, or lens-screen-film combinations—arise primarily in the preparation of a suitable test object.