Radiographic information theory and application to mammography

A theoretical estimate is made for the lowest exposure necessary to record the smallest feature of interest, using various radiographic detecting surfaces and allowing for the probability of making errors to be within certain specified bounds. These calculated exposures are compared with the errors to be within certain specified bounds. These calculated exposures are compared with the actual exposures given in clinical situations in order that the smallest feature of interest can be observed by the practitioner with the same probability of error. A measure of the viewing efficiency of a radiographic process is given by the ratio of these exposures. The application of these concepts to mammography for three different recording media--film, film/screen, and xeroradiography--shows that the minimum entrance exposure necessary for recording information by each receptor varies with the energy of the x-ray beam, and that the optimal photon energy lies in the 21- to 25-keV region. All three methods with their present modes of processing use at most only a few percent of the information they actually intercept. This argues, for example, that if some more efficient method of reading xerographic plates were found, entrance exposures as low as 9 mR per exposure would suffice for a mammogram.