Toward a better understanding of radiographic contrast

Contrast is a familiar term that relates directly to the ability of human observers to detect differences in optical density. It also has been used to describe changes in x-ray fluence determined by attenuation differences in tissues of diagnostic interest. Ambiguous conclusions in the literature can be often traced to a lack of specificity in what is being measured and the validity of the implicit assumptions involved. Particularly troublesome are nonspecific bases for normalization and the assumption that a finite change in fluence can be accurately represented as a differential. These practices also tend to obscure consideration of system nonlinearities which can result in contrast enhancement produced by exposure to fogging radiation. Precisely defined components of contrast are symmetrically determined by the concept of modulation. This concept provides a powerful basis for mathematical description of linear systems. Bases for linearization of radiographic systems are also confusing unless thay are explicitly stated. This is particularly true for transfer functions as defined in many radiographic applications. Special functions which account for nonlinearity commonly associated with contrast gain in the detector simplify description and provide a quantitatively meaningful basis for specifying system performance in terms of information theory.