Resolution and contrast reduction.

Abstract

Lack of resolution (unsharpness) can reduce contrast in diagnostic radiography if the proper conditions of magnification and unsharpness are met. To describe this phenomenon, a modification of the contrast reduction factor (CRF) was introduced which used the response function of a semi-opaque edge to predict contrast reduction for small bar-shaped objects. To predict CRF, unsharpness is employed as a single-term description of resolution and is obtained experimentally from the edge response function. The unsharpness term is defined as the distance over which the response goes from 16.5% to 83.5% of the maximum. Measured and predicted CRFs were compared and the CRF concept was found to be an excellent predictor of contrast reduction. The individual components of unsharpness were determined experimentally and the sum-of-squares rule predicted adequately their combination. Three methods to measure unsharpness were compared: (a) the ICRU prescription using pinhole radiographs of the focal spot, (b) one-dimensional integration of the focal-spot pinhole radiograph, and (c) the unsharpness term produced by a semi-opaque edge. The latter two were measured using a microdensitometer.