Visual optimization of radiographic tone scale

Radiographic tone scale can be optimized for visual perception by mapping equal log-exposure difference in the transmitted radiation field to equal discriminable brightness difference in the displayed image. The mapping will render objects equally visible independent of its exposure level and background. This visually optimized tone scale can be readily achieved in computed radiography. It can also be applied to the conventional screen/film system to provide an ideal aim curve for screen/film design. One of the key components in the construction of such a tone scale curve is the function that relates the physical luminance to the perceptual brightness. It is well known that the perceived brightness is a function of the viewing condition. By comparing the predicted brightness differences from the various brightness models, we found that the Michaelis-Menten function provides a visual tone scale that significantly improves the visibility of details in the high density regions over that produced by existing screen/film systems. Based on the basic visual tone scale, we parameterize the curve shapes at the toe and the shoulder, and thus obtain a family of tone scale curves for computed radiography. The toe and shoulder shapes can be customized for different examination types for achieving the best rendition under the constraints of the limited dynamic range of the output display and the minimum required image contrast.