Modulation transfer function associated with geometrical unsharpness in medical radiography.

The geometrical unsharpness caused by the size of the focal spot and the geometry of the exposure set-up is expressed in terms of the modulation transfer function (MTF) by means of Fourier techniques. The theoretical MTF associated with geometrical unsharpness has been verified experimentally with light and a small aperture to simulate a uniform focal spot. The MTF associated with the unsharpness produced by an actual focal spot was measured by means of edge exposures. To compare these results with the effect of focal-spot size on image sharpness in practice, a chest phantom was radiographed with screens and a knee phantom without screens, and with two focal-spot sizes. The combined MTF's of geometrical unsharpnesses and of the recording systems were compared qualitatively with the apparent sharpness of the radiographs. The MTF's calculated from an average object-film distance agreed qualitatively with the overall visual impressions of sharpness.