Choice of line spread function sampling distance for computing the MTF of radiographic screen-film systems

Abstract

When the modulation transfer function (MTF) of a radiographic screen- film system is computed digitally from experimental line spread function (LSF) data, the LSF must be sampled at a discrete set of positions and the continuous Fourier integral approximated by a sum. The resulting computed MTF will, in general, be an imperfect representation of the true system MTF because of the necessarily finite distances between LSF samples. The fundamental nature of errors in the computed MTFs has been investigated both theoretically and experimentally. Using the trapezoidal rule to approximate the required integrals, errors in MTFs, computed both from simple mathematical functions representing the screen-film LSF and from experimental screen-film LSF data, were less than 0.005 at all spatial frequencies below 1/(2*sampling distance), if the sampling distance was less than about 25% of the LSF half-width at half-maximum.