A simple method for determining the modulation transfer function in digital radiography

The authors developed a simple method for determining the presampling modulation transfer function (MTF). which includes the unsharpness of the detector and the effect of the sampling aperture, in digital radiographic (DR) systems. With this method, the presampling MTF is determined by the Fourier transform of a `finely sampled´ line spread function (LSF) obtained with a slightly angulated slit in a single exposure. Since the effective sampling distance becomes much smaller than the original sampling distance of the DR system, the effect of aliasing on the MTF calculations can be eliminated. The authors applied this method to the measurement of the presampling MTF of a compound radiographic system and examined the directional dependence, the effect of exponential extrapolation, and the effect of different sampling distances. It is shown that the technique of multiple slit exposure and exponential extrapolation of the LSF tail, which has been commonly used in analog seven-film systems, can be employed in DR systems. The authors determined the glare fraction in order to estimate the component of low-frequency drop mainly due to `glare´