A SIMPLE METHOD FOR DETERMINING THE MODULATION TRANSFER-FUNCTION IN DIGITAL RADIOGRAPHY

We developed a simple method for determining the presampling modulatio n transfer function (MTF), which includes the unsharpness of the detec tor and the effect of the sampling aperture, in digital radiographic ( DR) systems. With this method, the presampling MTF is determined by th e Fourier transform of a "finely sampled" LSF obtained with a slightly angulated slit in a single exposure. Since the effective sampling dis tance becomes much smaller than the original sampling distance of the DR system, the effect of aliasing on the MTF calculations can be elimi nated. We applied this method to the measurement of the presampling MT F of a computed radiographic (CR) system and discussed the directional dependence, the effect of exponential extrapolation, and the effect o f different sampling distances. We showed that the technique of multip le slit exposure and exponential extrapolation of the LSF tail, which has been commonly used in analog screen-film systems, can be employed in DR systems. Furthermore, we determined the glare fraction, by means of a lead-disk method, in order to estimate the component of low-freq uency drop mainly due to "glare." The results showed that the value of glare fraction was 5-6.5% and slightly dependent on the sampling dist ance.