The spatial-resolution properties in a digital radiographic system can be quantified by means of a “presampling” modulation transfer function (MTF). The theory and technique for determining the presampling MTF are illustrated based upon our recent research. With this technique, the presampling MTF, which includes the geometric unsharpness, the detector unsharpness, and the unsharpness of the sampling aperture, can be measured beyond the Nyquist frequency. An image of a slightly angulated slit is employed in order to obtain Fourier transforms of line spread functions (LSFs) at different alignments of the slit relative to the sampling coordinate. The presampling OTF up to 2
n (
n: positive integer) times the Nyquist frequency can be determined by averaging of the 2
n Fourier transforms of 2
n LSFs which are obtained from 2
n shifts of the sampling coordinates at increments of Δx/2
n(Δx: pixel size). The presampling MTF can then be calculated form the absolute value. As an example, the presampling MTFs for a digital subtraction angiographic (DSA) system are measured in two orthogonal directions for three different image-intensifier modes. It is also demonstrated that an “overall” MTF of a digital imaging system may incorrectly indicate the resolution properties of the system if the aliasing effect is not negligible, and thus the overall MTF of a digital system cannot be used in the same manner as that for an analog imaging system.
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