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Optical spatial filtering of radiographic images with binary filters.

An optical spatial filtering technique was used to remove grid lines by eliminating the transmission of the corresponding Fourier spectrum in the Fourier transform plane. Studies of the effect of band-pass filtering on cerebral angiograms showed that a recognizable image of the blood vessels is produced even when the filter is placed in a spatial frequency range beyond that at which the Fourier spectrum of the radiographic image would presumably be essentially zero. These results indicate that radiographic mottle carries image information, supporting an image-dependent (multiplicative) noise theory.