subtraction radiography

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The digital or photographic manipulation of a radiograph in which background images are eliminated to highlight areas for pre- and postoperative comparison.

Highlighting the differences between two radiographs by covering one film of a part with a diapositive film of the same part. A photograph of this compilation will produce one which has grayed out the background but reinforced the differences. The method removes distracting material in the ordinary film. Used in angiography where the vessels are difficult to see, e.g. in the skull.