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Digital subtraction radiography requires close matching of the contrast in the films to be subtracted. A digital method is presented permitting the retrospective correction of film contrast differences. The method is nonparametric and derives the required gray level transformation directly from the histograms associated with the radiographs. This transform is shown to be unique and monotonic. It is based on fewer theoretical assumptions than a previously described parametric correction method, and it performs significantly better in reducing the contrast mismatch measured by the standard deviation of the gray levels in the subtraction image.
Ruttimann et al. (Mon,) studied this question.