Reciprocity-Law Failure in Medical Screen-Film Systems and Its Effects on Patient Exposure and Image Quality
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Reciprocity effect or the failure of the reciprocity law is of considerable importance in medical radiography, because most radiographs are made with a pair of intensifying screens in intimate contact with both emulsions of an X-ray film. This failure of the law means that the photographic density depends upon both intensity and exposure time. Recently we have developed an easy and practical method to determine the reciprocity-law failure curve in medical screen-film systems [1–3]. By use of this method, two kinds of reciprocity-law failure curves, expressed as photographic density vs. exposure time for constant exposure and relative exposure vs. exposure time for constant density, can readily be obtained from several time-scale characteristic curves taken experimentally for several focus-film distances(FFDs) . In the present paper, this new method is described with its application to a green-sensitive X-ray film for use in combination with rare-earth screens. The reciprocity effect on the patient exposure will be discussed based on the results of our study.
KeywordsExposure Time Reciprocity Effect Patient Exposure Constant Exposure Relative Exposure
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