Exposure Geometry And Film Contrast Differences As Bases For Incomplete Cancellation Of Irrelevant Structures In Dental Subtraction Radiography

Subtraction radiography for longitudinal studies requires both reproducible imaging geometry and film contrast characteristics to permit perfect alignment of the radiographs, and achieve good cancellation of diagnostically irrelevant background structures. The standard deviation (SD) of gray levels about the mean in a subtraction image was used as a relative measure of the residual structured noise. In order to estimate the effects of improperly standardized radiographs on SD in the subtraction image, both the imaging angle and film exposure time were systematically varied. The results showed that SD changed linear for small misalignment angles of the central beam, the variance attributable to this error source reaching about the same magnitude as the variance due to anatomical differences for angulation errors within ±20. The SD increase due to large film contrast disparity could be partly reverted for angulation errors within this bound by using a quadratic transformation which matched the first two moments of the gray level distributions in the two parent radiographs. Therefore, in order to use some of the retrospective data obtained under less stringent standardizations for subtraction imagery, it appears possible to adjust for differences in film contrast, and, perhaps, correct for geometric misalignment by a separate algorithm.