QUANTITATIVE SUBTRACTION WITH DIRECT DIGITAL DENTAL RADIOGRAPHY

Objectives: To elucidate the accuracy in vitro of quantitative digital subtraction for direct digital dental radiography using a commerciall y available image analysis system. Methods: Images were corrected firs t for dark current and sensitivity variation among pixels of the CCD s ensor (first correction), and corrected further for interimage variati ons due to the variations in exposure conditions (second correction), A copper stepwedge with six steps was used in the second correction. T hree corrected images derived from three original images at each diffe rent exposure time were examined to confirm the reliability of the sec ond correction. Correlation between bone mineral content and pixel val ue was assessed in subtracted images of an artificial bone phantom. Re sults: The first and second corrections significantly improved the int ra-and inter-image variation respectively, The second corrected images obtained at three different exposure times showed almost the identica l grey levels, An original image was transformed to a copper equivalen t image with a high reliability by our original method. The pixel valu e in the subtracted image between two copper equivalent images of a bo ne phantom was linearly related to the bone mineral content, There was a slight overestimate at lower pixel values but it was accurate at hi gher pixel values. Conclusions: Quantification of digital subtraction using this method is sufficeintly reliable for its application to clin ical practice to be evaluated.