COMPUTER-ASSISTED DENSITOMETRIC IMAGE-ANALYSIS OF DIGITAL SUBTRACTIONIMAGES - IN-VIVO ERROR OF THE METHOD AND EFFECT OF THRESHOLDING

THE AIM OF THE PRESENT STUDY was to assess the in vivo error of the me thod as well as the effect of thresholding when obtaining and evaluati ng standardized periapical radiographs for computer-assisted densitome tric image analysis (CADIA). Twenty healthy volunteers participated in an experimental gingivitis study in which neither mechanical nor chem ical plaque control was performed for 21 days. Two pairs of standardiz ed periapical radiographs were taken at days 0 (baseline) and 21 (foll ow-up), one from a maxillary area (15 volunteers) and one from a mandi bular molar/premolar area (17 volunteers). Each baseline radiograph wa s digitized and its image displayed on a mon itor. The follow-up radio graph was then superimposed and digitized as well. After gray level co rrection, subtraction radiographic images were produced. The differenc e in gray level between the baseline and the follow-up image was calcu lated within each region of interest (ROI) at each picture point (pixe l). In bone ROI, changes in density reflected the amount of change due to methodological errors plus the basic bone remodeling over 3 weeks. For gingival ROI, changes in density reflected the methodological err or plus a possible change in soft tissue density during the experiment al gingivitis. Within all of the ROI, some pixels indicated a change i n gray level. A change in gray level was then thresholded; i.e., only changes >5 and then >10 gray levels were registered and used for calcu lation of the CADIA values. With a threshold of 5, 44/45 maxillary bon e ROI and 60/66 mandibular bone ROI showed a change in density, while 41/45 maxillary gingiva ROI and 26/66 mandibular gingiva ROI indicated a change in density. With a threshold of 10, 16/45 maxillary bone ROT and 12/66 mandibular bone ROI indicated a change in density, while 13 /45 maxillary gingiva ROI and 1/66 mandibular gingiva ROI indicated a change. The amounts of changes in density calculated in the various RO I were low even when applying no threshold, ranging from -0.279 to 0.6 21, Applying a threshold of 5, the CADIA values ranged from -0.234 to 0.727. With a threshold of 10, the changes in density ranged from -0.3 18 to 0.133. In vivo, CADIA of standardized radiographs indicated chan ge in density due to methodological errors. Application of thresholds may avoid false-positive diagnoses. When applying CADIA in clinical re search, the range of change to be expected due to methodological limit ations as well as the threshold for true change should be evaluated. T hese thresholds may differ in various areas of the mouth, i.e., bone o r gingival, maxillary/mandibular, anterior/posterior ROI.