They are used to diagnosis of subtle changes in bone. Eg; it can be used to assess bone levels before and after periodontal therapy, study of periapical region, study of the superior surface of the condyle. The basic advantages includes overall contrast is improved and the structures are more closely visualized in the processed image, the trabecular fine marrow spaces are excellently visualized, low density as well as high density structures are equally enhanced and better visualized. The diagnostic problem in a radiographic examination lies primarily in the identification of image features, caused by a pathologic process, buried in a background of normal anatomical structures. During interpretation, the desired part has to be separated from the irrelevant distribution of other structures. The other structures which do not contain diagnostic information of interest have been termed as “noise.” Here the reference radiograph is digitized and converted into its positive image by the computer. The subsequent radiograph is then displayed on the same server and aligned to the reference image and then digitized. Substraction of the gray levels between the two images is then performed. Any change that has occurred between the original radiograph and the subsequent radiograph shows up as light or dark areas. Loss of bone is seen as dark areas and gain of bone as light areas. To compensate for variations in film response, oral hard tissue measurements with film are sometimes made with the aid of an intraoral stepwedge. Correct placement of the wedge is a problem, however, because it is difficult to match the cheek thickness and scattered radiation intensity without superimposing the step wedge image on the teeth, bone, film-positioning device, or occlusal registration material. Subtraction radiography with film is improved by contrast and exposure corrections. The process is tedious and is of questionable validity for detection of generalized bone loss (osteopenia). Loss of a substantial amount of bone would violate a fundamental assumption by changing the histogram and would therefore reduce the apparent bone loss or gain if histogram-based correction is used. ,No one has yet shown how to distinguish among changes in the histogram that are due to changes in bone mass, film, exposure, and processing. Indeed, an unambiguous distinction may not be possible from image histograms alone. If contrast corrections and the use of intraoral stepwedges are fraught with difficulties, then a better, more reproducible method of measuring x-ray attenuation is needed.
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