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Dentomaxillofacial Radiology, Vol 25, Issue 1 42-47, Copyright © 1996 by British Institute of Radiology

ARTICLES

Histogram-matching and histogram-flattening contrast correction methods: a comparison

C. F. Hildebolt, R. K. Walkup, G. L. Conover, N. Yokoyama-Crothers, T. Q. Bartlett, M. W. Vannier, M. K. Shrout and J. J. Camp
Washington University, St. Louis, Missouri, USA.

OBJECTIVES: To compare the results or two methods of histogram matching and two methods of histogram flattening for their ability to correct for contrast variations in digital dental images. METHODS: A custom-built, aluminium stepwedge with 0.1, 0.5 and 1.0 mm steps was placed over Ektaspeed films and exposed for 0.06, 0.12 and 0.25 s, respectively. Radiographs were digitized at 50 microns spatial resolution and 12-bit contrast resolution. Contrast corrections were performed using Ruttimann et al.'s algorithm (1986) for one method of matching (RM) and flattening (RF) and Castleman's algorithm (1979) for the other method of matching (CM) and flattening (CF). Mean pixel grey-scale values were determined for each step. The 0.12 s exposure was considered to be the target image exposure. Absolute differences in pixel grey-scale values between the target images and the modified images were determined. RESULTS: The median values of the absolute differences in pixel grey-scale values between the target images and the contrast corrected images were: CM = 4.3; RM = 4.1; CF = 70.2 and RF = 70.2. CONCLUSION: Castleman's and Ruttimann's matching algorithms perform equally well in correcting digital image contrast. Histogram flattening was less effective.


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[Abstract] [Full Text] [PDF]


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