Histogram matching and histogram flattening contrast correction methods: A comparison

Objectives. To compare the results of 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 mu m 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.