Quantitation of vital bleaching by computer analysis of photographic images

Background. The authors investigated the use of computer processing of photographic images to monitor changes in tooth brightness after nightguard vital bleaching, or NGVB. Methods. Photographs of shade guides and clinical cases (patients' teeth) were taken on 35-millimeter film with electronic flash illumination and processed commercially. A slide scanner was used to digitize images as red, green and blue, or RGB, files, with constant brightness, contrast and linearity settings; the images were then analyzed with commercial software. Relevant image components (that is, teeth or shade guide tabs) were separated, and histograms of various numerical color descriptors were generated for each image component. Results. Analysis of shade tab images showed that the mean pixel intensity for the RGB blue channel, or MPIb, was the most satisfactory brightness descriptor, with clear sequential MPIb increments from lighter to darker shades in each series of colors (A through D) and close correlation with the manufacturer's brightness scale (r = .83). Mathematical analysis of MPIb data for shade tabs in the same image yielded a brightness index that was reproducible and correlated well with the manufacturer's brightness scale. Sequential measurements of this index in three subjects whose teeth were bleached with carbamide peroxide for 14 days correlated well with assessments made by visual shade guide comparisons. Conclusions. The authors conclude that computer analysis of digitized photographic images with internal color controls provides an index of tooth brightness that is reproducible from image to image. Clinical Implications. A brightness index derived from computer analysis of digitized photographic images may be useful for monitoring the effectiveness of NGVB.