Digital image ratio: a new radiographic method for quantifying changes in alveolar bone. Part 1: Theory and methodology.

A new world, digital image ratio (DIR), has been developed for directly measuring changes in alveolar bone. The image on the computer monitor represents the relative mass change between two radiographs. Fourier filtering is used to reduce noise artefacts. This method is validated through an experiment with a step wedge. DIR needs only a preliminary calibration of the experimental conditions of operation and avoids tedious calibrations for each measurement as in the case of digital image substraction. Low-voltage X-ray techniques are suggested for long-term quantitative studies of patients to minimize irradiation doses.