[Digital subtraction technique in dentistry]

By computerized subtracting the grey values of geometrically corresponding pixels in consecutively taken digital X-ray images, it is possible to visualize or measure changes in radiodensity over time. This can be very useful, both in clinical and research applications, for the follow-up of tooth mineralization or bone changes. A major problem, however, resides in obtaining a sufficient geometrical standardization of the images. Several techniques have been proposed to this end, most of them still being in development, or too cumbersome to find application in daily routine.