This project is an extension of previous work directed toward the study of noninvasive methods to determine spatial and temporal relationships existing between tissues of clinical interest. The approach involves analytic formulation of the diagnostic task, computer simulations, in vitro measurements, and the development of prototypes suitable for clinical evaluation. Recent work has focussed mostly on studies directed toward development of a versatile computerized dental radiographic system to produce images which can be subtracted to show small changes in tissue occurring over long intervals of time, and which can be combined in ways permitting synthesis of desired projections or tomosynthetic display of specific slices of the teeth and jaws. The main difficulty with subtraction radiography is to attain proper control of the x-ray projection geometry. Therefore, work continued ont he design of a head holder system using a cephalostat and closed-circuit video feedback to stabilize the projection geometry in film-based radiography. Measurements on a proto-type design demonstrated that it was possible to reposition patients within acceptable tolerances necessary for subtraction radiography with sagittal as well as frontal projections. Bitewing radiographs are commonly used for detecting interproximal carious lesions, with little firm knowledge about the best projection directions. A controlled psychophysical study revealed best detection performance with angulation in the horizontal plane perpendicular to the tooth surfaces, and positive or negative tilts of 12 to 15 degrees from the perpendicular in the vertical direction. Deviation from the optimal horizontal angle resulted in an increased number of missed lesions, i.e. loss of sensitivity, whole vertical deviations caused mainly an increase in false positive diagnoses, i.e., a drop of specificity. New calibration methods are developed to control for nonlinear effects caused by differential x-ray beam hardening in the tissues of interest in order to achieve a more precise quantitation of bone mass loss by subtraction radiography.
