Effect of number of projections on accuracy of depth discrimination using tuned-aperture computed tomography for 3-dimensional dentoalveolar imaging of low-contrast details

Abstract

Objective. The purpose of this study was to test the hypothesis that the number of projections influences the accuracy of a simple depth discrimination task when tuned-aperture computed tomography is used. Study design. In each of 4 partially edentulous mandibles, 2 radiopaque steel spheres were attached to the facial and lingual surfaces and 1 ceramic sphere was place in the apical region of an open tooth socket. Errors in estimates of the depth of the apically positioned ceramic sphere relative to the 2 steel spheres were determined from 3-dimensional tuned-aperture computed tomography reconstructions. These data were compared with actual measurements produced independently by means of an optical micrometer. Multiple projections were produced from radially symmetric exposures bearing an angular disparity of 15 degrees. The number of symmetrically dispersed projections per tuned-aperture computed tomography reconstruction was varied systematically (2, 4, 8, 12, and 16 projections). The consequences of this variable, as well as specimen and observer effects, were evaluated in a balanced factorial experimental design. Depth estimates were performed by 10 trained observers. The depth reported was that corresponding to the tuned-aperture computed tomography slice perceived to yield the image of the ceramic sphere in sharpest focus. Resulting data were normalized by logarithmic transformation and analyzed statistically by analysis of variance. Results. No statistically meaningful effects were found for the number of projections ( P = .607) or for different observers ( P = .093), but a significant specimen effect was demonstrated ( P = .006). Conclusions. Factors other than high contrast limit the perception of image sharpness under these conditions. Depth may be estimated accurately from relatively small numbers of projections. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1999;88: 100-5)