title: 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.
  
  
  METHODS
  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.
authors.affiliation: School of Dentistry, University of Louisville, Ky, USA.
authors.forename: K
authors.initials: K
authors.lastname: Yamamoto
authors.forename: A G
authors.initials: AG
authors.lastname: Farman
authors.forename: R L
authors.initials: RL
authors.lastname: Webber
authors.forename: R A
authors.initials: RA
authors.lastname: Horton
authors.forename: K
authors.initials: K
authors.lastname: Kuroyanagi
mesh_terms: D000704:Analysis of Variance; D016477:Artifacts; D002516:Ceramics; D003287:Contrast Media; D003867:Depth Perception; D006801:Humans; D007576:Jaw, Edentulous, Partially / Q000000981:diagnostic imaging; D011856:Radiographic Image Enhancement / Q000379:methods*; D011861:Radiography, Dental / Q000379:methods*; D015203:Reproducibility of Results; D013232:Steel; D014057:Tomography, X-Ray Computed / Q000379:methods*; D020390:Tooth Socket / Q000000981:diagnostic imaging*
chemical_list: D003287:Contrast Media; D013232:Steel