title: Effect of cephalometer misalignment on calculations of facial asymmetry.
abstract: BACKGROUND
  In this study, we evaluated errors introduced into the interpretation of facial asymmetry on posteroanterior (PA) cephalograms due to malpositioning of the x-ray emitter focal spot. We tested the hypothesis that horizontal displacements of the emitter from its ideal position would produce systematic displacements of skull landmarks that could be fully accounted for by the rules of projective geometry alone.
  
  
  METHODS
  A representative dry skull with 22 metal markers was used to generate a series of PA images from different emitter positions by using a fully calibrated stereo cephalometer. Empirical measurements of the resulting cephalograms were compared with mathematical predictions based solely on geometric rules. The empirical measurements matched the mathematical predictions within the limits of measurement error (x= 0.23 mm), thus supporting the hypothesis. Based upon this finding, we generated a completely symmetrical mathematical skull and calculated the expected errors for focal spots of several different magnitudes. Quantitative data were computed for focal spot displacements of different magnitudes.
  
  
  CONCLUSIONS
  Misalignment of the x-ray emitter focal spot introduces systematic errors into the interpretation of facial asymmetry on PA cephalograms. For misalignments of less than 20 mm, the effect is small in individual cases. However, misalignments as small as 10 mm can introduce spurious statistical findings of significant asymmetry when mean values for large groups of PA images are evaluated.
authors.affiliation: Department of Orthodontics, School of Dentistry, Chonnam National University, Gwangju, Korea.
authors.forename: Ki-Heon
authors.initials: KH
authors.lastname: Lee
authors.forename: Hyeon-Shik
authors.initials: HS
authors.lastname: Hwang
authors.forename: Sean
authors.initials: S
authors.lastname: Curry
authors.forename: Robert L
authors.initials: RL
authors.lastname: Boyd
authors.forename: Kevin
authors.initials: K
authors.lastname: Norris
authors.forename: Sheldon
authors.initials: S
authors.lastname: Baumrind
mesh_terms: D002508:Cephalometry / Q000295:instrumentation / Q000706:statistics & numerical data*; D003951:Diagnostic Errors; D005146:Facial Asymmetry / Q000175:diagnosis* / Q000000981:diagnostic imaging; D006801:Humans; D008962:Models, Theoretical; D011858:Radiographic Magnification; D012886:Skull / Q000000981:diagnostic imaging