Objective: To examine the effect of misalignment of the central ray upon calculation of facial asymmetry in Roentgenographic cephalometrics. Quantitative assessments of facial asymmetry are typically made by measuring postero-anterior (PA) skull x-ray images based on the assumption that the central ray from the x-ray emitter to the film plane travels along the mid-sagittal plane. Empirical observation has demonstrated that most cephalometers don't satisfy this requirement. Methods: By the method of similar triangles, it was calculated that, for objects lying in the ear rod plane, a lateral emitter displacement of 1 cm would produce a projective displacement of 1mm at the film plane. Other landmarks should displace proportional to their spatial locations between the emitter and the film plane. This calculation was checked empirically using a dried skull with radiopaque markers and a calibrated cephalometer that permitted measured lateral displacement of the emitter. The relative 3D locations of the markers were known by prior measurement. Data were evaluated from a series of PA cephalograms, each generated with the emitter at a different lateral displacement. Results: The observed positions of the markers on the PA images compared closely with their predicted positions. Representative values for two projections of three key landmarks are shown in the table. Note that in each case the marker at menton shifted less than the gonion markers that were further from the film plane.

Conclusion: Misalignment of the central ray introduces systematic errors into the interpretation of skull asymmetries on PA cephalograms. The effect is usually small in individual cases but may introduce statistically significant systematic errors when sets of PA images made on different cephalometers are compared statistically.

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