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Validity of Cephalometrics

      • It occurs because x ray beams are not parallel with all points on the object to be examined .
      • The use of long focus object and short object film distance have been recommended in order to minimize such projection errors (Franklin 1952)
      • Focus film distance > 280 cms doesn’t alter magnitude of projection error (Ahlquvist et al 1986)
      • Enlargement compensation for lateral ceph. Radiograph is 4.6 to 7.2 % & for frontal film 0.3 to 9.2 % at 60” anode to midsaggital plane distance (Bergersen O E 1980)
    • Misalignment of patient head and X-ray apparatus (Ahlquvist et al 1983 , 1986)
    • Different magnification between different planes
    • Bilateral landmarks – “dual image” on the radiograph
    • Mild asymmetry- difficult to differentiate between geometric distortion and true subject asymmetry (Cook 1980)

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