The cephalometric projection. Part II. Principles of image distortion in cephalography.

Theprinciple effects of image distortion in cepha/ography are presented. The effects of misalignment between the ray source, the cepha/ostat, and the film are considered as well as ma/positioning of the object. In cephalometry, angles, distances, and rela­ tions between defined anatomical landmarks are established. However, cephalometric mea­ surements are performed in a two-dimensional image of a three-dimensional object placed in the radiographic system. Also when alignment of the radiographic equipment, and positioning of the patient are ideal, projection errors may arise affecting cephalometric measurements. In a methodological study Slagsvold & Peder­ sen (5) found significant differences between measurements of the gonial angle in skulls and lateral radiographs of the same individual. Ber­ gersen (2) studied magnification and distortion in cephalometric radiographs and found devia­ tions from true values in linear measurements when the landmarks were situated outside the sagittal plane. van Aken (1) demonstrated that projectional errors are usually small but may be significant for landmarks positioned outside the sagittal plane in cases having asymmetrical skulls. There are few analyses of the principles underlying image distortion in cephalography. The cephalographic system consists of three main components: the ray source, the cephalo­ stat, and the film. The distances between these components are strictly defined. In the ideal