Image Projection

Panoramic radiography is based on the principles of tomography and slit radiography. Tomography is a body-sectioning technique involving the simultaneous movement of the x-ray tubehead and image receptor in opposite directions to produce an image at a depth of tissue. Slit radiography involves the acquisition of an image of a large structure by the scanning movement of a slit beam. These two processes produce a curved focal trough that theoretically corresponds to the average jaw shape. The anterior part of the layer is unavoidably narrower than the posterior part of the layer. Therefore, some patient’s jaws will not exactly match the predetermined form of the image layer. Correct patient positioning is essential for optimal results. In general, image distortion occurs when structures are positioned anterior or labial (narrows and blurs), posterior or lingual (widens and blurs) or a combination thereof relative to the focal trough (Figures 18A-C).

Figure 18A.
Focal Trough
Resultant curved image layer that corresponds to arch shape.
Figure 18B.
Focal Trough
Malpositioning relative to the focal trough alters structures most significantly in width.
Figure 18C.
Resultant curved image layer that corresponds to arch shape

Horizontal malpositioning results in one side appearing narrow and the other wider.