Image Projection

Panoramic radiography is based on the principles of tomography and slitradiography. Tomography is a body-sectioning technique involving the simultaneous movement of the x-ray tubehead and film cassette 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 match the predetermined form of the image layer. Correct patient positioning is essential for optimal results. Generally speaking, image distortion occurs when structures are positioned anterior (narrows and blurs), posterior (widens and blurs) or a combination thereof relative to the focal trough (Figure 18A and B).

Figure 18. A
Figure 18. B
Resultant curved image layer that corresponds to arch shape.  Mal-positioning relative to the focal trough alters structures most significantly in width. (Figure 18B)