Normally, the person bites on a plastic spatula so that all the teeth, especially the crowns can be viewed individually. The whole orthopantomogram process takes about one minute.
Because the collimation, while rotating, the X-rays projects on the film only a limited portion of the anatomy, at every instant but, as the rotation progresses around the skull, the whole maxillo-facial block is scanned. While the arm rotates, the film moves in a such way that the projected partial skull image (limited by the beam section) scrolls over it and exposes it entirely. Because the beam travels across the skull, the partial image it projects on the film every instant contains all the anatomical elements it crossed in the skull, overlapped. Not all the overlapped element images projected on the film have the same magnification because the beam is divergent and the elements are at different distances from the generator focus. Also not all the element images move with the same velocity on the target film as some of them are far and other closer to the instant rotation center. The velocity of the film is controlled in such fashion to fit exactly the velocity of projection of the anatomical elements of the dental arch side which is closer to the film. Therefore they are recorded sharply while the elements in different places are recorded blurred as they scroll at different velocity.
The dental panoramic image suffers from important distortions because a vertical zoom and a horizontal zoom both varying differently along the image. The vertical and horizontal zooms are determined by the relative position of the recorded element versus film and generator. Closer to the generator means bigger vertical zoom. More, the horizontal zoom is also dependent on the relative position of the element to the focal path. Inside the focal path arch means bigger horizontal zoom and blurred, outside means smaller horizontal and blurred.
The result is an image showing sharply the section along the mandible arch and blurred the rest. For example, the more radio-opaque anatomical region, the cervical vertebras (neck), shows as a wide and blurred vertical pillar overlapping the front teeth. The path where the anatomical elements are recorded sharply is called "focal path".
