Section III. PARALLELING (LONG-CONE) PERIAPICAL EXPOSURE TECHNIQUES
A long cone is used to take x-rays with paralleling exposure techniques.
Periapical film is held parallel to the long axis of the tooth using film-holding instruments.
The central ray is directed to pass at a perpendicular angle to both the tooth and the
film. Since the slope and curvature of the dental arches and the alveolar processes will
not permit the film to be held close to the teeth and still be parallel to their long axes, the
film must be held away from the teeth. This method provides a target-film distance of
approximately 16 inches, in contrast to 8 inches for the bisecting technique (see figure
4-1). The increase in the target-film distance is related to the size of the image
produced. If the film is held away from the tooth and the target-film distance kept at 8
inches, enlargement of the image would be unavoidable. Enlargement is minimized,
however, by increasing the target-film distance to 16 inches, thus using the parallel
rays. An extension cone is used (see figure 4-18) to increase the target-film distance.
Figure 4-18. Long cone on x-ray unit.
4-19. ASSEMBLY OF FILM-HOLDING INSTRUMENTS
The instruments used to hold the film parallel to the teeth are plastic bite-blocks,
indicator rods, and plastic locator rings.
a. Anterior Instrument. The anterior instrument (see figure 4-19) is assembled
and used as follows.