Figure 4-3. Head position for making mandibular periapical radiographs.
c. Angulation. When the cone is adjusted to project the central beam upward, it
will be set at a negative (-) degree angulation. When it is adjusted to project the central
beam downward, it will be set at a positive (+) angulation.
CENTRAL RAY ANGULATION
The angle of the x-ray beam, the average angle of projection of the central ray, is
essential for successful use of the bisecting exposure techniques. Both vertical and
horizontal angulations must be considered.
a. Vertical angulation is the up-and-down movement of the tube head or x-ray
beam. The correct vertical angulation exists when the central ray is directed
perpendicular to the bisector of the angle formed by the long axis of the tooth and the
plane of the film (see figure 4-4). When this angulation is correct, the vertical dimension
of the tooth will be as realistic as possible. Incorrect vertical angulation may cause two
(1) Foreshortening exists if the vertical angulation is larger than necessary.
The image of the teeth appears smaller than normal (see figure 4-5).
(2) Elongation exists if the vertical angulation is less than is necessary. The
image of the teeth appears larger than normal (see figure 4-6).