C HAPT E R 6 Projection Geometry 87

Anode

10
5
0
Image
receptor
0 5 10 FIGURE 6-7 The central ray should be perpendicular to the long axes of both the tooth
and the image receptor. If the direction of the x-ray beam is not at right angles to the long axis
of the tooth, the appearance of the tooth is distorted, typically by apparent elongation of the
FIGURE 6-5 Foreshortening of a radiographic image results when the central ray is per- length of the palatal roots of upper molars and distortion of the relationship of the height of
pendicular to the image receptor but the object is not parallel with the image receptor. the alveolar crest relative to the cementoenamel junction.

Central axis of tooth

Anode
Imaginary bisector

0 5 10
25
20
15
10
5
0

FIGURE 6-6 Elongation of a radiographic image results when the central ray is perpen- FIGURE 6-8 In the bisecting-angle technique, the central ray is directed at a right angle
dicular to the object but not to the image receptor. to the imaginary plane that bisects the angle formed by the image receptor and the central axis
of the object. This method produces an image that is the same length as the object but results
in some image distortion.

position of alveolar crest with respect to the cementoenamel junc- loss of sharpness. To overcome these limitations, the paralleling
tion of a tooth. In recent years, the bisecting-angle technique has technique also uses a relatively long open-ended aiming cylinder
been used less frequently for general periapical radiography as use (“cone”) to increase the focal spot-to-object distance. This “cone”
of the paralleling technique has increased. directs only the most central and parallel rays of the beam to the
The paralleling technique is the preferred method for making image receptor and teeth and reduces image magnification, while
intraoral radiographs. It derives its name as the result of placing increasing image sharpness. Because it is desirable to position
the image receptor parallel to the long axis of the tooth (Fig. 6-9). image receptors near the middle of the oral cavity with the paral-
This procedure minimizes image distortion and best incorporates leling technique, image receptor holders should be used to support
the imaging principles described in the first three sections of this the image receptor in the patient’s mouth (see Chapter 7).
chapter.
To achieve this parallel orientation, the practitioner often must
position the image receptor toward the middle of the oral cavity,
OBJECT LOCALIZATION
away from the teeth. Although this allows the teeth and image In clinical practice, the dentist often must derive from a radiograph
receptor to be parallel, it results in some image magnification and three-dimensional information concerning patients. For example,
88 PAR T I I Imaging

the dentist may wish to use radiographs to determine the location radiograph, the dentist may take a mandibular occlusal view to
of a foreign object or an impacted tooth within the jaw. Three identify its mediolateral position. The occlusal film may reveal a
methods are frequently used to obtain such three-dimensional calcification in the soft tissues located laterally or medially to the
information. The first is to examine two images projected at right body of the mandible. This information is important in determin-
angles to each other. The second method is to use the tube-shift ing the treatment required. The right-angle (or cross section) tech-
technique employing conventional periapical views. Third, in nique is best for the mandible (see Figs. 22-8, A, 22-15, and 22-23,
recent years, the advent of cone-beam imaging has provided a new B). On a maxillary occlusal view, the superimposition of features
tool for obtaining three-dimensional information. In this chapter, in the anterior part of the skull frequently obscures the area of
we discuss the first two of these methods. These techniques are interest.
valuable because cone-beam CT may not be available or even The second method used to identify the spatial position of an
necessary if the dentist already has multiple periapical views of the object is the tube-shift technique. Other names for this procedure
region of interest. Cone-beam CT is discussed in Chapters 11-13. are the buccal-object rule and Clark’s rule (Clark described this
Figure 6-10 shows the first method, in which two views made method in 1910). The rationale for this procedure derives from the
at right angles to one another localize an object in or about the manner in which the relative positions of radiographic images of
maxilla in three dimensions. In clinical practice, the position of two separate objects change when the projection angle at which
an object on each radiograph is noted relative to the anatomic the images were made is changed.
landmarks; this allows the observer to determine the position of Figure 6-11 shows two radiographs of an object exposed at dif-
the object or area of interest. For example, if a radiopacity is ferent angles. Compare the position of the object in question on
found near the apex of the mandibular first molar on a periapical each radiograph with the reference structures. If the tube is shifted
and directed at the reference object (e.g., the apex of a tooth) from
a more mesial angulation and the object in question also moves
mesially with respect to the reference object, the object lies lingual
Central axis of tooth to the reference object.
Alternatively, if the tube is shifted mesially and the object in
question appears to move distally, it lies on the buccal aspect of
the reference object (Fig. 6-12). These relationships can be easily
remembered by the acronym SLOB: same lingual, opposite buccal.
Thus if the object in question appears to move in the same direc-
tion with respect to the reference structures as does the x-ray
tube, it is on the lingual aspect of the reference object; if it appears
to move in the opposite direction as the x-ray tube, it is on the
buccal aspect. If it does not move with respect to the reference
object, it lies at the same depth (in the same vertical plane) as
the reference object.

FIGURE 6-9 In the paralleling technique, the central ray is directed at a right angle to
the central axes of the object and the image receptor. This technique requires a device to support
the film in position.

A
B FIGURE 6-11 The position of an object may be determined with respect to reference
structures with use of the tube shift technique. A, A radiopaque object on the lingual surface
FIGURE 6-10 A, Periapical radiograph shows impacted canine lying apical to roots of of the mandible (black dot) may appear apical to the second premolar. B, When another
lateral incisor and first premolar. B, Vertex occlusal view shows that the canine lies palatal to radiograph is made of this region angulated from the mesial, the object appears to have moved
the roots of the lateral incisor and first premolar. mesially with respect to the second premolar apex (“same lingual” in the acronym SLOB).
 C HAPT E R 6 Projection Geometry 89

A
B

FIGURE 6-12 The position of an object can be determined with respect to reference
structures with use of the tube shift technique. A, An object on the buccal surface of the
mandible may appear apical to the second premolar. B, When another radiograph is made of
this region angulated from the mesial, the object appears to have moved distally with respect
to the second premolar apex (“opposite buccal” in the acronym SLOB).

Examination of a conventional set of full-mouth images with

this rule in mind demonstrates that the incisive foramen is located
lingual (palatal) to the roots of the central incisors and that the B
mental foramen lies buccal to the roots of the premolars. This
technique assists in determining the position of impacted teeth, FIGURE 6-13 The position of the maxillary zygomatic process in relation to the roots
the presence of foreign objects, and other abnormal conditions. It of the molars can help in identifying the orientation of views. A, The inferior border of
works just as well when the x-ray machine is moved vertically as the zygomatic process lies over the palatal root of the first molar. B, The inferior border
horizontally. of the zygomatic process lies posterior to the palatal root of the first molar. This difference
The dentist may have two radiographs of a region of the in position of the zygomatic process in relation to the palatal root indicates that when the
dentition that were made at different angles, but no record image in A was made, the beam was oriented more from the posterior than when the
exists of the orientation of the x-ray machine. Comparison of image in B was made. The same conclusion can be reached independently by examining
the anatomy displayed on the images helps distinguish changes the roots of the first molar. The palatal root lies behind the distobuccal root in the image
in horizontal or vertical angulation. The relative positions of in A, but it lies between the two buccal roots in the image in B.
osseous landmarks with respect to the teeth help identify changes
in horizontal or vertical angulation. Figure 6-13 shows the
inferior border of the zygomatic process of the maxilla over
the molars. This structure lies buccal to the teeth and appears
to move mesially as the x-ray beam is oriented more from
the distal. Similarly, as the angulation of the beam is increased than photons traveling at right angles to the surface. Figure 6-14,
vertically, the zygomatic process is projected occlusally over B, shows an expansile lesion on the buccal surface of the mandible
the teeth. on an occlusal view. The periphery of the expanded cortex is
more opaque than the region inside the expanded border. The
EGGSHELL EFFECT cortical bone is not thicker on the cortex than over the rest of
the lesion, but rather the x-ray beam is more attenuated in this
Plain images—images that project a three-dimensional volume onto region because of the longer path length of photons through the
a two-dimensional receptor—may produce an eggshell effect of bony cortex on the periphery. This eggshell effect accounts for
corticated structures. Figure 6-14, A, shows a schematic view of why normal structures such as the lamina dura, the border of the
an egg being exposed to an x-ray beam. The top photon has a maxillary sinuses and nasal fossa, and abnormal structures, includ-
tangential path through the apex of the egg and a much longer ing the corticated walls of cysts and benign tumors, are well
path through the shell of the egg than does the lower photon, demonstrated on plain images. Soft tissue masses, such as the
which strikes the egg at right angles to the surface and travels nose and tongue, do not show an eggshell effect because they
through two thicknesses of the shell. As a result, photons traveling are uniform rather than being composed of a dense layer sur-
through the periphery of a curved surface are more attenuated rounding a more lucent interior.