CHAPTER 2 The Skull

OVERVIEW

The skull is the most complex and specialized part of the skeleton; it is a challenging region to evaluate with conventional radiography. Superimposition of the many complex osseous structures limits the usefulness of radiography. Imaging the skull is much better suited to the cross-sectional modalities of computed tomography and magnetic resonance imaging, which solve the issue of superimposition and are characterized by superior contrast resolution. However, well-positioned radiographs with good detail and contrast can provide information that can help in directing appropriate patient management or selecting the best alternative imaging modality.

Although it is often more convenient to acquire radiographs of the skull with the patient conscious or minimally sedated, this rarely results in well-positioned radiographs. Poorly positioned skull radiographs are particularly challenging to interpret and usually have no diagnostic value. Because the skull is so complex, standardization of views becomes more important than in any other location. Taking the time to ensure adequate patient restraint and optimal positioning significantly increases the chances of obtaining meaningful information.

The skull varies more in size and shape among domestic dogs than in any other mammalian species. Various measurement parameters are used to group dogs based on skull morphology. The term dolichocephalic describes long, narrow-headed breeds, like collies and wolfhounds. The term mesaticephalic describes breeds that have heads of medium proportions, such as German shepherds, beagles, and setters. Brachycephalic describes breeds with short and wide heads, such as Boston terriers and Pekingese.

A radiographic study of the skull should always include a lateral and dorsoventral (or ventrodorsal) view (Figures 2-1 and 2-2).

Figure 2-1 A, Lateral radiograph of the skull of an 8-year-old golden retriever. A1, The same image as in A, with labels identifying major structures. B, A dorsoventral radiograph of a 9-year-old mixed breed dog. B1, The same image as in B, with major structures identified. TM, Temporomandibular.

Figure 2-2 A, Lateral radiograph of a 3-year-old domestic shorthair cat. The osseous tentorium, which separates the caudal aspect of the cerebrum from the rostral aspect of the cerebellum, is particularly well-developed in cats compared to dogs. An endotracheal tube is present. B, Dorsoventral radiograph of an 18-year-old domestic shorthair cat. TMJ, Temporomandibular joint.

Additional views of the skull are often acquired depending on the region of interest. They should, however, always be acquired in concert with standard lateral and dorsoventral (or ventrodorsal) views. Oblique views should not be used to replace the standard views because standard views often provide information that allows more effective interpretation of oblique views. In addition, oblique views are impossible to interpret correctly unless a readily understandable external radiographic marking system is used. This allows correct identification of surfaces projected in profile. This is particularly important when there is no obvious unilateral disease. This is discussed in the “Dentition” section.

Approximately 50 bones, many of which are paired, compose the skull. Many are not identifiable with survey radiographs, and most fuse with adjoining bones precluding specific delineation. The important larger bones of the skull that are visible radiographically are the incisive, nasal, maxillary, lacrimal, frontal, zygomatic, pterygoid, sphenoid, parietal, temporal, and occipital bones. Figure 2-3 identifies these bones diagrammatically.

Figure 2-3 Three-dimensional volume-rendered computed tomography images of the skull. A, Lateral view; B, dorsal view; C, ventral view.

DENTITION

The upper incisors are located within the incisive bone, and the canine teeth, premolars, and molars are located within the maxilla bone (Figure 2-4). The maxilla is the large bone that, in addition to the much smaller incisive and nasal bones, encases the majority of the nasal cavity and the blind-ended maxillary recesses located ventrolaterally on each side of the nasal cavity. Ventrally, the maxilla forms the hard palate, which is the physical separation between the nasal and oral cavities. Caudally, the maxilla fuses with the frontal bone, which encompasses the frontal sinuses and the rostral aspect of the cranial vault.

Figure 2-4 Oblique view of a maxillary dental arcade of an 8-month-old mixed breed dog. The four premolars and two molars are readily identifiable. The fourth upper premolar (08) and the first lower molar (09) are sometimes referred to as carnassial teeth.

In the dog, the anatomic dental formulas are as follows (I, Incisors; C, canine; PM, premolars; M, molars):

Deciduous: I3/3, C1/1, PM3/3

Permanent: I3/3, C1/1, PM4/4, M2/3

The fourth maxillary premolar and first mandibular molar in the dog are sometimes called carnassial teeth, which refers to their shearing functions. Canine tooth eruption times are given in Table 2-1.

Table 2-1 Canine Dental Eruption Times ¹

	Deciduous Teeth (Weeks)	Permanent Teeth (Months)
Incisors	3-4	3-5
Canines	3	4-6
Premolars	4-12	4-6
Molars	n/a	5-7

In the cat, the anatomic dental formulas are as follows:

Deciduous: I3/3, C1/1, PM3/2

Permanent: I3/3, C1/1, PM, 3/2, M1/1.

In the cat, the roots of the upper fourth premolar extend into the wall of the orbit. The carnassial teeth in the cat are the same as in the dog: the last upper premolar and the first lower molar. Feline tooth eruption times are given in Table 2-2.

Table 2-2 Feline Dental Eruption Times ¹

	Deciduous Teeth (Weeks)	Permanent Teeth (Months)
Incisors	2-3	3-4
Canines	3-4	4-5
Premolars	3-6	4-6
Molars	n/a	4-5

The modified Triadan system of dental nomenclature is an alternative system for tooth identification ² (Figure 2-5). The first digit denotes the quadrant. The right upper quadrant is designated 1, the left upper quadrant is designated 2, the left lower quadrant is designated 3, and the right lower quadrant is designated 4. The second and third digits denote positions within the quadrant, and numbering always begins at the midline; thus the central incisor is always 01, the canine is always 04, and the first molar is always 09. For example, the right upper fourth premolar would be tooth number 108 in the Triadan system.

Figure 2-5 The modified Triadan system of dental identification. In A, each quadrant of the mouth is designated by a number; this is the first digit in identification. With respect to permanent teeth, the right maxilla is designated 1, the left maxilla 2, the left mandible 3, and the right mandible 4. When describing deciduous teeth, the right maxilla is designated 5, the left maxilla 6, the left mandible 7, and the right mandible 8. B, Schematic model of a canine jaw with each permanent tooth identified. The numbering always begins on midline; the central incisor is designated 01, the canine 04, and the first molar 09. C, Schematic model of a feline jaw, showing tooth identification.

Radiography of the dental arcade is commonly performed using dedicated dental x-ray machines, which generally have relatively low mA and kVp capabilities, used with intraoral film or a small dedicated intraoral dental imaging plate. In the absence of a dental radiographic system, lateral oblique views can be used to assess each dental arcade.

As previously noted, it is critical that labeling of oblique radiographs be accurate and that the radiographer understands what will be projected when the skull is angled. This has been described elsewhere,³ but a brief example follows. Assume the dog is in right lateral recumbency on the x-ray table and the goal is to evaluate the right upper arcade. By elevating the mandible with a nonradiopaque triangular sponge and using a vertical x-ray beam, the right maxillary arcade can be projected ventral relative to the rest of the maxilla without superimposition (Figure 2-6). To evaluate the right mandibular arcade, use the triangular sponge to elevate the maxilla, not the mandible. To evaluate the left arcades, reposition the dog in left recumbency and repeat the procedure.

Figure 2-6 A, Schematic showing the oblique positioning for radiography of the right maxillary dental arcade. Padding has been placed under the mandible to elevate the mandible and to rotate the left side of the patient’s head dorsally. The left side of the patient is subsequently projected dorsally and the right maxillary and mandibular arcades are projected ventrally. Both a left and a right marker should be used to avoid confusion as to which arcade is highlighted. B, Radiograph of an 8-month-old mixed breed dog. positioned as in A. A small plastic sleeve has been inserted between the left canines to hold the mouth open. This reduces mandibular and maxillary superimposition and maximizes the chances of obtaining an unobstructed view of the dental arcade of interest.

(A, From Owens JM, Biery DN: Radiographic interpretation for the small animal clinician, Baltimore, 1999, Williams & Wilkins.)

When a tooth is evaluated, the structures that can be visualized are the roots and crown, the pulp cavity, and the lamina dura. Although the periodontal membrane cannot be visualized directly, its location can be inferred (Figure 2-7).

Figure 2-7 Oblique view of the maxilla of an 8-month-old mixed breed dog. The thin radiopaque lines delineated by the hollow white arrows are the laminae dura, which is solid compact bone that surrounds the dental alveolus and provides an attachment surface for the periodontal ligament. The periodontal ligament in health is not visible radiographically but is in the radiolucent space between the tooth root and lamina dura (solid white arrow). The hollow black arrow is the apex of the tooth and the base of the alveolus or socket. An asterisk is superimposed over the lucent pulp cavity within the fourth premolar.

Visualization of both deciduous teeth and permanent teeth buds is normal in adolescent patients (Figure 2-8). Molars have no associated deciduous tooth. Normally, the deciduous teeth are shed as permanent teeth erupt. A retained deciduous tooth can impede eruption of permanent incisors, canines, or premolars. Alternatively, retained deciduous teeth may not impede eruption of the subjacent permanent tooth but become displaced as the permanent tooth erupts (Figure 2-9). Retained deciduous teeth are readily apparent upon visual inspection of the oral cavity. Radiographs are not needed for their detection but will be helpful in determining whether the eruption disorder is altering the underlying tooth roots or bone structure.

Figure 2-8 Lateral view of the mandible of a 3-month-old bassett hound. The hollow white arrow is the deciduous fourth lower premolar. Subjacent to the deciduous fourth lower premolar is the tooth bud for the permanent fourth lower premolar (solid black arrow). The solid white arrow is the tooth bud for the permanent first lower molar.

Figure 2-9 Lateral (A), dorsoventral (B), and oblique (C) radiographs of a 10-month-old Bichon Frise. There is bilateral retention of the deciduous maxillary canine tooth (solid white arrow).

NASAL CAVITY AND SINUSES

The nasal cavity is normally bilaterally symmetric, divided in the midsagittal plane by the vomer bone and nasal septum. Each nasal cavity contains a myriad of fine turbinate bones, collectively known as the ecto- and endoturbinates (Figure 2-10). These fine turbinates support the large mucosal surface area that is responsible for filtering, warming, and moisturizing inspired air. Standard views for the nasal cavity are lateral and dorsoventral views (see Figures 2-1 and 2-11), intraoral views with the film placed in the mouth, and oblique open mouth views. With analog (film-based) imaging systems, the best way to evaluate the nasal cavity is by using an intraoral film encased in a vinyl cassette system (3M Company, St Paul, Minn.) (Figure 2-12). By placing the film in the mouth and directing the x-ray beam onto the dorsal aspect of the maxilla, superimposition of the mandible is avoided. With digital systems, because the plate cannot be placed in the oral cavity, the best view to evaluate the nasal cavity is an open-mouth ventrodorsal oblique view (V20°R-DCdO). To acquire this view, the dog is anesthetized and placed in dorsal recumbency; the mouth is pulled open by placing tape or a small rope around the maxillary and mandibular incisors while maintaining a parallel relationship between the hard palate and x-ray table; the tongue is held against the mandible using mandibular gauze; and the x-ray beam is angled 20° rostrally and directed into the mouth. This eliminates superimposition due to the mandible but does result in some anatomic distortion (Figure 2-13).