Conventional Radiography

The five radiographic projections needed to visualize the soft tissue of the external acoustic canal and the bone of the tympanic bullae and petrous temporal bones are referred to collectively as a bulla series.

Technique and Normal Radiographic Findings

High-detail film screen combinations (Kodak Lanex fine screens with Ektascan M film or 3M SE plus film with Assymetrix detail screens) are required to provide the contrast and spatial resolution needed to recognize the typical abnormalities seen in the canine and feline ear.

Lateral View

The animal is placed in lateral recumbency with the nasal septum parallel and the hard palate perpendicular to the tabletop cassette (Figure 4-1, A and B). The primary beam is centered on the external acoustic canal. A foam wedge or gauze roll placed beneath the rostral third of the nose is needed to maintain proper alignment. The head should be slightly extended to avoid superimposition of the bullae on the pharynx. Portions of the larynx and pharynx should be included in this view (Figure 4-1, C) to assess the temporomandibular joints and the nasopharynx because as some diseases, such as nasopharyngeal polyps and craniomandibular osteopathy, can also affect the middle ear. This author prefers a subtle rostral offset of one bulla from the other to compare them. To achieve this, the primary beam can be centered just rostral to the external acoustic meatus, or the rostral third of the nose can be elevated slightly from

A, Patient positioning to obtain a lateral view of the middle ear.

Continued

parallel to the cassette (Figure 4-1, D). In animals with a large pinna covering the acoustic canal, such as hound or spaniel breeds, the pinna should be unfolded and placed dorsal to the skull to avoid allowing skin artifacts to obscure the area of inter­est. The endotracheal tube can be left in place. The external acoustic meatus is a circular to oval, gas-filled structure with well-defined inner borders. The tympanic bullae have smooth, thin-walled bone margins and a gas-filled lumen (see Figure 4-1, C

Figure 4-1—cont'd

C, Lateral radiograph of the middle ear. The left and right tympanic bullae are superimposed (white arrowheads). The petrosal portions of the temporal bones (asterisk) are located dorsal to the bullae. The soft palate is indicated with black arrowheads. The external acoustic meatus (arrows) is dorsal to the tympanic bullae. D, Lateral radiograph of the middle ear with one bulla rostrally positioned (arrowhead). The petrosal portions of the temporal bones (asterisk) are located dorsal to the bullae. The stylohyoid bones (white arrow) overlie the caudal aspect of the nasopharynx. The external acoustic meatus (black arrows) is also visible. and D). Thickness of the wall varies between breeds. There is less variation in the thickness of the bullae walls between breeds in cats. The petrosal portions of the temporal bones are highly radiopaque and superimposed on each other in this view; therefore, they cannot be fully assessed. In cats the bullae appear larger in propor­tion to the head than in dogs.

Oblique Views

Two opposite oblique views are taken.