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. These views are: lateral, two opposite obliques (left 20-degree ventral–right dorsal oblique and right 20-degree ventral– left dorsal oblique), open-mouth (rostral 30-degree ventral–caudodorsal open-mouth oblique), and a ventrodorsal (VD) or dorsoventral (DV).

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 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 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.

Figure 4-1 A, Patient positioning to obtain a lateral view of the middle ear. The asterisk indicates the point where the primary x-ray beam enters the patient. A foam wedge under the maxillary bones ensures the nasal septum is parallel to the table. B, The arrow indicates the trajectory of the primary beam. The skull should be positioned with the hard palate parallel to the primary beam. 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.

Oblique Views

Two opposite oblique views are taken. With the animal in lateral recumbency, the bulla to be imaged is placed closer to the cassette. The thoracic limbs, sternum, nasal cavity, and mandible are rotated 20 degrees from the horizontal plane and are held in position with foam wedges (Figure 4-2, A and B). The mouth is closed to avoid superimposition of the mandible on the area of interest. The primary beam is centered at the base of the ear, ventral to the tragus. The primary beam travels through the patient in a lateral 20-degree ventral–left dorsal oblique direction. The tympanic bulla to be assessed is projected ventrally, while the contralateral bulla is superimposed over the caudal third of the calvarium and therefore cannot be assessed fully (Figure 4-2, C). Portions of the stylohyoid bone may be superimposed over the bulla of interest. The tympanic bullae have smooth, thin-walled bone margins and a gas-filled lumen. The external acoustic meatus is projected on the bulla as a circular to oval, gas-filled structure with well-defined inner borders.

Figure 4-2 A, Patient positioning to obtain an oblique view of the middle ear. The resultant radiograph is named the left 20-degree ventral–right dorsal oblique view, which describes the entrance and exit points of the primary beam. The asterisk indicates the point where the primary x-ray beam enters the patient. B, Patient positioning to obtain a right 20-degree ventral–left dorsal oblique radiograph. The arrow indicates the trajectory of the primary beam. C, Left 20-degree ventral–right dorsal oblique radiograph of the middle ear. The right bulla is displaced ventrally (white arrowheads). Only a faint outline of the wall of the left bulla can be seen (black arrowheads) with the left external acoustic meatus located dorsally (black arrows). There is superimposition of the right stylohyoid bone over the right tympanic bulla. The angular process of the right hemimandible (white arrow) is visible ventral to the right temporomandibular joint (curved arrow).

VD and DV Projections

Choosing between a DV and VD projection is more a function of hospital protocol or personal preference than clinical need. To obtain the VD view, the animal is placed in dorsal recumbency. The primary beam enters the patient in a VD orientation at the midline halfway between the external acoustic meatuses (Figure 4-3, A). The animal is placed in sternal recumbency to obtain a DV view. The primary beam is directed vertically centered at a point where an imaginary line connecting the bullae intersects with the midsagittal plane. The body of the mandible should be parallel to the cassette, to avoid distortion. The tongue should be pulled forward and maintained on the midline. These views are used to assess the ear canals and to compare symmetry of the bullae and the petrosal portion of the temporal bones. The tympanic bullae cannot be evaluated fully in this view because they are superimposed over the petrosal portions of the temporal bones (Figure 4-3, B). There is no specific bone pattern associated with the petrous temporal bones, however; they should exhibit a symmetrical shape, size, and opacity on these projections. Reducing the milliamp seconds (mAs) by half to highlight the soft tissues allows visualization of the horizontal portion of the external acoustic canals (Figure 4-3, C), which are noted as well-defined lucent structures. The canals tend to be wider laterally as the auricular cartilage expands to form the pinna. The average diameter of the proximal end of the annular cartilage is 4.1 ± 0.7 mm in dogs in which the tympanic membrane is visible otoscopically.¹

Figure 4-3 A, Patient positioning to obtain a VD view of the middle ear. The asterisk indicates the entrance point of the primary x-ray beam. Choosing between a VD and a DV view is more a function of hospital protocol or personal preference than clinical need. B, VD radiograph of the middle ear. The most caudal walls of the tympanic bullae (black arrowheads) are noted caudal to the more radiopaque petrosal portions of the temporal bones (asterisks). Rostral to the mastoid process (white arrowheads) lies the external acoustic meatus. The paracondylar process (white arrows) projects caudally. C, DV radiograph of the middle ear. The soft tissue exposure technique allows visualization of the external ear canals (arrows). The walls of the tympanic bullae (arrowheads) surround the petrosal portions of the temporal bones (asterisks).

Open-Mouth View

A commercially available U-shaped acrylic head rack can be used to facilitate positioning (Figure 4-4, A). Without a positioning device, medical-grade adhesive tape can be used to separate the mandible from the maxilla (Figure 4-4, B). With the animal in dorsal recumbency, the head is acutely flexed toward the thoracic inlet. The vertical primary beam is directed rostroventral to caudodorsal and centered immediately ventral to the hard palate. The hard palate and mandible are 30 degrees from the vertical plane. This will highlight both tympanic bullae with minimal superimposition from the surrounding structures. The endotracheal tube should be removed or secured against the mandible. To avoid increased bullae opacity due to superimposition, the tongue should be pulled rostrally and secured to the mandible on the midline. The normal bullae are noted as thin-walled structures with a lucent center ventral to the base of the skull (Figure 4-4, C). Increasing the angle of the hard palate relative to the primary beam can be used as an alternative to the open-mouth projection (Figure 4-4, D). This projection is easier to perform because it is a closed-mouth view that highlights the most caudal surface of the tympanic bullae. Caution should be taken in assessing abnormal findings on this projection because its clinical value has not been studied as extensively as the open-mouth view.² Normal bullae are thin-walled structures with a lucent center ventral to the base of the skull (see Figure 4-4, C). Their walls are of uniform thickness. They are symmetrical in size, shape, and opacity when compared with one another.

Figure 4-4 A, Patient positioning to obtain a rostral 30-degree ventral–caudodorsal open-mouth oblique projection. The arrow indicates the trajectory of the primary beam. The positional device is known as an acrylic head rack, which is radiolucent on radiographs. B, Patient positioning to obtain a rostral 30-degree ventral–caudodorsal open-mouth oblique projection without the aid of a positional device. Medical-grade adhesive tape is used to keep the mouth open. The arrow indicates the trajectory of the primary beam. C, Rostral 30-degree ventral–caudodorsal open-mouth oblique radiograph of a normal dog. The tympanic bullae (arrowheads) are located ventral to the petrosal portions of the temporal bones (asterisks). The external acoustic meatus (arrows) is partially obscured by the overlying coronoid process of the mandible. D, Patient positioning to obtain a rostroventral-caudodorsal closed-mouth oblique radiograph. There is an increased angle of the hard palate in relationship to the primary beam represented by the arrow. This projection is technically easier to perform, but its clinical value has not been studied as extensively as the open-mouth projection. It highlights the caudal aspect of the tympanic bullae. E, Rostral 30-degree ventral–caudodorsal open-mouth oblique radiograph of a normal cat. An osseous septum (white arrowheads) separates the dorsolateral compartment (white asterisk) from the larger ventromedial compartment (black asterisk). The white arrows indicate the walls of the medioventral compartment. The external acoustic meatus (black arrowheads) is visualized overlying the dorsolateral compartment.

In cats an osseous septum divides the bullae into two separate but communicating tympanic cavities—a smaller dorsolateral compartment and a larger ventromedial compartment (Figure 4-4, E). There is less variation in the thickness of the bulla walls between breeds in cats. The external acoustic meatus is sometimes superimposed on the dorsolateral compartment.

Abnormal Radiographic Findings

Otoscopic evaluation is the method of choice to evaluate the external ear canal. However, radiographs can reveal narrowing of its lumen by soft-tissue proliferation from extraluminal masses in cases of neoplasia or by inflammatory tissue, exudates, or debris in cases of otitis externa or trauma (Figure 4-5, A). Dystrophic calcification can be seen associated with chronic otitis externa (Figure 4-5, B).

Figure 4-5 A, VD radiograph of a 4-year-old Doberman diagnosed with chronic bilateral ear infections. Both external acoustic canals (arrowheads) are narrowed and somewhat tortuous. The right canal is smaller and less defined than the left. There is an increased opacity associated with the right tympanic bulla and petrosal portion of the temporal bone (asterisk). Compare the canals with the normal external acoustic canals depicted in Figure 4-3, C. B, VD radiograph of a 2-year-old Bulldog diagnosed with chronic bilateral otitis. Radiographs were taken prior to a total ear canal ablation. There is exuberant bilateral dystrophic calcification of the external acoustic canals (asterisks). The visible walls of the tympanic bullae (arrowheads) appear normal.

Diseases affecting the middle ear, such as otitis media, neoplasia, and craniomandibular osteopathy, as well as polyps can be evaluated with a bullae series. Radiographic findings are nonspecific; therefore the list of differential diagnoses should be generated in light of the clinical history and not radiographic findings alone.

Common findings in otitis media include thickening of the wall of the bullae, increased soft tissue opacity within the bullae, and increased size of the bullae (Figure 4-6). In the large majority of cases it is not possible to differentiate a fluid-filled bulla from one with a thickened wall. If the process is chronic, the increased opacity is likely the result of both thickening and fluid accumulation. Rare mineral concretions within the bullae, also known as middle ear otoliths, have been reported in four dogs.^3,⁴ Middle-ear otolithiasis may be associated with nonactive or active cases of otitis media. If the otitis media is secondary to otitis externa, narrowing and mineralization of the external acoustic canal can also be seen.