Lateral Projection

• The patient is placed in either a right or left lateral recumbent position, and x‐rays traverse the patient lateromedially. This projection shows alignment of the maxilla and mandible, and generally provides an overview of skull morphology (Figure 11.3).

Closed Mouth Ventrodorsal Projection

• The patient is placed in dorsal recumbency and x‐rays transverse the patient from ventral to dorsal. This projection is useful to display left/right alignment of the mandible with the skull and for evaluating congruency of the temporomandibular joints (TMJs) (Figure 11.4). Superimposition of the mandible with the nasal cavity limits the ability to interpret intranasal pathology on this projection.

Open Mouth Ventrodorsal Projection

• This projection is performed with the patient in dorsal recumbency. The mandible is maximally retracted (often using gauze or other material) and the x‐ray beam is directed through the nasal cavity without superimposition of the mandible. This projection is the most useful to diagnose pathology in the nasal passage (Figure 11.4). If the patient is anesthetized, the endotracheal tube should be distracted to the side to prevent superimposition with the nasal cavity.

Frontal Sinus Skyline Projection

• The patient is placed in dorsal recumbency. The neck is flexed, so that the rostral tip of the nose is directed toward the x‐ray tube, with x‐rays directed in a rostral to caudal direction. This projection is useful for showing pathology within the frontal sinus (Figure 11.5).

Left 45° Ventral‐Right Dorsal Oblique and Right 45° Ventral‐Left Dorsal Oblique

• These are often referred to as “45° oblique projections.” With the patient in a laterally recumbent position, the head is rotated in one direction along its axis. The x‐ray beam is then directed at an approximately 45° angle relative to the nasal cavity from left ventral to right dorsal and vice versa (Figure 11.6). The orthogonal (opposite 45°) projection is then acquired by rotation of the head in the other direction along its long axis. These projections are useful for overall survey of the maxilla, but also useful when evaluating the bullae as one will be more ventrally located on each oblique, allowing evaluation without other osseous superimposition.

Left 10° Rostral‐Right Caudal Oblique and Right 10° Rostral‐Left Caudal Oblique [1]

• These are often referred to as the “TMJ oblique projections.” They show the TMJs in a lateral‐type position, without superimposition of the contralateral side. Starting with the patient in a lateral recumbent position, the nose is slightly elevated (approximately 10°) from the radiographic table (Figure 11.7). This places one TMJ rostral to the other. The patient is then placed in the opposite recumbency and the same procedure followed to provide a projection of the contralateral TMJ.

Bulla Projections

• Though the 45° oblique projections and ventrodorsal projections allow evaluation of the bullae, additional projections may be performed when middle or inner ear disease is suspected. In both dogs and cats, the patient is placed in dorsal recumbency. The neck is flexed, and the mouth opened; the x‐ray beam is then directed in a rostral to caudal direction through the mouth, highlighting the bullae. In the cat, an alternative projection is available. Rather than opening the mouth, the neck may be slightly less flexed, resulting in an approximately 10° ventral angulation of the x‐ray beam relative to the nasal planum (Figure 11.8).

As the head is symmetric, images must be labeled with indicators for sidedness. On ventrodorsal and rostrocaudal projections, a single L or R indicator is sufficient to unambiguously label the side of the head. On oblique images, double simultaneous marking with both L and R indicators leaves no question as to which is left and which is right anatomy (Figure 11.6).

Typically, closed mouth ventrodorsal and lateral projections are the minimum requirements for a study of the skull, but additional projections are recommended depending on the clinical question to answer (Figure 11.9). Below are recommended imaging protocols for each area of interest.

Nasal Disease

• Closed mouth ventrodorsal projection, open mouth ventrodorsal projection, lateral projection, orthogonal 45° projections, frontal sinus skyline projection.

Middle Ear Disease

• Closed mouth ventrodorsal projection, lateral projection, left/right 45° orthogonal projections, rostrocaudal bulla projection.

TMJs

• Lateral projection, closed mouth ventrodorsal projection, left/right temporomandibular oblique projections.

Routine Trauma Survey

• Lateral, closed mouth ventrodorsal projection, orthogonal 45° projections.

Rhinitis

There are many causes of nasal inflammation, including infectious rhinitis, immune‐mediated rhinitis, and foreign body rhinitis, though many cases are idiopathic in nature. Rhinitis typically results in bilateral increase in soft tissue within the nasal cavity, without destruction of the turbinates or surrounding bones. Fine turbinate detail is retained but with chronic rhinitis, mild turbinate atrophy may occur. Concurrent sinusitis causes mild fluid accumulation within the sinuses and/or thin soft tissue thickening along the luminal margin of the sinus due to inflamed or hyperplastic sinus mucosa.

The appearance of Aspergillus rhinitis in dogs is unique, and does not conform to the description of other causes of rhinitis described above. While Aspergillus does cause increased opacity in the nasal cavity, it also causes extensive turbinate lysis (Figure 11.15) and tends to be unilateral. Unlike neoplasia (which also causes turbinate lysis), the turbinate loss due to Aspergillus is more multifocal, resulting in larger gas‐filled voids throughout the nasal cavity. A solitary, large mass is not present but instead, the soft tissue thickening is more peripheral among the margins of the nasal cavity and along the residual turbinates. Fungal plaques in the frontal sinuses are common, and appear as heterogeneous mixed gas and fluid (unlike other causes of sinusitis where the thickening is homogeneous). The frontal sinus skyline projection in particular is useful to identify these changes. Other less common fungal agents may also cause rhinitis, and can appear mass‐like or more diffuse [3].

Fungal rhinitis in cats is rare, but Aspergillus and Cryptococcus are most common and appear as a solitary mass or more diffuse disease (Figure 11.16) [4].

Nasal Foreign Bodies

Foreign body rhinitis typically localizes around the foreign body, resulting in a more focal and unilateral abnormality compared to other causes of rhinitis (Figure 11.17). The rostral nasal cavity is typically affected, as the nasal aperture is the point of entry for most foreign bodies (Figure 11.18). The foreign body establishes conditions ideal for secondary Aspergillus infection, which can result in more extensive turbinate lysis in the area of the foreign body. Unless mineral or metallic, the foreign body is not often easily identified, particularly for small foreign bodies such as plant material (grass) or other soft tissue material (such as nasal parasites) (Figure 11.19).

Nasal Neoplasia

Neoplasms arise from the soft tissues or osseous structures of the nasal cavity. There is a wide variety of tumor types reported, with epithelial origin (carcinoma) being the most common tumor type in dogs (up to 75% of cases), but tumors arising from connective tissue (sarcomas) are also common. Lymphoma, osseous tumors, and other uncommon tumor types (such as leiomyoma and transmissible veneral tumor) also occur [5–7].

Most nasal neoplasms result in a solid, solitary mass that causes local turbinate destruction (Figures 11.20 and 11.9). If the mass become very large, it causes lysis of the larger osseous structures surrounding the nasal cavity, such as the nasal or maxillary bones, but this tends to be reserved for only the largest, most advanced masses. Commonly, there will be a large amount of fluid among the remaining turbinates due to secondary hemorrhage or rhinitis. Large masses trap fluid caudally in the nasal cavity or frontal sinus due to obstruction of nasal drainage.

Most nasal tumors (carcinomas, sarcomas) do not have unique features that allow further differentiation of tumor type, though some have unique characteristics that can be identified on CT. For instance, nasal chondrosarcoma typically has patchy mineral regions within it, a feature not expected in carcinomas [8]. Nasal lymphoma may be a solitary mass but tends to be more widespread than other nasal tumors and can cause mild diffuse thickening that mimics rhinitis (Figure 11.21). Tumors (e.g., osteosarcoma) arising from the larger bones surrounding the nasal cavity are centered on the osseous structures rather than within the nasal cavity. Olfactory neuroblastomas are uncommon tumors that arise from the sensory neuroendocrine olfactory cells in the upper part of the nasal cavity. These center on the cribriform plate with invasion into the cranial vault and caudal nasal cavity (Figure 11.22) [9]. Most of these features are not distinguishable on radiography, but readily are identified on CT, allowing more specific prioritization of neoplastic differential diagnoses [10].

CT allows assessment of draining lymph nodes in cases with nasal neoplasia, which can guide the decision on whether to sample them for complete staging purposes. CT lacks specificity to distinguish inflammatory/reactive lymph nodes from neoplastic lymph nodes, so ultimately image‐guided aspiration and cytologic/histopathologic evaluation may be necessary to make the final determination [11].

Though MRI can be used to image nasal neoplastic disease, it is less commonly employed compared to CT, due to the necessity of general anesthesia, greater cost, and longer scan times. MRI provides additional information about structures that are difficult to assess on CT, such as reactive meningeal changes surrounding a tumor or bone marrow involvement of the calvarium [12] (Figure 11.23).

Intranasal Epidermoid Cysts

Intranasal epidermoid cysts are rare causes of mass effects within the nasal cavity of brachycephalic dogs. They have a characteristic appearance on CT and MRI, being entirely fluid attenuating on CT or homogenously fluid hyperintense on MRI (Figure 11.24). They cause resorption of adjacent osseous structures via pressure necrosis [13].