Indications for imaging the nasal chambers and paranasal sinuses include investigation of:

 Nasal discharge

 Epistaxis

 Facial deformity

 Stertor

 Sneezing

 Exophthalmos

 Dyspnoea of upper airway origin

 Ocular discharge

The primary aims of imaging sinonasal disease are to:

 Determine the extent of any pathology

 Create a ranked differential diagnosis list based on likely aetiology

 Aid planning for further investigation or therapy (e.g., radiotherapy)

Radiography

Radiography is the most commonly used modality and is relatively sensitive and specific for the diagnoses of neoplasia and rhinitis. Radiography of the nasal chambers requires high-quality radiographs taken under general anaesthesia and should be performed prior to rhinoscopy or flushing.

At least four projections are required to fully evaluate the nasal chambers and sinuses. The most informative projection is the dorsoventral intra-oral which allows assessment of the nasal conchae without superimposition of the mandibles. This projection should be taken using dental film or detail film-screen combination in a flexible cassette. The natural symmetry of the nasal chambers is used to detect lesions and accurate positioning is vital as rotation of the image makes evaluation difficult. The laterolateral projection allows assessment of the nasopharynx and dorsal parts of the nasal and frontal bones. Assessment of the frontal sinuses often requires oblique projections (rostrocaudal or lateral obliques).

The radiographs should be evaluated for (Figure 1):

 Integrity of the turbinates

 Location of any increased soft tissue opacity or reduced opacity

 Destruction/deviation of the vomer/septum

 Involvement of the bony case

 Distortion/changes of the soft tissues and nares

 Sinus opacification

 Patency of the nasopharynx

 Dental changes

Figure 1. Summary of radiographic changes seen with chronic nasal disease in dogs.

In cases with acute onset of nasal signs, imaging is often unrewarding unless the signs are secondary to trauma in which case fractures may be seen. Radiolucent foreign bodies (grass awns) are rarely visible radiographically and are often not seen on magnetic resonance imaging (MRI).

With chronic disease in dogs, radiography provides a high diagnostic yield. It is usually possible to categorise the radiographic changes into one of three broad categories:

 Changes suggestive of neoplasia

 Changes suggestive of non-destructive non-neoplastic disease

 Changes suggestive of destructive non-neoplastic disease

In referral practice nasal tumours are the most common cause of chronic nasal discharge in dogs. The features most specific for neoplasia are:

 Invasion of adjacent bony structures

 Ipsilateral turbinate destruction with increased opacity

 Ipsilateral frontal sinus opacification

 Generalised uni- or bilateral soft tissue opacity

 Destruction/deviation of the vomer/septum

Whilst radiography is relatively accurate (>80% for the diagnosis of nasal tumours) it is not possible to differentiate between tumour types based on imaging features.

Invasion of the brain cannot be identified radiographically unless there is marked destruction of the cribriform plate or inner table of the frontal bone. Limitations of radiography are that identification of neoplasia may be difficult early in the disease process as small masses may not be visible and it may not be possible to differentiate polyps from malignant masses.

Destructive rhinitis occurs most commonly secondary to fungal infection. The radiographic changes may be bilateral or unilateral. In contrast to neoplasia, deviation of the vomer/septum and gross destruction of the bony case are rare, although small punctate lysis is often present with aspergillosis. The key feature of destructive rhinitis is loss of turbinates with reduced opacity but there is often a mixed pattern of ill defined increased and decreased opacity due to turbinate destruction and accumulation of discharge and fungal granuloma. In early cases the changes may be subtle and computed tomography (CT) or MRI is more sensitive for showing areas of turbinate loss. Where infusion techniques are used for therapy advanced imaging allows assessment of integrity of the cribriform plate and is helpful to rule out intra cranial extension.

Non-destructive non-neoplastic changes can occur with many diseases and are non-specific. The severity of changes generally reflects the severity and chronicity of the signs. It is important to recognise that normal nasal radiographs do not rule out nasal disease. With chronic foreign bodies localised changes due to inflammation may be seen (localised increased opacity); however, radiographs are often normal.

The interpretation of feline nasal radiographs is less straightforward. There is greater overlap in the radiographic features of neoplasia and inflammatory disease than in dogs. The principles of interpretation are the same but nasal radiography is less accurate in cats compared to dogs. As with dogs the features suggestive of neoplasia are displacement of midline structures, unilateral generalised soft tissue opacity with loss of turbinates and bone invasion. There is, however, marked variation in appearance of rhinitis which can range from normal to resembling neoplasia.

Computed Tomography

In many studies CT has been shown to be more accurate and sensitive than radiography for the investigation of nasal diseases (accuracy >90%) and in some cases may be more accurate than biopsy or rhinoscopy. In cases where the detection of intracranial involvement or extension outside the nasal chambers is important or suspected (e.g., presence of neurological signs, exophthalmos, nasopharyngeal disease) then either MRI or CT are indicated.

Magnetic Resonance Imaging

MRI gives the most information about nasal disease due to the inherently good soft tissue contrast. MRI allows clear differentiation of fluid from soft tissues and comparison of radiography and MRI shows that radiography tends to overestimate tumour size due to the inability of radiography to differentiate between fluid and soft tissues. Interpretation of CT and MRI is relatively straightforward with the imaging features similar to those seen radiographically. MRI images should be obtained in all three planes and post-contrast T1-weighted images should be obtained in the dorsal plane to allow evaluation of intracranial spread of disease.

Sinus Disease

Isolated disease of the sinuses is rare. Sinus involvement is usually secondary to nasal disease and it is common with nasal disease to see opacification of the frontal sinuses radiographically. Unless there is involvement of the bones it is not possible to differentiate between trapped fluid and extension of masses. MRI allows differentiation between solid tissue and fluid. Ultrasonography of the frontal sinuses is possible using a direct approach through the frontal bones. In normal animals air within the sinus prevents assessment of the sinus contents. If there is soft tissue or fluid present within the sinuses then this may be visible ultrasonographically.

There may be fluid present within the tympanic bullae associated with sinonasal disease. This is not usually visible radiographically but may be seen with MRI or CT. The significance of this is uncertain but is probably secondary to auditory tube dysfunction.

References

1.  Lamb CR. Skull--nasal chambers and frontal sinuses. In: Barr, FJ; Kirberger, RM. eds. BSAVA Manual of canine and feline musculoskeletal imaging. Quedgeley: BSAVA, 2006; 192-205.

2.  Lamb CR, Richbell S, Mantis P. Radiographic signs in cats with nasal disease. Journal of Feline Medicine and Surgery 2003; 5: 227-235.

3.  Saunders JH, Clercx C, et al. Diagnostic value of computed tomography in dogs with chronic nasal disease. Veterinary Radiology and Ultrasound 2003; 44: 409-413.