Introduction

Historically, the skull has been a challenging region to radiograph and interpret, largely due to the fact that there is marked superimposition of about 50 bones, teeth, cartilage and soft tissue structures.¹ The inherent benefit of imaging the skull, however, is that the left and right sides allow for symmetry and thus comparison of the two sides aids in identifying abnormalities.¹

Since the accessibility of computed tomography has increased, this modality has become superior to radiography, as the tomographic nature of the image eliminates the superimposition that inhibits radiographic interpretation. Thorough knowledge of the anatomy, in particular the osseous anatomy, is vital when viewing CT or radiographic images of the skull.

Both modalities, however, employ the use of x-rays to generate the image and thus stringent radiation safety principles should be enforced. With radiography, in particular, it is necessary to have knowledge of the most applicable views required to best highlight specific conditions.

The main indications for radiographic and CT studies of the skull include masses/swellings/deformity of the skull, trauma to the head, ear conditions, ocular abnormalities affecting the symmetry of the skull, nasal disease, dental disease, problems involving the jaw (problems with apprehension, mastication, swallowing, etc.) and any pain of the skull region.¹

The basic standard views of the skull are the laterolateral and the dorsoventral (ventrodorsal) views.¹ For the laterolateral view, it is essential that the head is in a true lateral position, achieved by utilising positional aids (foam wedges, etc.).¹ Similarly for the DV/VD view, the palate should be parallel to the x-ray table/cassette.¹ Masking tape/Elastoplast®-type material can aid in taping the muzzle into place. Collimation of both views will depend on the area of interest. If pathology is suspected within midline structures, it is vital to remove the endotracheal tube prior to imaging.¹

Contrast studies are not routinely employed; however, sialography and fistulography can be considered if the need arises.¹ Sialography can, however, be technically challenging and it is beneficial to practice cannulising the salivary ducts on cadavers prior to performing this procedure.

This paper will approach each structure/region of interest individually, discussing best radiographic techniques and CT findings unique or specific to the region.

Nasal Studies

Radiographic Studies

Special views that are required to fully evaluate the nasal passages radiographically include the dorsoventral intraoral view or alternatively the ventral-20°-rostro-dorsocaudal view.² The latter view allows improved imaging of the cribriform plate and is easier to perform, especially in smaller dogs and cats, as the cassette is not placed intraorally. The rostro-caudal skyline view offers a skyline view of the nasal passages and frontal sinuses (depending on where the view is centered).² This view is technically challenging in brachycephalic and patients with dome-shaped heads. Finally, as the maxillary arcade can be a site of primary or secondary disease when presented with a patient with a complaint of nasal disease, the left/right 45° ventral right/left dorsal oblique views allow for visualisation of the left and right maxillary arcades without the same degree of superimposition as is noted on the lateral and DV view.²

When assessing radiographs of the nasal passages, one must look carefully for any loss of turbinate structures, increased or decreased opacity in a focal or diffuse zone, periodontal disease, midline nasal septum shift or lysis, cribriform and maxillary and nasal bone lysis.²

In dogs, sinonasal neoplasms and fungal rhinitis represents 75% of chronic nasal disorders whereas nonspecific chronic and foreign body rhinitis account for approximately 20% of disorders. In cats, 75% of nasal disorders is due to neoplasia and chronic rhinosinusitis. One of the biggest challenges when interpreting nasal radiographs is distinguishing rhinitis from neoplasia. In a study assessing 42 dogs with rhinitis and 30 with neoplasia, the authors found that the signs which predicted neoplasia included lysis of bone, entire ipsilateral nasal passage involved, generalised unilateral or bilateral soft tissue opacity, opacification of the frontal sinuses and generalised loss of turbinate detail. In cats, if there was displacement of midline structures, this too was a predictor of neoplasia.³

Computed Tomography

Similar interpretative principles apply when evaluating CT images of the nasal passages; however, it is easier to appreciate distribution, areas of lysis and involvement of the surrounding structures on CT studies.⁴ In our institution (Onderstepoort Veterinary Academic Hospital) as both skull radiographs and CT require general anaesthesia and several radiographic views are required when investigating nasal disease, CT has replaced radiography for investigating nasal disease.

Ultimately, however, often it is not possible on radiography or CT studies to differentiate with 100% certainty between neoplastic and chronic rhinitis conditions without biopsy. A benefit of CT over radiography is the ability to closely investigate regional lymph nodes for signs of metastasis when neoplastic disease is suspected.

Temporomandibular Joint and Masticatory Apparatus

Radiographic Studies

Although the TMJs can be visualised on the DV and VD views, the joint space and retroarticular processes are best visualised on oblique studies.^1,5

Several oblique projections have been suggested to image the left and right TMJ whilst attaining minimal superimposition of overlying structures. These include comparing the latero 20° rostral-laterocaudal oblique and latero 10° rostral-laterocaudal oblique view (achieved by tilting the nose up by 10° and 20°, respectively) and latero 20° rostral-laterocaudal oblique and latero 10° rostral-laterocaudal oblique view achieved by angling the x-ray beam 10° and 20°, respectively, and finally a parallax view with the beam centered over C2 and collimated to include the TMJ region.⁶ The last view uses divergence of the x-ray beam to project the TMJs separately. The view that was ascertained to be the best in this study was the nose-up latero 20° rostral-laterocaudal oblique view. However, this view was also deemed to be the most technically challenging to achieve.⁵

Computed Tomography

For reasons not dissimilar to nasal studies, CT is fast becoming the modality of choice when investigating conditions which may be attributable to TMJ disorders. CT allows excellent visualisation of the anatomical structures comprising this joint, supreme assessment of the joint space and articular surfaces (although arthroscopy is likely still the gold standard for this purpose) and evaluation of the subchondral bone plate - a region which cannot be adequately assessed radiographically. CT is superior to radiography to detect small fissure fractures or assess the involvement of the joint surface when fractures are present, which may be missed radiographically.⁶ More specifics of the CT features of conditions of the TMJ will be discussed in more detail during the oral presentation.

External, Middle and Inner Ear

The external ear canals can be easily assessed on DV/VD radiographs and extensive, unilateral or asymmetrical mineralisation of the ear cartilages are often incidental findings in older dogs.¹ Special views to highlight the bullae include the rostrocaudal open-mouth views with the hard palate positioned vertical to the cassette. For feline patients, with their differing anatomy, the rostral-10° ventral-caudodorsal oblique view is required to image the tympanic bullae. Left or right 30° dorsal-right or left ventral oblique views can also be used to image the tympanic bullae.¹

Radiography is insensitive for middle ear disease, and increased opacification of the bullae and irregular thickening of the bullae walls are indicative of chronic otitis media.¹

CT allows differentiation of the contents of the bullae (fluid versus soft tissue) by assessing the Hounsfield unit of the material as well as by assessing any post contrast enhancement that may occur.⁷ This modality also allows superior assessment of the bony margins of the middle ear and the fine structures of the inner ear. The surrounding soft tissue structures and regional lymph nodes can also be assessed during CT studies. For similar reasons as with nasal imaging, CT is the imaging modality of choice in the OVAH for middle and inner ear disease. Unfortunately, CT does not have the superior sensitivity of MRI for detecting intracranial extension of middle and inner ear disease and if this is suspected, MRI should be performed.¹

References

1.  Dennis R. Skull - general. In: Barr FJ, Kirberger RM, eds. BSAVA Manual of Canine and Feline Musculoskeletal Imaging. 1st ed. Gloucester: British Small Animal Veterinary Association; 2006.

2.  Lamb CR. Skull - nasal chambers and frontal sinuses. In: Barr FJ, Kirberger RM, eds. BSAVA Manual of Canine and Feline Musculoskeletal Imaging. 1st ed. Gloucester: British Small Animal Veterinary Association; 2006.

3.  Russo M, Lamb CR, Jakovljevic S. Distinguishing canine nasal neoplasia and rhinitis by radiography. Vet Radiol Ultrasound. 2000;41(2):118–124.

4.  Saunders J, Schwarz T. Nasal cavities and frontal sinuses. In: Schwarz T, Saunders J, eds. Veterinary Computed Tomography. 1st ed. Iowa: Wiley-Blackwell.

5.  Hammond G, King A, LaPaglia J. Assessment of five oblique radiographic projections of the canine temporomandibular joint. Vet Radiol Ultrasound. 2012;53(5):501–506.

6.  Schwarz T. Temporomandibular joint and masticatory apparatus. In: Schwarz T, Saunders J, eds. Veterinary Computed Tomography. 1st ed. Iowa: Wiley-Blackwell.

7.  Travetti O, Giudice C, Greci V, Lombardo R, Mortellaro M, Di Giancamillo M. Computed tomography features of middle ear cholesteatomy in dogs. Vet Radiol Ultrasound. 2010;51(4):374–379.