Radiology of the Skull
Ruth Dennis, MA, VetMB, DVR, DipECVDI, MRCVS
Animal Health Trust
Newmarket, Suffolk UK
INDICATIONS FOR SKULL RADIOGRAPHY
Common and important indications for radiography of the skull include nasal disease, dental disease, ear problems, cranial, facial or mandibular swelling, oral masses, exophthalmos and trauma. Animals with neurological signs rarely show radiographic changes, exceptions being seizures or central blindness due to intracranial extension of a nasal tumour, and feline meningiomas that may cause cranial hyperostosis. Magnetic resonance imaging (MRI) or computed tomography (CT) is of course required for investigation of most neurological cases.
Positioning for all views requires chemical restraint; oblique laterals and the dorsoventral view (DV) may be performed under sedation but other views need general anaesthesia.
1. Lateral view: requires padding beneath nose and mandible to bring skull into true lateral position.
2. Dorsoventral/ventrodorsal: must be symmetrical; endotracheal tube removed for assessment of midline structures.
3. Lateral oblique: side of interest closest to table and skull rotated about its long axis in the appropriate direction, to remove superimposition of other structures. Used particularly for tympanic bullae, frontal sinuses, teeth and mandibles.
4. Intra-oral or occlusal (usually using flexible cassette or non-screen film): dorsoventral view for upper jaw and nasal chambers, and ventrodorsal view for lower jaw. N.B. must increase the mAs considerably when using non-screen film.
5. Open mouth ventrodorsal: an alternative to the intra-oral view of the nasal chambers and has the advantage of imaging the nasal area further caudally. Patient in dorsal recumbency with upper jaw horizontal and mouth open; beam angled in a caudodorsal direction to miss the lower jaw.
6. Sagittal oblique: special view for temporomandibular joints (TMJs). Position for lateral view then tip nose up 10-20 degrees to bring lower TMJ space into a vertical position. Useful to perform both open and closed mouth views.
7. Rostrocaudal: animal in dorsal recumbency with head flexed on neck to varying degrees. Used for frontal sinuses, cranium and foramen magnum.
8. Rostrocaudal open mouth: as above but mouth open (exact orientation of hard palate depends on shape of skull). Used for tympanic bullae and odontoid peg. The endotracheal tube must be removed.
9. Rostro-10o-ventral-dorsocaudal oblique: a special view for the tympanic bullae in cats, used as an alternative to the open mouth RCd as it is easier to obtain. The anaesthetised cat lies on its back with the mandible 10o from the vertical in a less flexed direction. The large bullae of the cat project sufficiently far below the cranium to be visible.
10. Lesion-oriented oblique (LOO): oblique view as required to skyline areas of swelling, depression or other deformity.
11. Horizontal beam frontal sinus view: a useful technique for detecting frontal sinus fluid and an easier projection to obtain than the rostrocaudal view. The patient lies in sternal recumbency with the nose elevated slightly; a horizontal beam is directed from caudal to rostral such that the sinus outline is projected onto the cassette in the light from the LBD. Take care with the use of the horizontal X-ray beam.
The small animal skull is an extremely complicated anatomical structure consisting of about 50 bones, numerous teeth and soft tissue and cartilaginous structures. There is very great variation between species (dog and cat) and between breeds, and since the skull is roughly spherical or cylindrical there is much superimposition of different areas. A large number of standard radiographic views are employed for imaging different areas of the skull and any deviation from the standard positioning (such as slight obliquity) poses extra problems in interpretation.
Doliocephalic (dolichocephalic)-nasal area longer than cranium; e.g., greyhound, rough collie, borzoi;
Mesaticephalic-nasal area and cranium equal in length; e.g., Labrador, spaniel, most crossbreeds;
Brachycephalic-nasal area shorter than cranium; frontal sinuses small or absent; e.g., boxer, bulldog, Pekinese. Many other abnormalities are present in these deformed animals, including inferior prognathus, dental displacement, soft palate hyperplasia, tracheal hypoplasia, tracheal collapse, etc.
Domed frontal and nasal bones producing convex shape on the lateral view
Laterally-protruding zygomatic arches
Large post-orbital processes
Reduced nasal chambers and prominent ethmoturbinates
Large, "double-walled" tympanic bullae
Prominent tentorium cerebelli osseum.
INTERPRETATION OF SKULL RADIOGRAPHS
Although the skull is very complex, most lesions are unilateral or asymmetrical and so comparison of the right and left sides on DV/VD, RCd and lateral oblique views is very helpful. Reference to a bone specimen is also very helpful.
During the course of the lecture examples of important radiological diagnoses will be shown.
Nasal disease in dogs
Chronic hyperplastic rhinitis is usually bilateral and results in increased opacity in the nasal cavity with blurring of the turbinate pattern. There is no turbinate lysis, although the turbinate pattern may be obscured due to surrounding fluid. Nasal foreign bodies produce a subtle, localised rhinitis.
Destructive rhinitis (fungal disease, e.g., Aspergillosis) usually affects young adult dogs of doliocephalic breeds and may be unilateral or bilateral. There are ill-defined areas of increased opacity due to nasal discharge and sometimes fungal granulomas, but the main pattern is of turbinate destruction. In severe cases there is also involvement of the surrounding bones (hard palate, nasal and maxillary bones) or a frontal sinusitis. If bilateral, the midline shadow of the vomer bone and nasal septum may be interrupted.
Nasal neoplasia usually arises in older dogs and is initially unilateral although in later cases both nasal cavities may be affected. It usually arises in the mid third of the nasal cavity near the 4th premolar tooth. Radiographically, there is a soft tissue opacity in the affected area with loss of turbinate pattern and nasal septum/vomer shadow. Commonly, the ipsilateral frontal sinus is opacified due to trapped fluid. Loss of the outline of the cribriform plate suggests intracranial extension. Rarely, benign mass lesions may occur but these cannot be differentiated from malignant tumours radiographically.
Nasal disease in cats is much harder to diagnose. Chronic bacterial rhinitis appears similar to fungal rhinitis in dogs. Nasal lymphoma may occur. Generally, bilateral disease is more likely to be inflammatory and unilateral disease neoplastic.
Tumours may also affect the cranium, facial bones, tympanic bullae and jaws. General principles of interpretation apply. Most are malignant, as benign skull tumours are rare in small animals. Most lesions show a combination of both osteolysis and new bone proliferation with overlying soft tissue swelling and differentiation of histological types cannot be made radiographically. Many cranial tumours extend inwards but this cannot be seen radiographically and requires MRI or CT. Chest radiographs should always be performed in animals with tumours, although skull neoplasms metastasise less commonly than those of the appendicular skeleton.
Craniomandibular osteopathy causes bony proliferation on the mandibles, tympanic bullae and occasionally frontal and cranial bones. It is seen in young dogs, especially the West Highland white terrier, and should not be mistaken for neoplasia because of its typical appearance.
Middle ear disease (otitis media)
MED is often present without radiographic changes, but in chronic and severe disease, opacification of the affected tympanic bulla and thickening of the bony wall may be evident.
Periodontal disease usually results in osteolysis around the affected tooth root, and surrounding sclerosis may also be seen. Radiography is especially helpful to assess the teeth in animals with facial swellings. Other signs of dental disease include widening of the tooth's pulp cavity and irregularity of the outline of the tooth root. In the mandible, pathological fracture of the bone is possible in severe cases. In animals with generalised rarefaction of bone around the teeth, renal secondary hyperparathyroidism due to kidney failure should be considered.
Dental eruption cysts and tumours of dental tissue also occur.
Skull fractures may be visible radiographically, especially if displacement has occurred. Oblique views are required, skylining the area of deformity. Open cranial suture lines in young animals or in dogs with domed head conformation such as Chihuahuas should not be mistaken for fractures, as these will be bilaterally symmetrical. If there is mandibular deviation or difficulty opening or closing the mouth, special radiographs for the temporomandibular joints should be obtained. MRI and CT are required for assessment of intracranial trauma such as subdural haemorrhage or brain contusion.