The equine skull has a very complex morphology and its radiographic appearance provides unique diagnostic information that is not readily available on clinical examination. Radiographic changes associated with disease in the skull may be apparent, ranging from fluid levels within the sinuses, to opacification of the nasal septum, distortion or new bone formation in the skull and the presence of circumscribed masses. The ability to interpret these changes can provide a clinical diagnosis and allow the clinician to monitor progress of the patient.
For radiographic evaluation of the cranium, place the animal in dorsal recumbency with the head extended and the external occipital protuberance resting on a sponge placed beneath the table/cassette/detector. This allows the occipital protuberance to be positioned at the level of the film. This position is very important to minimize geometric magnification and allow clear definition of the skull bones, temporomandibular joint (TMJ) and the mandibles.
A normal lateral radiograph of the skull shows the bony structures of the skull, including the frontal and parietal bones, the sphenoid bone, the palatine bone and the vomer bone (Fig. 1). The clivus of the skull is also well defined. The cerebellar tonsils are also visible.
When a thoracic mass is present it is usually obvious on a lateral projection. If the shape of the lesion is irregular or nonuniform, a radiograph of a 30° dorsal-ventral oblique is needed to evaluate volumetric characteristics. When a solitary spherical mass is encountered, it may be termed a coin lesion although orthogonal projections are required to confirm the shape and size of the lesion.
The thoracic spine of the horse is different from that of small animals. The spine has two scapular processes and is more robust than that of small animals, reflecting its function as a support structure for the neck. It also has large transverse processes that form a articulation with the caudal aspect of C1.