Imaging - The Brain

Diagnostic imaging is important in the characterization and identification of gross structural abnormalities affecting the nervous system. All imaging studies in the neurological patient should be preceded by clinical assessment aiming at ruling out non-neurological causes of signs or systemic disease, determining the lesion localization within the nervous system, and identifying possible concurrent injuries.  As with anatomical imaging elsewhere in the body, functional disorders and diseases that do not result in a gross structural change may not be visible on images. Such imaging is only useful if interpreted along with the patient’s signalment, history and with the information provided by a comprehensive neurological examination.

1. Survey Radiography

Survey radiography of the cranium provides information largely limited to the osseous component of the skull. Nonetheless, plain radiographs are quick to obtain and relatively cheap but in many cases of neurological emergency have a low diagnostic yield. There is often a poor correlation between radiological abnormalities and neurological status, and fractures are often missed. 

Radiography has a very low diagnostic yield for the diagnosis of intracranial pathology and survey radiographs are not usually indicated unless there is external swelling or a known history of severe head trauma. Even in cases of skull fractures, radiography will not provide information on the severity of brain injury and many skull fractures may be missed. Depressed fractures or swellings will only be visible if the X-ray beam is tangential to the lesion. A specific lesion-orientated oblique view may be required. This is obtained by angling the X-ray beam so that it skylines the swelling or depression.

Skull radiographs can be used in the investigation of peripheral vestibular syndromes and facial nerve paresis due to otitis media-interna (albeit with relatively low accuracy) but have limited value in the investigation of most cranial nerve or peripheral nerve lesions. Survey radiography to assess the bullae in cases of peripheral vestibular disease involves a rostrocaudal open mouth oblique or lateral obliques and a dorsoventral (DV) view. The sensitivity of radiography for the diagnosis of otitis media compared to CT was only 0.85 in one study with a specificity of 0.68. Bullae radiographs may be difficult to interpret in large dogs due to large amounts of overlying soft tissue and radiographs provide no information about the intracranial extension (Figure 2) of otitis media. Soft tissue/fluid opacity within the bullae may also be non-significant as primary secretary otitis is a common, apparently incidental finding in brachycephalic dogs.