Neuroradiology
Ordinary X-rays show only the bony structures, giving no direct information about the nervous system itself for which X-ray computerized tomography (CT) or magnetic resonance imaging (MRI) are required. MRI is the best technique for imaging most central nervous system (CNS) pathology of both the brain and spinal cord.
Contrast media may be used to enhance diagnosis of CT and MRI scans, particularly highlighting defects in the blood–brain barrier (BBB). While different agents are used in each (iodine-based preparation for CT, gadolinium for MRI), the principle is the same. They are injected into a vein, usually in the arm, and become concentrated in vascular structures or where the BBB is disrupted, making these areas appear brighter. For example, a tumour is vascular with an impaired BBB and will therefore enhance, while the centre of the tumour is necrotic with no circulation and will not enhance, giving a characteristic pattern of ‘ring enhancement’ with contrast on the scan (Fig. 1).
Computerized axial tomography
CT scan is the most commonly available form of neuroimaging in the UK (Fig. 2a). The patient lies with the head in a ring which contains both X-ray emission and detection apparatus. Images are formed in slices, as the head is moved through the ring. The dose of X-rays is relatively large and CT is contraindicated in pregnancy except in emergencies. CT scan remains the method of choice, however, for the demonstration of acute intracranial haemorrhage and intracranial calcification. CT is relatively insensitive at detecting pathology of the spinal cord but can detect most herniated lumbar intervertebral discs and is useful in delineating bony abnormalities. Modern CT can be used to examine arterial and venous structures and provides a non-invasive high-quality alternative to conventional angiography.
