Computed tomography detects differences in X-ray density between bone, brain, blood and CSF. There is a difference in healthy tissue and infarcted, infected or oedematous tissue. It may be possible to differentiate between old lesions and acute lesions – including infarcts and bleeds.
· Spaceoccupying lesions: cerebral tumours (may miss low-grade astrocytomas), abscesses.
· Cerebral infarction.
· Extracerebral or intracerebral haematomas.
· Subarachnoid haemorrhage (may miss up to 15%).
· Pituitary for tumours, pituitary infarction (although
MRI is preferred if available).
Intravenous contrast injections show areas of increased blood supply, breakdown of the blood-brain barrier and oedema.
Lesions under 1 cm in diameter may be missed, although improvements in technology are increasing the sensitivity of scans. Small lesions close to the skull may be missed.
CT of the skull is also useful for detecting minor fractures particularly basal skull fractures, which may be difficult to see on skull X-ray.
Newer CT scanners are able to diagnose cerebellar lesions more reliably than in the past, but MRI is still preferred if brainstem or cerebellar lesions are suspected.
Faster scans are now possible – particularly helpful for patients unwilling or unable to lie flat for long, although in some cases general anaesthetic may be necessary for uncooperative patients.