Basic imaging - CT of the head

Role

CT of the head is the first-line investigation for acute neurological presentations (suspected stroke, head trauma, reduced consciousness, suspected raised intracranial pressure before lumbar puncture) given its speed and wide availability, particularly for excluding haemorrhage and mass effect
Extremely widely used in emergency and inpatient settings given the frequency of acute neurological presentations requiring rapid exclusion of a surgical/time-critical diagnosis

Non-contrast CT is highly sensitive for acute haemorrhage (hyperdense on CT) and adequate for detecting significant mass effect/midline shift, but has limited sensitivity for early ischaemic stroke (often normal in the first few hours) and for posterior fossa/small lesions where MRI is superior
Contrast-enhanced CT improves detection of infection (abscess, empyema) and some tumours through enhancement patterns

Essential before lumbar puncture when there are risk factors for raised intracranial pressure (focal neurological signs, reduced consciousness, papilloedema, new seizure) - to exclude a mass lesion/shift that would make LP unsafe
A normal CT does not exclude meningitis (which is a clinical/CSF diagnosis) or early ischaemic stroke - clinical correlation and, where relevant, MRI remain necessary

Suspected haemorrhage: proceed to urgent neurosurgical/stroke team involvement based on findings
Suspected abscess/empyema: contrast-enhanced imaging and urgent neurosurgical/infectious diseases input
Normal scan with high clinical suspicion of an alternative diagnosis (early ischaemic stroke, meningitis, encephalitis): proceed with the clinically indicated pathway (thrombolysis assessment, lumbar puncture) rather than being falsely reassured

Intracranial haemorrhage, cerebral abscess, raised intracranial pressure contraindicating lumbar puncture, early ischaemic stroke (a key CT limitation)

A normal non-contrast CT head is a common source of false reassurance in early ischaemic stroke and in meningitis/encephalitis, both of which are frequently normal or subtle on CT - the scan's primary job in the acute setting is excluding haemorrhage and mass effect, not "clearing" the brain of all pathology

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