🔬 CT Head

A Computed Tomography (CT) scan of the head uses a series of X-rays to create detailed cross-sectional images of the brain, skull, and associated structures. It is the primary imaging modality for acute neurological emergencies due to its speed and high sensitivity for detecting acute intracranial haemorrhage and skull fractures. It is essential in the immediate triage of stroke and head trauma.

Acute neurological deficit or suspected stroke. Severe, sudden-onset 'thunderclap' headache (suspected SAH). Head injury meeting NICE criteria (e.g., GCS <13, focal deficit, vomiting, or anticoagulation). Unexplained persistent altered consciousness or GCS drop. Suspected intracranial mass or abscess. Pre-lumbar puncture screening for raised intracranial pressure. Post-operative monitoring of neurosurgical patients. Chronic or progressive headache with 'red flag' features.

The patient lies on their back on a motorized table that moves through a circular scanner (the gantry). Multiple X-ray beams rotate around the head, and detectors measure the attenuation. A computer processes this data to create cross-sectional 'slices.' The test is very fast, usually taking less than a minute for a plain scan. For certain indications (like suspected tumour or vascular malformation), an iodine-based contrast agent may be injected intravenously to highlight blood vessels and areas of blood-brain barrier breakdown.

Symmetrical brain parenchyma with no areas of abnormal density (haemorrhage or infarct). Normal grey-white matter differentiation. No midline shift or mass effect. Ventricles and sulci are of appropriate size for the patient's age. No intracranial air or fluid collections. All basal cisterns are open and symmetrical. The skull vault and base are intact with no fractures. No signs of pathological calcification.

A systematic approach using the 'Blood Can Be Very Bad' mnemonic: Blood (look for white areas/haemorrhage); Cisterns (are they open?); Brain (symmetry, sulci, grey-white matter differentiation); Ventricles (size, symmetry, shift); Bone (fractures or erosions). Compare sides for symmetry. Evaluate the density of tissues—acute blood is bright, while old infarcts or cerebrospinal fluid are dark. Check for 'midline shift' which suggests significant herniation risk.

Acute haemorrhage appears as high-density (white) areas; subarachnoid haemorrhage (SAH) shows blood in the cisterns/sulci, while intraparenchymal haemorrhage is a discrete mass. Ischaemic stroke appears as low-density (dark) areas, though it may be normal in the first few hours. Midline shift and effacement of ventricles indicate mass effect (oedema, tumour, or bleed). Fractures of the skull vault or base may be seen. Subdural haematomas are crescent-shaped, while extradural haematomas are biconvex/lens-shaped and usually associated with skull fractures.

The CT head is the definitive tool for differentiating between ischaemic and haemorrhagic stroke, which is essential before administering thrombolysis. It is the primary investigation for acute head trauma to identify intracranial bleeds or fractures requiring neurosurgical intervention. It also screens for raised intracranial pressure before performing a lumbar puncture. its speed and availability make it the 'workhorse' of emergency neurological imaging.

Misinterpreting physiological calcification (e.g., pineal gland or choroid plexus) as acute haemorrhage. Missing a subtle subarachnoid haemorrhage (CT is ~95% sensitive in the first 6 hours, dropping thereafter). Overlooking fractures by not viewing the scans in 'bone windows.' Failing to appreciate that a normal CT does not exclude an ischaemic stroke in the hyperacute period. Assuming a normal CT rules out meningitis (clinical/CSF diagnosis).

CT is relatively poor at visualising the posterior fossa (cerebellum and brainstem) due to bone artifact. It has low sensitivity for hyperacute ischaemic stroke (within 3-6 hours). It involves significant ionising radiation to the head and lenses of the eyes. Small lesions or subtle neuroinflammatory conditions (like MS) are better seen on MRI. The use of IV contrast (if required) carries risks of nephrotoxicity and anaphylaxis, though most head CTs are 'plain' (non-contrast).

Students must know the NICE Head Injury criteria for when a CT is indicated within 1 hour versus 8 hours. Understand the 'time is brain' principle in stroke—CT Head is used primarily to 'rule out' haemorrhage rather than 'rule in' ischaemia in the hyperacute phase. Be able to differentiate between subdural (crescent/crosses sutures) and extradural (lentiform/limited by sutures) haematomas. Always consider the patient's renal function and allergy history if contrast is needed.