CT head vs MRI brain: which scan for which problem?

When someone arrives with sudden severe headache, collapse, new weakness, or a head injury, the main concern is bleeding or a large blockage, and CT can answer that within minutes. In these situations speed saves brain, and CT is available around the clock in acute hospitals.

When the issue is subacute or chronic, such as unexplained persistent neurological symptoms, suspected multiple sclerosis, memory change or a pituitary question, the finer contrast of MRI is more likely to find the cause.

CT is excellent at detecting acute blood, skull fractures, large tumours, hydrocephalus (enlarged fluid spaces) and established large strokes. It is less sensitive for early or small ischaemic strokes, for the brainstem and cerebellum (where bone creates artefacts), for small tumours and for inflammatory conditions.

CT is often performed without contrast, and the radiation dose is higher than plain X-ray but is accepted when it is likely to change care.

MRI can show small strokes within hours using diffusion imaging, demyelinating lesions of multiple sclerosis, tumours, infection, pituitary disease, inner ear tumours and structural causes of epilepsy. Different sequences highlight different tissue properties, which is why the scan takes longer.

It requires lying still in a noisy tube, with metal safety checks. Some people with pacemakers, certain implants or severe claustrophobia need alternatives or special arrangements. Sedation or an open scanner may be possible in some units.

NICE head injury guidance (NG232) sets out criteria for when a CT head scan is needed after head injury, such as loss of consciousness with other risk factors, vomiting or signs of skull fracture, because the aim is to find treatable bleeding quickly while avoiding unnecessary radiation. Stroke pathways similarly start with immediate brain imaging to separate bleeding from blockage before treatment is decided.

In non-urgent settings, headache guidelines focus on identifying red flags. Without them, imaging is not advised because it rarely changes management and can lead to incidental findings.

• Sudden severe headache, head injury with warning signs, new weakness or speech problems: call 999 or go to A&E; CT is likely to be the first test.

• Persistent but stable neurological symptoms such as tingling, visual disturbance or balance change: MRI is more likely to find the cause.

• Typical migraine pattern with a normal neurological examination: imaging is usually not needed.

• Suspected pituitary or inner ear problem: MRI, with a specialist protocol.

• Cannot have an MRI: CT, sometimes with contrast, is the practical alternative.

A normal CT does not exclude all brain problems. It excludes bleeding and large abnormalities but can miss small or early lesions. A normal MRI is more reassuring for structural disease, but even it does not explain every symptom, and a headache with a normal scan is still real.

Another is that a private MRI is "more thorough" for headaches. If there are no red flags, a scan is unlikely to help and can produce incidental findings of no significance.