CT Head Scan Explained: When It Is Needed for Stroke, Trauma and Headache
A non-contrast CT head is the first imaging test in stroke and head trauma. Learn NICE criteria, radiation dose, and what CT misses.
Non-contrast CT head is fast, widely available, and excellent for acute haemorrhage, mass effect, and skull fracture. It is the first imaging in stroke (to exclude bleed before thrombolysis) and head trauma. Radiation dose is ~2 mSv (roughly 8 months background). It is INSENSITIVE for early ischaemic stroke (first 6 h), posterior fossa lesions, and small subarachnoid haemorrhage after 6–12 hours.
CT head is the workhorse of acute neuroimaging. In suspected stroke, non-contrast CT is performed within 20 minutes of arrival (national target for thrombolysis pathways) primarily to exclude haemorrhage; ischaemic changes are often subtle in the first hours (loss of grey–white differentiation, hyperdense MCA sign). NICE CG176 defines urgent (within 1 hour) CT criteria in head injury: GCS < 13 on arrival, GCS < 15 at 2 hours, focal neurology, seizure, suspected open/depressed skull fracture, signs of basal skull fracture, vomiting more than once (adults), or age > 65 + amnesia > 30 minutes. For thunderclap headache, CT within 6 hours has near-100% sensitivity for aneurysmal subarachnoid haemorrhage; beyond 6 hours a lumbar puncture (looking for xanthochromia) is required if CT negative. Radiation dose is not trivial: apply justification and optimisation, particularly in children and pregnancy. MRI is superior for posterior fossa, small ischaemic strokes, cavernomas, and demyelination but slower and not always available.
- Radiation dose
- ~2 mSv (~8 months background)
- Excellent for
- Acute haemorrhage, mass effect, skull fracture
- Misses
- Early infarct, posterior fossa, late SAH
- Stroke pathway
- CT within 20 min of arrival
- Head injury guide
- NICE CG176
CT vs MRI head — when to choose
| Question | CT | MRI |
|---|---|---|
| Acute haemorrhage | First-line | Susceptibility-weighted MRI equivalent |
| Acute ischaemic stroke | First (to rule out bleed) | MRI-DWI more sensitive at 24h |
| Posterior fossa | Limited (bone artefact) | Superior |
| Skull fracture | Superior | Poor for bone |
| Multiple sclerosis | Insensitive | First-line |
NICE head-injury CT criteria (within 1 hour)
GCS < 13 on initial assessment, or GCS < 15 at 2 hours post-injury.
Suspected open or depressed skull fracture, signs of basal fracture (haemotympanum, panda eyes, CSF rhinorrhoea/otorrhoea, Battle sign).
Post-traumatic seizure.
Focal neurological deficit.
More than one episode of vomiting.
CT for thunderclap headache
Non-contrast CT within 6 hours of onset has sensitivity approaching 100% for aneurysmal subarachnoid haemorrhage on modern scanners.
Beyond 6 hours, sensitivity falls to 85–90%; combine with lumbar puncture (xanthochromia after 12 h).
CT angiography follows a positive CT to define aneurysm anatomy for coiling or clipping.
- Sudden severe "worst-ever" headache — thunderclap SAH pathway, urgent CT.
- Head injury + any NICE red flag — CT within 1 hour.
- Stroke symptoms (FAST) — thrombolysis pathway, target CT within 20 min.
Which head scan?
- 1Acute trauma?Non-contrast CT.
- 2Suspected acute stroke?Non-contrast CT + CT angiogram (if thrombectomy possible).
- 3Thunderclap headache?CT within 6 hours; LP if > 6 hours + CT negative.
- 4Progressive neurological symptoms, MS or tumour suspected?MRI with contrast.
Related questions people ask
Frequently asked questions
- Non-contrast CT first for acute head presentations.
- NICE CG176 governs head-injury CT criteria.
- CT within 6 h has near-100% sensitivity for SAH.
- Radiation dose is real — justify each scan.
- MRI is superior for posterior fossa, small stroke, and MS.
References
2 sources- NICE CG176Head Injury: assessment and early management
Updated to NG232 (2023).
nice.org.uk
- RCRiRefer
UK imaging referral guidance.
irefer.org.uk
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