Non-contrast CT is most sensitive for subarachnoid haemorrhage in the first hours after headache onset, because blood is cleared from the subarachnoid space over time. The same normal scan therefore carries different weight at three hours and at three days.
The requesting clinician knows the time of onset; the radiologist often does not. Asking for it on every request for sudden severe headache, and stating it in the report, helps the clinician decide whether a normal CT is enough or whether further tests such as lumbar puncture or CT angiography are needed.
- Ask for the exact time of headache onset on CT requests for sudden severe headache.
- State the interval from onset to scan in the report.
- Remember that a normal CT becomes less reassuring as time from onset increases.
- Look carefully at the interpeduncular cistern and the occipital horns for small amounts of blood.
Why it matters
A normal CT is only as reassuring as its timing, and that timing is often missing from the request.
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