Almost nobody with a headache needs a brain MRI. That is the reassuring, boring, statistically honest lede. Migraine, tension headache, sinus pain, medication-overuse headache and cervicogenic headache together account for the overwhelming majority of headache in UK adults, and none of them need a scan to be diagnosed. But there is a small, specific list of features that do change the answer, and the point of this article is that if you can read that list before your GP appointment, you will spend a lot less time worrying and a lot less time in the wrong queue.
The one-sentence answer
If your headaches fit a familiar pattern, your neurological examination is normal, and none of the red flags below apply, a brain MRI is very unlikely to change what happens next. NICE guidance, the British Association for the Study of Headache and the international headache classification are all aligned on this. If any of the red flags do apply, then imaging moves from "reassurance" to "clinically indicated" and should not wait.
The SNOOP10 red flags neurologists actually use
SNOOP10 is the mnemonic most UK neurologists use to sift the headache clinic. It is not a test you take; it is a filter your GP applies. If you tick any single one, an MRI (and often an urgent same-week neurology review) is on the table.
- Systemic symptoms - fever, weight loss, night sweats, or a known systemic illness such as cancer, HIV or immunosuppression.
- Neurological signs - weakness, numbness, visual loss, double vision, speech disturbance, or a new seizure.
- Onset sudden - a true thunderclap headache reaching maximum intensity within seconds. This is treated as a subarachnoid haemorrhage until proven otherwise.
- Older age, new onset - a genuinely new headache pattern appearing over the age of 50. Giant cell arteritis and secondary causes are more likely at this age.
- Pattern change - a lifelong migraine sufferer whose headache has changed in character, frequency or severity in a persistent, unfamiliar way.
- Positional - headache clearly worse when standing (low-pressure headache) or worse when lying flat (raised-pressure headache).
- Precipitated by Valsalva - triggered by coughing, sneezing, straining or exercise. Can point to a posterior fossa or Chiari-type lesion.
- Papilloedema - swelling of the optic disc, seen on fundoscopy. This is one of the clearest signals of raised intracranial pressure.
- Progressive - a headache that has been steadily and relentlessly worsening over weeks, without remission.
- Pregnancy or postpartum - the risk of cerebral venous sinus thrombosis, pre-eclampsia-related headache and pituitary events all rise in this window.
- Painful eye - a red, painful eye with autonomic features, or unilateral headache with visual disturbance suggesting acute glaucoma, cluster or an orbital process.
If none of these are true and your examination is normal, statistically your risk of an important intracranial lesion is very low, and a scan will most likely return a normal result you paid or waited for.
What a brain MRI can rule in and rule out
A modern 1.5T or 3T brain MRI is the most sensitive non-invasive test we have for intracranial disease. In a headache work-up it is looking for a specific short list:
- Brain tumour - primary and secondary. Rare as a cause of everyday headache but the reason most people ask about a scan.
- Aneurysm or arteriovenous malformation - suggested by thunderclap headache or a strong family history. Usually needs MRA sequences added.
- Multiple sclerosis lesions - FLAIR sequences pick up demyelinating plaques with high sensitivity.
- Hydrocephalus - obstruction of cerebrospinal fluid flow, often presenting with progressive headache and cognitive change.
- Cerebral venous sinus thrombosis - a serious cause of headache in pregnancy, postpartum and in patients on oestrogen-containing contraception. Usually needs MRV sequences.
- Pituitary lesions - typically microadenomas or macroadenomas, sometimes causing headache and visual field loss. Best characterised with contrast.
Just as important is what an MRI is not. It is not a functional test. It will not diagnose migraine, tension-type headache or medication-overuse headache. It will not show why your muscles hurt. A normal MRI does not mean nothing is wrong. It means nothing structural is wrong.
Migraine vs tension vs cluster: which one warrants imaging?
In classic form, none of them. Migraine is diagnosed clinically by unilateral, throbbing, moderate-to-severe pain with photophobia, phonophobia, nausea or aura, in a recognisable, repeating pattern. Tension-type headache is bilateral, band-like, pressing rather than throbbing. Cluster headache is severe, unilateral, orbital, with autonomic features such as tearing, ptosis and nasal congestion, in stereotyped bouts.
All three have well-established diagnostic criteria and none requires imaging when the picture is textbook and the examination is normal. Where imaging enters is when the story is atypical, the pattern has changed, or a red flag is present. Cluster headache is the most likely of the three to trigger a scan on first presentation, because it can rarely mimic secondary causes and because pituitary lesions can present with cluster-like symptoms.
The scan you probably actually need
For most headache indications the study of choice is a brain MRI without contrast, on a 1.5T or 3T scanner, with standard sequences (T1, T2, FLAIR, DWI). That is enough to see or rule out a mass, a stroke, MS lesions and hydrocephalus.
Add-ons are decided by clinical context, not by default:
- Contrast (gadolinium) - added when a suspected tumour, infection, inflammation or pituitary lesion needs to be characterised.
- MRA (magnetic resonance angiogram) - added when there is concern for an aneurysm, arterial dissection or vascular malformation. Standard after a thunderclap headache work-up.
- MRV (magnetic resonance venogram) - added when cerebral venous sinus thrombosis is on the differential, particularly in pregnancy, postpartum, or oestrogen-containing contraception users.
- Dedicated pituitary sequences - thin-slice, contrast-enhanced sequences added when a pituitary lesion is suspected.
A well-run private clinic will not upsell any of these. They will run the study that answers the specific clinical question your referring clinician asked, and add sequences only when the picture in front of the radiologist calls for them.
