MRI for Stroke: What It Shows

What Is a Stroke?

A stroke occurs when the blood supply to part of the brain is interrupted, causing brain cells to die. There are two main types: ischaemic stroke (a blocked artery, accounting for about 85% of cases) and haemorrhagic stroke (a ruptured blood vessel, about 15%).

Stroke is the fourth leading cause of death in the UK and the single largest cause of adult disability. Around 100,000 people in the UK have a stroke each year — roughly one every five minutes. About 1.3 million stroke survivors are living in the UK, many with lasting effects including paralysis, speech difficulties, and cognitive impairment.

A transient ischaemic attack (TIA, or "mini-stroke") causes temporary symptoms that resolve within 24 hours but signals a high short-term risk of full stroke — about 10% within the first 90 days if left untreated.

Why MRI Is Used for Stroke

In the acute setting, CT is usually the first scan performed — it's fast, widely available, and rules out haemorrhage within minutes. But MRI provides information that CT simply cannot match:

• Detecting early ischaemic stroke: Diffusion-weighted imaging (DWI) can detect an ischaemic stroke within minutes of onset — hours before CT shows any abnormality. This is MRI's greatest advantage.
• Differentiating old from new strokes: By comparing DWI, FLAIR, and T2 sequences, radiologists can determine whether an area of brain damage is hours, days, or weeks old. This distinction is clinically important.
• Identifying the cause: MRA (MR angiography) shows vessel narrowing or blockage. High-resolution vessel wall imaging can distinguish atherosclerosis from dissection from vasculitis.
• Assessing the penumbra: Perfusion MRI shows the area of brain tissue that is threatened but not yet irreversibly damaged — the "ischaemic penumbra." This tissue is potentially salvageable with treatment and helps guide decisions about thrombolysis and thrombectomy.
• Post-stroke workup: After the acute event, MRI helps classify the stroke subtype, guides secondary prevention, and provides prognostic information.
• TIA assessment: After a suspected TIA, MRI with DWI can show evidence of small areas of restricted diffusion — confirming that a genuine ischaemic event occurred even though symptoms have resolved.

MRI Sequences Used

• Diffusion-Weighted Imaging (DWI): The single most important sequence for acute stroke. When brain cells are deprived of blood, they swell (cytotoxic oedema), restricting the movement of water molecules. DWI detects this restricted diffusion as bright areas within minutes of onset. The corresponding ADC (Apparent Diffusion Coefficient) map confirms true restriction by showing dark signal in the same area.
• FLAIR (Fluid-Attenuated Inversion Recovery): Shows brain oedema and established damage. In acute stroke, the "DWI-FLAIR mismatch" is clinically valuable: a bright DWI lesion with a normal FLAIR suggests the stroke occurred within the last 4-6 hours — useful when the exact time of onset is unknown (e.g. wake-up strokes).
• T2-weighted: Shows established oedema and old strokes. Chronic strokes appear as areas of high signal with tissue loss (encephalomalacia).
• T1-weighted: Anatomical reference. Subacute haemorrhage (a few days to weeks old) appears bright on T1 due to methaemoglobin.
• SWI/GRE (Susceptibility-Weighted or Gradient Echo): Highly sensitive to blood products. Detects haemorrhagic transformation of an ischaemic stroke, old microbleeds, and primary haemorrhage.
• MR Angiography (MRA): Time-of-flight or contrast-enhanced imaging of the intracranial and/or cervical arteries. Shows the site of vessel occlusion, stenosis, or dissection.
• Perfusion-weighted imaging (PWI): Uses a bolus of gadolinium to map blood flow to different brain regions. The area where DWI is abnormal shows irreversibly damaged tissue; the larger area where perfusion is reduced but DWI is still normal represents the ischaemic penumbra — salvageable tissue.

What Radiologists Look For

• DWI lesion location and size: Which arterial territory is affected? The pattern of DWI abnormality can indicate whether the stroke is from large vessel occlusion, small vessel disease (lacunar stroke), or cardioembolic origin.
• Vascular territory: Anterior cerebral artery, middle cerebral artery, posterior cerebral artery, basilar artery — each has a characteristic territory. The affected territory guides treatment and prognosis.
• DWI-FLAIR mismatch: As described above, this helps estimate stroke age and eligibility for treatment in patients with unknown onset time.
• Vessel status on MRA: Is a large vessel occluded? Is there significant stenosis? Is there evidence of dissection (a torn vessel wall) in the carotid or vertebral arteries?
• Haemorrhagic transformation: Ischaemic strokes can bleed, particularly after thrombolysis. SWI detects this early.
• White matter disease burden: The extent of pre-existing small vessel disease (leukoaraiosis) on FLAIR provides context about the patient's overall cerebrovascular risk.
• Old strokes: Evidence of previous infarcts suggests recurrent disease and influences secondary prevention strategy.
• Cerebral microbleeds: Multiple microbleeds on SWI may suggest cerebral amyloid angiopathy or hypertensive small vessel disease, which affects anticoagulation decisions.

How to Prepare for the Scan

• In the acute setting, there's no preparation — speed is everything. The stroke team will handle everything.
• For planned post-stroke or TIA assessment MRI: no fasting required, take all medications as normal (especially blood thinners and blood pressure medication)
• Remove all metal items — jewellery, glasses, hearing aids, dentures with metal clasps
• Inform staff about any metallic implants, pacemakers, or cochlear implants
• If you have weakness or difficulty lying flat, let the team know — they can adapt positioning

What Happens During the Scan

For an acute stroke MRI, the protocol is streamlined for speed. A focused "stroke MRI" covering DWI, FLAIR, SWI, and MRA can be completed in 10-15 minutes. Every minute saved is brain tissue preserved.

For a planned post-stroke workup, the scan is more thorough and takes 30-45 minutes. You'll lie on your back with your head in a coil. If contrast is needed for perfusion imaging or enhanced MRA, a cannula will be placed.

If you have post-stroke weakness, the radiographer will help position you comfortably and may use supports to keep affected limbs still. Communication through the intercom is available throughout.

What MRI Can't Show

• Hyperacute haemorrhage (first few minutes): CT remains superior for detecting very acute bleeding. However, by the time most patients reach the scanner, MRI sequences like SWI are extremely sensitive to blood.
• Cardiac source of embolism: MRI of the brain doesn't image the heart. Echocardiography (TTE/TOE) is needed to look for cardiac sources of clot — atrial thrombus, patent foramen ovale, valve vegetations.
• Real-time clot assessment: While MRA shows vessel occlusion, the detailed composition and length of an intravascular clot (relevant for mechanical thrombectomy) is better assessed with CT angiography in the acute setting.
• Functional recovery prediction: MRI shows the structural damage but doesn't reliably predict individual functional recovery, which depends on rehabilitation, neuroplasticity, and patient-specific factors.

Treatment Pathways After Diagnosis

• Acute ischaemic stroke — thrombolysis: Intravenous alteplase (clot-busting drug) can be given within 4.5 hours of symptom onset (or within 4.5 hours of the DWI-FLAIR mismatch suggesting recent onset).
• Mechanical thrombectomy: For large vessel occlusion strokes, a catheter is threaded to the clot and the clot is physically removed. Effective up to 24 hours after onset in selected patients, guided by perfusion imaging showing salvageable tissue.
• Secondary prevention: Antiplatelet therapy (aspirin, clopidogrel), statin therapy, blood pressure management, and addressing modifiable risk factors (smoking, diabetes, atrial fibrillation).
• Carotid intervention: If MRA or ultrasound shows significant carotid artery stenosis (>50-70%), carotid endarterectomy or stenting may be recommended.
• Anticoagulation: For strokes caused by atrial fibrillation, long-term anticoagulation (typically a DOAC) prevents recurrence.
• Rehabilitation: Multidisciplinary stroke rehabilitation — physiotherapy, occupational therapy, speech and language therapy — begins as soon as the patient is medically stable.

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