How MRI shows MS
How MRI makes a picture
The body is full of water, and every water molecule contains hydrogen nuclei, which behave like tiny magnets. In the strong field of the scanner, usually 1.5 or 3 tesla, they line up. A pulse of radio waves knocks them out of line, and as they relax back they give off a faint radio signal that the scanner records. How quickly they relax depends on the tissue around them, so different tissues give different signals. MRI uses no X-rays or other ionising radiation.
Relaxation has two parts, called T1 and T2. By changing the timing of the pulses, the scanner can make a picture in which differences in T1 or in T2 dominate. These are the sequences, and each shows MS lesions differently.
The main sequences
- T2 and FLAIR. Lesions contain more free water than healthy tissue, so they look bright. FLAIR is a T2 sequence that darkens the fluid in the ventricles, so lesions next to them stand out. On T2 and FLAIR, old and new lesions look alike. International guidelines now recommend three-dimensional FLAIR as the core brain sequence for diagnosis and monitoring.
- T1. Most lesions look grey or invisible. Lesions that stay dark on T1, called black holes, mark severe loss of tissue and axons.
- T1 after gadolinium. Gadolinium is a contrast agent injected into a vein. It normally stays in the blood, but where the blood-brain barrier is open it leaks into the tissue and the lesion lights up. Enhancement usually lasts a few weeks, so an enhancing lesion is a recent one.
- Spinal cord sequences. The cord is imaged separately and is often affected in MS. Lesions there cause much of the disability in walking and bladder control.
Where MS lesions sit
MS lesions have typical sites and shapes. They lie next to the ventricles, at or in the cortex, in the brainstem and cerebellum, in the spinal cord and in the optic nerves. Next to the ventricles they are often oval and point away from them, along the small veins, a pattern known as Dawson's fingers. The 2024 McDonald criteria count five sites: the four in the brain and spinal cord used before, and now the optic nerve.
Bright spots on MRI are common and many are not MS. Small vessel disease from age, high blood pressure, diabetes or smoking, and migraine can all produce white spots. The pattern, the central vein sign (lesson 6.2) and the clinical story are what make the difference, which is why the criteria insist that no other disease explains the findings better.
Measuring activity
To monitor MS, a new scan is compared with an earlier one, ideally from the same scanner with the same protocol. New or enlarging T2 lesions show activity since the last scan, and enhancing lesions show activity in the past few weeks. Guidelines recommend using gadolinium judiciously, for specific clinical questions rather than for every follow-up scan. One reason is that small amounts of gadolinium remain in the brain after repeated doses, first shown in 2015. No harm from these deposits has been shown, but caution is sensible.
Click the lesion that is new.
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Why trials love MRI
New lesions respond quickly to effective treatment and can be counted precisely. That is why phase 2 trials in relapsing MS usually use new enhancing or new T2 lesions as their main measure: a medicine that does not reduce them is unlikely to reduce relapses. In progressive MS the link is weaker, because much of the damage that drives progression does not appear as new lesions (lesson 3.2).
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What we know
- T2 and FLAIR show all lesions, gadolinium shows recent ones, and persistent T1 black holes show severe tissue loss.
- Lesion sites and shapes, read together with the clinical picture, separate MS from most mimics.
- New lesions on MRI respond to effective treatment and predict relapses at the group level.
What we don't know
- How to see cortical lesions reliably on routine scanners.
- How much of the disability in a given person their visible lesions explain (lesson 6.2).
- Whether the small deposits of gadolinium left in the brain have any long-term effect.
In the research now
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Key terms
- FLAIR
- Fluid-attenuated inversion recovery: an MRI sequence that shows lesions as bright spots and darkens the fluid around them.
- T1 and T2
- Two ways MRI can weight its pictures, based on how quickly hydrogen nuclei relax. Lesions look bright on T2 and grey or dark on T1.
- Gadolinium
- A contrast agent injected before an MRI scan. It leaks into lesions where the barrier is open.
- Dawson's fingers
- Oval lesions pointing away from the ventricles along small veins, typical of MS.
- Small vessel disease
- Damage to the brain's small blood vessels with age, high blood pressure or diabetes. It causes white spots on MRI that can be mistaken for MS.
- Cortical lesion
- A lesion in the cortex, the outer grey matter of the brain. Often hard to see on standard MRI.
- Axon (nerve fibre)
- The long extension of a nerve cell that carries its signal. Losing axons, rather than myelin alone, is thought to cause most lasting disability.
- Blood-brain barrier
- The tight lining of the small blood vessels of the brain and spinal cord, which keeps most immune cells and large molecules out.
- Lesion (plaque)
- An area of damage where myelin has been lost and the tissue scarred.
- Central vein sign
- A small vein running through the centre of a lesion on MRI. It can support an MS diagnosis in specified circumstances under the 2024 McDonald criteria.
- Relapse (attack)
- New or worsening symptoms lasting at least 24 hours, without fever or infection to explain them.
- McDonald criteria
- The international criteria for diagnosing MS, last revised in 2024 and published in 2025.
- Phase 1, 2 and 3
- The stages of testing in people: safety and dose, then an early sign of effect, then a large test of benefit.
References
- Wattjes MP, et al. 2021 MAGNIMS-CMSC-NAIMS consensus recommendations on the use of MRI in patients with multiple sclerosis. Lancet Neurol 2021.https://doi.org/10.1016/S1474-4422(21)00095-8
- Montalban X, et al. Diagnosis of multiple sclerosis: 2024 revisions of the McDonald criteria. Lancet Neurol 2025.https://doi.org/10.1016/S1474-4422(25)00270-4
- McDonald RJ, et al. Intracranial gadolinium deposition after contrast-enhanced MR imaging. Radiology 2015.https://doi.org/10.1148/radiol.15150025
Next · Lesson 2 of 2
Rims, central veins and atrophy
Standard MRI shows where lesions are. Newer methods show what they are: whether a vein runs through them, whether their edge is still inflamed, and how much tissue has been lost.
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