How an MRI Works and What It Tells Us

Reviewed by: HU Medical Review Board | Last reviewed: May 2021.

Magnetic resonance imaging (MRI) is a common tool for diagnosing MS. The majority of people with MS have abnormal MRI imaging. MRIs of both the brain and spinal cord can be helpful in looking for signs of MS.

How does an MRI work?

MRI machines use a powerful magnet and radio waves to take an image of the inside of the body. MRI does not use any radiation. This makes it a helpful imaging tool for people who should not receive radiation (like children or pregnant women).

The images created by MRI are very clear. They show much more detail than an x-ray or computed tomography (CT) scan. They also take longer to complete. X-rays and CT scans take seconds to finish. MRIs can take many minutes to an hour or longer depending on the part of the body imaged.

The magnet affects the hydrogen atoms that exist within the water molecules of our body. Different types of tissues have varying levels of water content. This is how the images created by the magnet highlight different structures. Fluid shows up differently than air, muscle shows up differently than bone, and so on.1

Radiologists are doctors trained in reading images of the body. They can look at the different structures to determine if there are any signs of inflammation, masses, or other issues.1

What can be seen on an MRI scan?

The most common signs of MS in MRI scans are brain and spinal cord lesions. These can look different depending on the type of MRI but usually, show up as small bright spots. These bright lesions are areas of active inflammation where MS is damaging the myelin (protective covering) of nerves. The brighter the spot, the more recent the inflammation. In some cases, lesions with different levels of brightness can be seen. Less bright lesions have been present for a longer time. More recent lesions are also often larger than older ones.

It is also possible to see MS-like lesions in a person who has no symptoms of MS. In these cases, a person may have gotten a brain or spinal MRI for another reason, like headaches.1-4

If you have no MS symptoms but show signs of MS on MRI, you have radiologically isolated syndrome (RIS). It is also possible for MS-like lesions to be a normal finding and mean nothing at all. This is especially true as you age or if you have migraine.1-4

MS is a clinical diagnosis

MRI findings, especially if you are having your first MS attack, can be helpful in diagnosis. The McDonald criteria are a set of guidelines that can be used to aid in the diagnosis of MS. The criteria take into account the number of attacks, symptoms, and findings on MRI. If you have lesions in more than 1 area of the CNS that appear to have shown up over time, it is more likely you have MS.5

MS is a clinical diagnosis. This means, there is no 1 test that confirms it. Instead, a doctor will take many factors into account. Type, timing, and recurrence of symptoms are all factored in. A person’s age, personal or family medical history, and physical exam findings also play a role. MRI findings and the McDonald criteria can help to make the final call.2,3

MRI is more useful in ruling out other conditions than in proving MS. For example, an MRI can help rule out a brain mass or stroke. After ruling out other causes, a diagnosis of MS may become easier.

Monitoring progression and treatment of MS

Repeat MRI scans can be helpful in keeping track of MS. As MS progresses, more lesions may develop. It is also possible to track existing lesions for improvement. This is helpful when trying to see if a treatment is working. If existing lesions improve and no (or few) new lesions develop, it most likely is. The timing of repeat MRIs may vary between doctors. Many experts suggest getting scans in the same machine whenever possible to best compare images over time.1-4

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