Magnetic resonance imaging (MRI): A diagnostic tool
Magnetic resonance imaging (MRI) is the diagnostic tool that currently offers the most sensitive, noninvasive way of imaging the brain, spinal cord or other areas of the body. It is the preferred imaging method to help establish a diagnosis of MS and to monitor the disease course. MRI has made it possible to visualize the effects of MS and understand much more about it.
How does MRI work?
Unlike a computed tomography (CT) scan or conventional X-ray, MRI does not use radiation. Instead, MRI uses magnetic fields and radio waves to measure the relative water content in the body’s tissues. Because the layer of myelin that protects nerve cell fibers is fatty, it repels water. In the areas where the myelin has been damaged by MS, the fat is stripped away. With the fat gone, the area holds more water and shows up on an MRI scan as either a bright white spot or a darkened area depending on the type of scan that is used.
To be very specific, MRI works in the following way:
- A very strong magnetic field causes a small percentage of the hydrogen protons in water molecules to line up in the direction of the magnetic field.
- Once the hydrogen protons have been lined up, radio waves and some additional but weaker magnetic fields are used to knock them out of line.
- When the radio waves are stopped, the protons relax back into line. As they relax, the protons release signals that are transmitted to a computer, analyzed and converted into an image.
MRIs for detecting demyelination and diagnosing MS
MRI is particularly useful in detecting central nervous system demyelination — that is, damage of the myelin sheath in the nervous system. This makes it a powerful tool in establishing the diagnosis of MS.
- You may not always see a direct correlation between the MRI scan and your symptoms. Lesions seen on MRI may be small or have caused little damage. Your brain may have developed a workaround.
- Generally, lesions in smaller areas, such as the brainstem, the spinal cord or the optic nerve are more likely to produce signs and symptoms.
- People over age 50 may have small areas on their MRIs that resemble those seen in MS but are actually related to the aging process. A similar thing happens with people who experience migraine headaches.
MRIs for assessing risk after clinically isolated syndrome (CIS) diagnosis
MRI is particularly helpful in people who have had a single demyelinating attack that is suggestive of MS, also called clinically isolated syndrome (CIS).
- The number of lesions on an initial MRI of the brain (or spinal cord) can help assess your risk of developing a second attack in the future and being diagnosed with MS. Some of the treatments for MS have been shown to delay a second episode in people who have CIS.
- The MRI can also be used to identify a second neurological event in a person who has no additional symptoms. This helps confirm an MS diagnosis as early as possible.
MRIs for tracking disease progress
After an MS diagnosis, MRI scans help healthcare providers track the progress of the disease and help you make the best treatment decisions. For example, you and your provider may consider disease activity on MRI as well as symptoms and relapses in determining whether your current treatment is working or not.
In 2021, MS experts from North America and Europe developed guidelines on the use of MRI to diagnose and monitor the MS disease course. Learn more about these guidelines through the Ask an MS Expert webinar below.
What are the different types and purposes of MRI scans?
Various types of MRI scans are used in MS. Sometimes gadolinium, a contrast agent, is injected into the vein during an MRI to help detect areas of new inflammation. Because gadolinium is a large molecule, it normally cannot pass through the blood-brain barrier, which prevents substances from passing from the bloodstream into the central nervous system. However, active inflammation can disrupt the blood-brain barrier. Then gadolinium can enter and highlight the inflamed areas.
Common MRI sequences used in MS include:
- T-1 weighted without gadolinium — may show dark areas (hypointensities) thought to indicate areas of permanent nerve damage
- T-1 weighted with gadolinium — may show bright areas (enhancing lesions) that indicate areas of active inflammation
- T-2 weighted — shows overall disease burden or lesion load (meaning the total number of lesions, both old and new)
- Fluid attenuated inversion recovery (FLAIR) — shows MS activity by reducing interference from the spinal fluid
For more on lesions and the use of a contrast agent, watch the short video “White Spots on an MRI.”
What are possible safety concerns and risks associated with gadolinium contrast agents?
There are several forms of gadolinium-based contrast agents (GBCAs). Although GBCAs can be helpful, there are some risks to using them that you should know about:
- People with poor kidney function have increased risk of developing nephrogenic systemic fibrosis (NSF) if they are given GBCAs. This rare and serious disease causes thickening of the skin and damage to internal organs.
- Small amounts of the GBCAs can stay in your body for several months to years. It is not yet known if this retention of GBCA is harmful.
What you should know about these risks:
- The FDA issued a safety communication on the use of GBCAs and recommended types of gadolinium less likely to be retained in the body.
- The FDA also required the makers of gadolinium contrast agents to do research to determine if GBCA deposits are harmful.
What you can do:
- Ask your doctor if you need to receive GBCA for your next MRI scan. It is not needed for all MRIs.
- Have your doctor check your kidney function with a blood test prior to receiving GBCA. This will reduce the risk of developing NSF.
Ask the center doing your MRI what type of GBCA they use. Macrocyclic agents are less likely to be retained in the body. Search the FDA’s Medication Guides Database for the latest on gadolinium.
Different magnets provide different information
The strength of the magnet used in the MRI machine is important to the quality of the images. Magnet strength is measured in Tesla (T).
- Most conventional MRI machines are 1.5T or 3T.
- Open MRIs are usually less than 1.5T and do not provide the best images for detecting MS activity although they may be used when someone has difficulty tolerating a closed MRI machine.
When possible, follow-up MRIs should be obtained on the same scanner, as this will help the radiologist and your healthcare provider make a comparison from one MRI to the next.