Magnetic resonance imaging (MRI) is the diagnostic tool that currently offers the most sensitive non-invasive 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 course of the disease. MRI has made it possible to visualize and understand much more about the underlying pathology of the disease.
How it works
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 tissues — both normal tissue and abnormal — in the body. 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. The percentage lined up is small, but large enough to give a strong signal for imaging.
- 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 resonance signals that are transmitted to a computer, analyzed and converted into an image.
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.
Diagnosis of MS
Because MRI is particularly useful in detecting central nervous system demyelination, it is a powerful tool in helping to establish the diagnosis of MS.
- Since many lesions seen on MRI may be very small, have caused very little damage, or the brain has developed a work around, it is not always possible to make a specific correlation between what is seen on the MRI scan and your clinical signs and symptoms. Generally lesions in smaller areas, such as the brainstem, the spinal cord or the optic nerve are likely to produce signs and symptoms.
- In addition, with advancing age (over age 50), there are often small areas seen on MRI in healthy people that resemble MS but are actually related to the aging process. Also, people who are diagnosed with migraine headaches may have abnormal areas on the MRI that might look similar to MS lesions.
Clinically Isolated Syndrome (CIS)
MRI is particularly helpful in patients who have had a single demyelinating attack that is suggestive of MS, also called a clinically isolated syndrome (CIS).
- The number of lesions on an initial MRI of the brain (or spinal cord) can help the healthcare provider assess your risk of developing a second attack (and therefore “clinically-definite MS”) in the future. Some of the treatments for MS have been shown to delay the occurrence of a second episode of symptomatic demyelination 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 — thereby helping to confirm a diagnosis of MS as early as possible.
Tracking disease progress
Once a diagnosis of MS has been clearly established, no additional MRI scans are needed for diagnostic purposes. However, subsequent scans are important for tracking the progress of the disease and making treatment decisions. For example, a healthcare provider may consider disease activity on MRI as well as your clinical symptoms and relapses in order to determine whether the current treatment is effective or a change in treatment needs to be considered.
Healthcare professionals differ in their opinion about how often an MRI should be done for MS. The Consortium of MS Centers (CMSC) published an MRI Protocol and Clinical Guidelines for the Diagnosis and Follow-up of MS. These guidelines provide more direction for the frequency of follow-up MRI after a diagnosis of MS. 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.
Different scan types provide different information
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 (specialized cells in the blood vessel wall around the brain and spinal cord that prevent substances from passing from the blood stream into the central nervous system). However, when there is active inflammation, the blood brain barrier is disrupted and 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) that are 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- show overall disease burden or lesion load (meaning the total number of lesions, both old and new)
- FLAIR (fluid attenuated inversion recovery)- shows MS activity by reducing interference from the spinal fluid
Spinal cord imaging can identify pathology in the cord. It can also help establish the diagnosis of MS by demonstrating that damage has occurred in different parts of the central nervous system (dissemination in space) at different points in time (dissemination in time). It is also useful to identify MS activity when symptoms point to a problem in the spinal cord.
You may hear about other types of MRI scans that are used for research. Those listed in this article are the most commonly used in clinical care.
Possible safety concerns with gadolinium contrast agents
A contrast agent, known as gadolinium, is often injected into the vein during an MRI scan. Gadolinium is used to identify areas of active inflammation that can be associated with MS. There are several forms of gadolinium-based contrast agents (GBCAs) used. Although the use of GBCAs can be helpful, there are some risks to using them that you should know about. The risks of using CBGAs are:
- Nephrogenic Systemic Fibrosis (NSF)- a rare and serious condition that causes thickening of the skin and damage to internal organs that can occur in people with poor kidney function who are given GBCAs.
- Retention of GBCAs- deposits have been found in the brains and other body tissues of some people who received GBCAs for an MRI. It is not known if these deposits are harmful.
What you should know about these risks:
- The FDA has issued a safety communication on the use of GBCAs and made recommendations for the types of gadolinium that are less likely to be retained in the body
- The FDA is also requiring the makers of gadolinium contrast agents to conduct research to determine if there are harmful effects of gadolinium deposits.
- The Consortium of MS Centers (CMSC) published an MRI Protocol and Clinical Guidelines for the Diagnosis and Follow-up of MS. These guidelines address the safety concerns with GBCAs and make recommendations for when using a contrast agent is necessary.
What you can do:
- Ask your doctor if you need to receive a GBCA for your next MRI scan. It may not be 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. Macrocyclic GBCAs are marketed under the names Dotarem®, Gadavist® and ProHance®.
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.
- MRI machines used for research purposes have much higher magnet strength.