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.
The various types of MRI scans that are used — most commonly the T1-weighted scan and the T2-weighted scan — measure this relaxation time in different ways. Computer programs translate these data into cross-sectional pictures of the water in human tissue.
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. However, approximately 5 percent of people with clinically-definite MS do not initially show lesions on MRI at the time of diagnosis. If repeat MRIs continue to show no lesions, the diagnosis of MS should be questioned.
- Since many lesions seen on MRI may be in so-called "silent" areas of the brain that don’t produce symptoms, it is not always possible to make a specific correlation between what is seen on the MRI scan and the person's clinical signs and symptoms.
- In addition, with advancing age (probably 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.
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 physician assess the person’s 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 had only one.
- 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 neurologist may consider disease activity on MRI as well as a person's 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, but most now recommend follow-up MRIs on a yearly basis. 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
A T1-weighted brain MRI scan, enhanced with gadolinium (injected intravenously to further enhance scan sensitivity), supplies information about current disease activity by highlighting areas of active inflammation. Because gadolinium is a large molecule, it normally cannot pass through the blood-brain barrier (a cell layer around blood vessels in the brain and spinal cord that prevents 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.
These areas of inflammation appear as active lesions, meaning that they are new or getting bigger.
T1-weighted images also show dark areas (hypointensities) that are thought to indicate areas of permanent nerve damage.
T2-weighted images provide information about disease burden or lesion load (meaning the total amount of lesion area, both old and new).
FLAIR (fluid attenuated inversion recovery) images are used to better identify brain lesions associated with MS.
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).
Although other types of scans are used for research purposes, these are the ones most commonly used in clinical care.
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 2.0T.
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 T.