How an MRI Works and What It Tells Us
MRI imaging of the brain and spinal cord is the most common diagnostic tool to confirm the diagnosis of MS and to monitor disease activity. MRI scans reveal abnormalities in the majority (90% to 95%) of people with MS. Although brain lesions are more common than spinal cord lesions, imaging of both brain and spinal cord are important in diagnosing and monitoring MS.
How does an MRI work?
To understand how an MRI machine works requires a basic understanding of physics on a molecular level. MRI uses a powerful magnetic field to align hydrogen atoms inside water molecules in tissues in the body. Consider how metal shavings react when you hold a magnet close to them. They stand up in response to the pull of the magnetic field.
The MRI machine uses the magnetic field, as well as radio waves, to manipulate the position of hydrogen protons. As protons change position, they give off signals that can be used by a computer to make an image of the tissue. Because some tissues in the body contain more water (and therefore more hydrogen) than others, changes in these tissues are more visible on MRI images Notice the different shades of tissue in the MRI images below.
To create images, MRI relies on the fact that most tissues of the body contain water molecules, which are made up of hydrogen and oxygen atoms (H2O). MRI creates a very powerful magnetic field around whatever is being scanned. This magnetic field causes hydrogen protons (a part of the atom) in water to line up in a certain way.
These hydrogen protons are then knocked out of alignment using radio waves that interrupt the magnetic field. When the radio waves are turned off, the protons relax and resume their aligned position, releasing resonance signals. These signals, which communicate the time it took for the proton to relax and line back up, are detected by the scanner and transmitted to a computer. The computer translates this information into cross-section pictures, such as those below which show contrasts in brightness corresponding to the different amounts of water contained in tissues.
Two types of MRI scanning techniques (T1-weighted and T2-weighted) are commonly used in MS protocols to measure the proton relaxation time in different ways and to record tissues with varying degrees of brightness.
Myelin, the fatty protective covering that insulates nerve fibers, repels water. Areas of damaged myelin hold more water and appear as bright white spots or dark areas, depending on the type of MRI scan (T1-weighted or T2-weighted) used. In some cases, a contrast agent (gadolinium or Gd, for short) can be given intravenously to enhance the contrast between tissues.
Gd enhanced T1-weighted MRI images highlight areas where the blood-brain-barrier (BBB), which is the layer of cells surrounding blood vessels in the brain and spinal cord that normally keeps substances from passing from the blood stream into the CNS, has been damaged. Small breaks in the BBB allow inflammatory cells to enter the CNS, resulting in the inflammation that damages nerve cells in the brain or spinal cord. Gd contrast, injected during an MRI procedure, passes through the same holes in the BBB entering areas of new inflammation. These small holes in the BBB eventually heal and close, making Gd particularly useful in highlighting current sites of active, ongoing CNS damage
What will my neurologist be looking for on the MRI scan?
The radiologist and your neurologist look for evidence of new damage, mostly lesions, and evidence of chronic damage to the CNS. New or ongoing tissue damage may appear as areas of brightness where inflammation is causing damage to the myelin coating on nerve fibers. Gadolinium enhancement allows new active lesions to be distinguished from old ones as recently formed lesions or plaques will appear brighter on the MRI scan. Areas of past nerve damage or where axons (nerve fibers) have died, may appear as black holes
Following your MRI scan, be sure to ask your neurologist to discuss the results with you and point out the places in your brain and spinal cord where damage or changes have occurred.
Why can’t MRI alone be used to diagnose MS?
While MRI imaging is a valuable tool in making an MS diagnosis, it cannot be used alone to make a diagnosis. Other diseases or even just natural aging, can cause similar lesions or plaques to those seen with MS. In addition, about 5% of people with MS have no visible lesions or brain tissue damage on their MRI scans (at least, initially).
How is MRI used to monitor progression of MS?
Repeat MRI scans are useful in monitoring disease progression in MS. While MRI cannot reliably distinguish between types of MS, changes in the CNS as seen on repeat scans following diagnosis, especially early in the disease, may be useful in predicting long-term prognosis.
Repeat MRI scans can also be used to monitor the efficacy of disease-modifying treatments, such as glatiramer acetate, natalizumab, fingolimod, or beta interferon, in preventing new lesions from forming. New lesions may form without causing new symptoms, so monitoring changes in brain volume and lesion-load becomes important
Is MRI used to diagnose clinically isolated syndrome?
MRI is particularly helpful in detecting areas of demyelination in persons who have a single neurological attack suggestive of MS. Patients who experience a single demyelinating attack are diagnosed with clinically isolated syndrome (CIS). Studies have shown that the presence or absence of plaques or lesions, in addition to the one(s) that cause symptoms of the CIS attack, can help predict whether a person with CIS will eventually develop MS.