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.
Unlike a CT scan or conventional X-ray, this type of scanning device does not use radiation. Instead, it uses a powerful magnetic field that makes the hydrogen protons in water molecules line up. Once lined up, they are then knocked out of line by radio waves. When the radio waves are stopped, the protons relax back into line, releasing 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 differently. Computer programs translate these data into cross-sectional pictures of the water in human tissue. The layer of myelin that protects nerve-cell fibers is fatty and therefore 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. Gadolinium (gd) can be injected intravenously to further enhance the sensitivity of the T1-weighted MRI scan.
A gd-enhanced T1-weighted MRI scan supplies information about current disease activity by highlighting areas of breakdown in the blood-brain barrier that indicate inflammation. The blood-brain barrier is a cell layer around blood vessels in the brain and spinal cord that prevents substances from passing out of the blood stream into the central nervous system. These areas of inflammation appear as active lesions-meaning that they are new, or getting bigger. T1-weighted images also show “black holes,” which are thought to indicate areas of permanent damage. T2-weighted MRI scans are used to provide information about disease burden or lesion load (meaning the total amount of lesion area). Although other types of scans are used for research purposes, these are the ones most commonly used in clinical care.
Because MRI is particularly useful in detecting central nervous system demyelination, it is a powerful tool in helping to establish the diagnosis of MS. It should be remembered, however, that approximately 5% of patients with clinically definite MS do not show lesions on MRI at the time of diagnosis. Also, since many lesions seen on MRI may be in so-called “silent” areas of the brain, it is not always possible to make a specific correlation between what is seen on the MRI scan and the patient’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.
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 clinician assess the patient’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 patients who have had only one. However currently in New Zealand only patients with clinically definite relapsing remitting MS are able to access treatment.
Subsequent scans, are useful in tracking the progress of the disease, or possibly helping to establish a prognosis-a prediction of the course of a disease. For example, researchers have demonstrated that the degree of cognitive impairment as demonstrated by neuropsychologic testing can be correlated with the total amount of demyelination seen in certain areas of the brain on MRI.