What is MRI and How Does it Work
MRI and MS
MRI has contributed to our understanding of MS in several key ways. It is valuable in the diagnosis of MS, in understanding the nature of the disease process and as a tool in clinical trials to provide a rapid and objective means of assessing the influence of treatment on the course of MS.
Diagnosis
An MRI scan (specifically, a T2-weighted scan, which is described below) is considered strongly suggestive of MS if it shows at least four lesions in the brain, or if three lesions are present but one of them is located in the regions surrounding the ventricles in the brain, the periventricular region. As these criteria are sensitive (i.e. require the detection of numerous lesions) but not very specific (i.e. certain other conditions can cause brain lesions resembling those seen in MS), more specific ones were developed.
Three lesions must be present, fulfilling two of the following criteria:
• a periventricular lesion
• a lesion which is at least 6 mm in diameter
• a lesion located on the lower surface of the cerebellum in the brain (an infratentorial lesion). The cerebellum is located in the lower back part of the head, near the nape of the neck.1
Understanding the disease process
Three types of MRI scans are commonly used to investigate MS, all of which are sensitive to the higher than normal water content found in MS lesions. They are obtained by manipulating the radiowave pulses used in MRI in different ways, and are called T1-weighted, T2-weighted, and proton density scans. Each provides complementary information for the physician about the nature of your MS.
T1-weighted scans provide a lot of anatomical information about the brain. In this type of scan, which is particularly useful for identifying old lesions, abnormal areas appear as dark spots.
Here is an enhanced version of the T1-weighted image, which shows lesions that are not visible on the unenhanced image. The places where the contrast agent have accumulated show up as bright areas - these are active MS lesions. The old, inactive lesions remain dark.
T1-weighted scans are often used in association with a contrast agent such as a gadolinium-containing compound (e.g. Magnevist®): this is injected into the bloodstream before scanning, and which can highlight (or enhance) any areas of recent inflammation that indicate active disease.
T2 and proton density imaging
T2-weighted image (right). This type of image does not have as much anatomical detail as does the T1-weighted image. It shows both new and old lesions,and is often used when diagnosing MS. T2-weighted scans can be repeated regularly over time to give the neurologist snapshots in time of the way the disease is developing.
Proton density scan (bottom). This type of scan can identify both old and new lesions, which are highlighted as bright spots. It is particularly useful for identifying lesions near the fluid-filled ventricles.
Diagnosing
These different types of scan can be used to obtain information about the disease process in the three different types of MS - relapsing-remitting, primary progressive and secondary progressive. For example, in primary progressive MS, unlike relapsing-remitting disease, lesions are usually small and do not enhance with a contrast agent, implying that little inflammatory activity is present.
The location of a lesion within the CNS will determine the extent of the physical symptoms it produces. For example, a lesion in the spinal cord may result in numbness in the limbs and in bladder disturbance because it interferes with nerve impulses from the brain to these areas. Lesions in the optic nerve often cause optic neuritis, which may cause blurred vision and a loss of colour perception because the lesion impairs the ability of the optic nerve to function correctly.
Many lesions - even large ones - are often clinically silent (i.e. do not produce symptoms), but the higher a person's 'lesion load', the higher the chance that a key part of the brain will be affected and result in clinical symptoms.
Assessing the influence of treatment by MRI
Because it is non-invasive, MRI can be used as an objective tool to repeatedly scan the same person over a period of time and thus monitor the effects of treatment on disease activity (the number of new, enlarging or recurrent lesions). Nowadays MRI is a standard diagnostic tool for MS. In clinical trials MRI is been used to evaluate the effect of treatment (e.g. of beta interferons) in the course of the disease.
References
1. Fazekas et al. Neurology 1999;53:1448-456.
