MS is a chronic disease of the central nervous system (CNS) where the immune system mistakenly attacks the brain and spinal cord, causing inflammation and damage. This damage disrupts the nervous system’s communication pathways, leading to various symptoms. Magnetic Resonance Imaging (MRI) is the primary tool for visualizing this damage, which appears on the scan as bright spots or lesions. These “white spots” represent areas of injury, and their presence, location, and number are crucial for diagnosing and monitoring MS activity.
Understanding White Matter Lesions on MRI
The white spots seen on an MRI are technically referred to as white matter hyperintensities, or lesions. These lesions are visible because MS involves the destruction of myelin, the fatty protective sheath that insulates nerve fibers. Myelin makes up the brain’s white matter, acting like insulation to speed up nerve signals.
Inflammation and subsequent damage lead to an accumulation of fluid and inflammatory cells. On specific MRI sequences, particularly T2-weighted and FLAIR scans, this increased water content appears brighter than the surrounding healthy tissue. Radiologists use the term “hyperintense” to describe these bright spots, which indicate the total burden of disease, showing both old and new areas of damage. Conversely, on a T1-weighted scan, areas of permanent tissue loss or atrophy, often called “black holes,” may appear dark, signifying older, more destructive lesions.
Characteristics of Multiple Sclerosis Lesions
The specific appearance and location of lesions are essential for confirming an MS diagnosis. The current diagnostic criteria, known as the McDonald criteria, require evidence that the disease is “disseminated in space” (DIS), meaning damage has occurred in multiple, specific areas of the CNS. These characteristic locations include the periventricular region, the white matter immediately surrounding the brain’s fluid-filled ventricles. Lesions in this area often align perpendicular to the ventricles, sometimes forming finger-like projections known as Dawson’s fingers.
Other required locations for meeting the DIS criteria include the juxtacortical area (beneath the cortex), the infratentorial region (brainstem and cerebellum), and the spinal cord. To determine if a lesion is currently active, a contrast dye called gadolinium is injected before the scan. If the lesion is actively inflamed, the dye leaks through the compromised blood-brain barrier, causing the lesion to “enhance” or light up brightly on the T1 scan. This enhancing activity indicates a new or ongoing attack, differentiating it from older, non-enhancing lesions.
Other Causes of White Spots (Differential Diagnosis)
The presence of white spots on a brain MRI is common and does not automatically lead to an MS diagnosis. The differential diagnosis for white matter hyperintensities is extensive, as many conditions cause similar-looking spots. In older individuals, the most frequent cause is age-related small vessel disease, or microvascular ischemic disease.
These spots are often related to vascular risk factors like uncontrolled hypertension, diabetes, and high cholesterol. Migraines are another common non-MS cause, though these spots generally lack the specific size and location patterns of MS lesions. Other inflammatory or infectious conditions, such as systemic lupus erythematosus or sarcoidosis, can also produce these spots. Neurologists rely on the unique combination of lesion shape, size, and distribution—especially the periventricular and juxtacortical patterns—to distinguish MS from these other conditions.
The Role of MRI in Monitoring MS Disease Activity
Once MS is diagnosed, MRI continues to be a standard tool for monitoring the disease and evaluating treatment effectiveness. Regular scans help track the overall burden of disease by counting the total number of T2 lesions and identifying the formation of any new or actively enhancing lesions. A reduction in new lesion formation is a primary goal of MS therapy, and MRI provides objective evidence of whether a chosen medication is suppressing inflammatory activity.
Despite its utility, the connection between imaging findings and a patient’s physical experience is not always direct, a concept called the clinico-radiological paradox. A patient may have many lesions but experience few symptoms, or conversely, have few lesions but significant disability. This disconnect suggests that factors like precise lesion location and the brain’s ability to compensate play a significant role in determining impairment.