Multiple Sclerosis (MS) is a chronic neurological condition affecting the central nervous system, including the brain, spinal cord, and optic nerves. It is characterized by areas of damage or inflammation known as lesions. These lesions are a hallmark of the disease, providing insights into its activity and progression. This article explains what MS lesions are, why brain MRI is a powerful detection tool, and what their presence signifies for diagnosis and monitoring.
What Are MS Lesions?
MS lesions are areas in the central nervous system where the myelin sheath, the protective covering around nerve fibers, has been damaged. This demyelination disrupts nerve signal transmission. Lesions also involve inflammation, with immune cells infiltrating affected areas.
Lesions can occur throughout the central nervous system, but are often found in brain white matter. They are categorized by activity: active lesions are new or show ongoing inflammation and demyelination, while inactive or chronic lesions are older, with less inflammation and scarring.
Why Brain MRI is Essential for MS
Magnetic Resonance Imaging (MRI) is essential for detecting and monitoring MS. It is non-invasive and excels at visualizing soft tissues of the brain and spinal cord. MRI can reveal subtle changes in brain tissue not visible with other imaging techniques. It is particularly useful for detecting lesions that may not cause noticeable symptoms, which is important for early diagnosis and understanding the full extent of disease activity. The high sensitivity of MRI allows for early detection of these demyelinating plaques.
Reading MS Lesions on a Brain MRI
On a brain MRI, MS lesions typically appear as bright white spots, also known as hyperintensities, on T2-weighted and FLAIR (Fluid-Attenuated Inversion Recovery) sequences. FLAIR sequences are particularly sensitive for detecting lesions near the ventricles and cortex, while T2-weighted images are effective for visualizing lesions in the infratentorial regions. The signal increase seen on these sequences indicates a combination of edema, inflammation, demyelination, gliosis, and sometimes axonal loss within the lesion.
To identify active lesions, gadolinium-enhanced T1-weighted sequences are used. Active lesions show enhancement after gadolinium injection, indicating a temporary breakdown of the blood-brain barrier. This enhancement is often incomplete, appearing as an “open ring sign” in active plaques. Typical locations for MS lesions in the brain include the periventricular white matter (around the brain’s fluid-filled cavities), juxtacortical areas (adjacent to the brain’s outer surface), infratentorial regions (brainstem and cerebellum), and the corpus callosum. A specific pattern often seen in the periventricular region is “Dawson’s fingers,” which are ovoid lesions arranged perpendicularly to the lateral ventricles, reflecting inflammation along deep medullary veins.
The Significance of Lesions for MS Diagnosis and Monitoring
The presence, number, and location of MS lesions on a brain MRI are fundamental for diagnosing MS, particularly within diagnostic criteria like the McDonald criteria. These criteria rely on “dissemination in space” (lesions in multiple characteristic central nervous system locations) and “dissemination in time” (lesions occurring at different points in time). For instance, dissemination in space typically requires at least one lesion in two or more of four characteristic locations: periventricular, juxtacortical, infratentorial, and spinal cord.
MRI also plays a significant role in monitoring disease progression and assessing treatment effectiveness. New or enhancing lesions on follow-up scans can indicate ongoing disease activity, even if a patient is not experiencing new symptoms. While brain lesions are a key diagnostic and monitoring tool, their presence does not always perfectly correlate with a patient’s clinical symptoms or disability progression. Some individuals may have numerous lesions with few symptoms, while others may experience significant symptoms with fewer lesions, highlighting the complexity of the disease.