Multiple Sclerosis (MS) is a chronic disease of the central nervous system where the immune system attacks myelin, the protective layer surrounding nerve fibers in the brain and spinal cord. This attack causes inflammation and damage, disrupting the flow of information throughout the body. Magnetic Resonance Imaging (MRI) is the primary tool neurologists use to visualize this damage when MS is suspected. MRI is highly sensitive and allows doctors to look for characteristic signs of the disease.
What MRI Detects in the Brain and Spinal Cord
An MRI scan locates and characterizes areas of demyelination, known as lesions or plaques, which are hallmarks of MS. These lesions are visualized as bright spots on specific sequences, such as T2-weighted images, due to increased water content caused by inflammation and tissue damage. The location of these lesions is specific to MS and is carefully examined by radiologists.
MS lesions often cluster in distinct regions. These include the periventricular white matter, the juxtacortical area beneath the brain’s surface, and the infratentorial region (brainstem and cerebellum). Spinal cord imaging is also routine, as lesions frequently occur there, particularly in the cervical segment.
A contrast agent, typically gadolinium, helps distinguish between new and old lesions. When injected, gadolinium leaks into areas where the blood-brain barrier is actively broken down by inflammation, highlighting them as “enhancing” lesions. Enhancing lesions indicate current disease activity, while non-enhancing lesions represent older damage. The appearance, shape, and distribution of these spots offer strong evidence for an MS diagnosis.
Why Initial Scans Might Not Reveal MS
Although MRI is the most sensitive test, a patient may experience MS symptoms even with an initially negative scan. This occurs because lesions in the early stages may be too small or too few to be registered by standard imaging protocols. For example, a patient experiencing a clinically isolated syndrome (CIS)—the first episode of neurological symptoms—may not have sufficient lesions to meet diagnostic imaging requirements.
The location of the damage also impacts visibility. If symptoms are caused by a lesion exclusively in the spinal cord, a brain MRI alone would miss the damage. Spinal cord lesions are often smaller and more challenging to detect than brain lesions, especially in the thoracic or lumbar segments.
A small percentage of individuals meeting clinical criteria for MS exhibit clinicoradiologic dissociation. They have clear neurological symptoms but few or no abnormalities on conventional brain MRI. This suggests damage may be occurring at a microscopic level not captured by routine imaging technology. In these cases, diagnosis relies more on the full picture of clinical signs and other laboratory findings.
Other Essential Components of Diagnosis
An MS diagnosis is clinical, relying on a comprehensive assessment that goes beyond imaging alone. A neurologist must establish objective evidence of damage in multiple areas of the central nervous system that occurred at different points in time. This requires a thorough neurological examination and a detailed review of the patient’s past and present symptoms.
Lumbar Puncture
Physicians frequently use a lumbar puncture (spinal tap) to analyze the cerebrospinal fluid (CSF). This procedure checks for oligoclonal bands (OCBs), which are specific proteins indicating an abnormal immune response within the central nervous system. OCBs are found in the CSF of approximately 90% of people with MS and provide strong supportive evidence of the disease.
Evoked Potentials
Another supplementary test is Evoked Potentials (EPs), which measure the speed at which electrical signals travel through nerve pathways. For instance, a visual EP test can detect slowed signal transmission from the optic nerve, even if the patient has recovered from optic neuritis. The combination of clinical findings, imaging results, and these supportive tests helps rule out other conditions that mimic MS, leading to an accurate diagnosis.
Using Subsequent Scans to Confirm Diagnosis
When an initial MRI is negative or ambiguous, subsequent imaging over time is necessary to confirm the diagnosis. The appearance of new lesions on a follow-up scan fulfills the temporal requirement of the diagnosis. This demonstrates that inflammatory activity has occurred at different moments in time, even without new reported symptoms.
After diagnosis is confirmed, repeat scans monitor the disease and assess treatment effectiveness. Guidelines recommend a follow-up brain MRI approximately once a year to look for new or newly enhancing lesions. This monitoring tracks disease activity and helps adjust therapies to minimize future damage. The goal of repeated imaging is often to achieve “No Evidence of Disease Activity” (NEDA). NEDA means the absence of new clinical symptoms, new enhancing lesions, and new or enlarging T2 lesions on the MRI.