Multiple Sclerosis (MS) is a chronic, unpredictable disease affecting the central nervous system, including the brain and spinal cord. The immune system mistakenly attacks the myelin sheath, the protective layer around nerve fibers, causing damage known as lesions or plaques. While a definitive diagnosis requires a comprehensive clinical evaluation, imaging is indispensable for identifying these characteristic lesions. Computed Tomography (CT) scans, however, are generally not the primary tool for confirming an MS diagnosis.
Why CT Scans Are Not Effective for Detecting Demyelination
CT technology utilizes X-rays to create cross-sectional images, excelling at differentiating structures based on density, such as bone, air, and acute hemorrhage. This mechanism limits its ability to visualize the subtle changes associated with demyelination in MS, as MS lesions primarily involve changes in the soft tissue of the brain’s white matter.
These demyelinated plaques often appear isodense, meaning they have a density similar to the surrounding healthy brain tissue on a standard CT scan. Because the contrast between the lesion and the background is poor, the lesions are often invisible or difficult to distinguish, especially in the early stages of the disease. CT scans may fail to detect plaques smaller than seven millimeters, and even larger plaques can be misinterpreted as normal structures.
A CT scan is sometimes used early in a neurological workup, but its purpose is typically one of exclusion, not detection. Clinicians may order a CT to quickly rule out acute, life-threatening conditions that mimic MS symptoms, such as stroke, intracranial hemorrhage, or a large brain tumor. Once these other conditions are excluded, the physician must turn to a more sensitive imaging modality to search for the small, soft tissue lesions characteristic of MS.
The Role of MRI in Identifying MS Lesions
Magnetic Resonance Imaging (MRI) is considered the gold standard for diagnosing MS because it offers far superior soft tissue contrast compared to CT. This technology uses strong magnetic fields and radio waves to generate detailed images, providing a clear visualization of the demyelinated plaques in the central nervous system. MRI is significantly more sensitive, showing abnormalities in the brain of over 90% of MS patients and spinal cord lesions in up to 75% of cases.
The ability of MRI to visualize MS lesions is due to specialized sequences that highlight water content and inflammation. T2-weighted and Fluid-Attenuated Inversion Recovery (FLAIR) sequences are routinely used, with the lesions appearing as bright spots (hyperintensities). FLAIR is particularly useful as it suppresses the bright signal from cerebrospinal fluid (CSF), allowing lesions near the ventricles or brain surface to be clearly seen.
To assess whether a lesion is actively inflamed, a contrast agent, most commonly Gadolinium, is injected intravenously before a T1-weighted sequence is performed. In an active MS lesion, inflammation temporarily damages the blood-brain barrier, allowing the Gadolinium to leak into the tissue and “enhance” the lesion. Identifying these enhancing lesions is crucial for establishing disease activity and demonstrating that demyelination has occurred at different times, a requirement for diagnosis.
Completing the Diagnosis: Clinical Criteria and Supplemental Tests
While MRI is the most sensitive tool for visualizing MS lesions, the disease is not diagnosed by imaging alone; it remains a clinical diagnosis requiring a combination of evidence. The current diagnostic framework, known as the McDonald Criteria, requires both clinical and radiological evidence to confirm the diagnosis and rule out other conditions. This framework relies on confirming the presence of lesions that are separated in both location and time.
The concept of “Dissemination in Space” (DIS) is fulfilled by finding at least one lesion in specific areas of the central nervous system, such as the periventricular, juxtacortical, infratentorial regions, or the spinal cord. “Dissemination in Time” (DIT) is met by identifying both active (Gadolinium-enhancing) and inactive (non-enhancing) lesions on a single scan, or by showing new lesions on a follow-up scan compared to a previous one. The patient’s history, symptoms, and neurological examination are integrated with these imaging findings.
Supplemental tests are frequently used to support the diagnosis, particularly when imaging evidence is not fully conclusive. A Lumbar Puncture (spinal tap) is performed to collect and analyze cerebrospinal fluid (CSF). The presence of Oligoclonal Bands (OCBs)—specific proteins that indicate an immune response confined to the central nervous system—is a hallmark finding in MS.
Evoked Potential (EP) tests measure the speed of electrical signals traveling along sensory pathways, detecting delays caused by demyelination. The Visual Evoked Potential (VEP) test, which measures the brain’s response to visual stimuli, is often used, as optic nerve involvement is common in MS. The combination of clinical symptoms, characteristic MRI findings, and supporting laboratory evidence provides the definitive basis for an MS diagnosis.