A stroke is a medical emergency that happens when the blood supply to part of the brain is interrupted, either by a blockage or bleeding. This interruption prevents brain cells from receiving the oxygen and nutrients they need, causing them to die within minutes. Timely and accurate diagnosis is important for effective treatment and minimizing long-term damage. Magnetic Resonance Imaging (MRI) is a powerful imaging tool used for stroke diagnosis and evaluation.
How MRI Visualizes Stroke
MRI uses strong magnetic fields and radio waves to create detailed images of the brain’s soft tissues. It detects changes in brain tissue indicating a stroke. Different MRI sequences highlight various tissue properties for comprehensive assessment.
A key MRI sequence for detecting acute ischemic stroke is Diffusion-Weighted Imaging (DWI). DWI is sensitive, detecting brain ischemia minutes to hours after onset. It measures water molecule movement in brain tissue.
During an ischemic stroke, brain cells swell from cytotoxic edema, restricting water molecule movement. This restricted diffusion appears as a bright signal on DWI images and a dark signal on Apparent Diffusion Coefficient (ADC) maps. These early changes allow for rapid diagnosis, often before other imaging techniques detect abnormalities.
Distinguishing Stroke Types with MRI
Strokes are categorized into two main types: ischemic and hemorrhagic. Ischemic strokes, which account for about 87% of all strokes, occur when a blood clot blocks blood flow to a part of the brain. Hemorrhagic strokes, less common, result from bleeding within or around the brain from a ruptured blood vessel.
MRI plays a key role in differentiating between these two types, essential due to differing treatments. For ischemic stroke, DWI rapidly shows affected brain tissue where blood flow has been cut off, pinpointing the area of damage. For hemorrhagic stroke, different MRI sequences detect blood products.
Sequences like T2-weighted gradient echo (GRE) and Susceptibility-Weighted Imaging (SWI) are sensitive to blood, which appears as dark areas due to its magnetic properties. SWI offers superior sensitivity for detecting microbleeds and other blood products compared to T2 sequences. Distinguishing between ischemic and hemorrhagic stroke is important because clot-busting medications used for ischemic strokes can be dangerous if given for a hemorrhagic stroke.
The Value of MRI in Stroke Diagnosis
MRI offers advantages in stroke diagnosis compared to Computed Tomography (CT) scans. It provides superior soft tissue contrast, visualizing more subtle brain changes. This detail means MRI, especially with DWI, detects acute ischemic strokes earlier than CT, which may not show signs for several hours. MRI is also more effective at visualizing strokes in certain brain regions, such as the brainstem and cerebellum, known as the posterior circulation. These areas can be challenging for CT due to bone artifacts.
MRI can provide information about the age of a stroke, classifying it as hyperacute (0-6 hours), acute (6-24 hours), subacute (1-21 days), or chronic (over 21 days). Changes in signal intensity on various MRI sequences, including T1, T2, FLAIR, and ADC maps, evolve predictably over time, helping clinicians estimate when the stroke occurred. Beyond identifying the stroke, MRI detects conditions that mimic stroke symptoms but are not actual strokes. These “stroke mimics,” such as seizures, migraines, or tumors, are differentiated by MRI, ensuring correct treatment. MRI’s ability to provide detailed information about stroke presence, type, location, and age makes it a valuable tool for comprehensive evaluation and guiding patient care.