A magnetic resonance imaging (MRI) scan uses magnetic fields and radio waves to create detailed images of the body’s soft tissues. In neurological medicine, this capability makes MRI invaluable for assessing the brain and diagnosing acute conditions, such as stroke. The detailed visualization of brain tissue is essential for making time-sensitive treatment decisions. Interpreting these complex images requires a methodical understanding of the different sequences and what specific tissue changes look like.
Why MRI is Essential for Stroke Diagnosis
Magnetic resonance imaging holds distinct advantages over other common imaging methods, such as computed tomography (CT), especially in the immediate evaluation of stroke. MRI is significantly more sensitive in detecting acute tissue changes caused by a blockage, known as ischemia, often within minutes of symptom onset. This superior sensitivity is particularly important for small strokes or those affecting the posterior circulation, which includes the brain stem and cerebellum.
The ability to quickly and accurately identify the affected tissue is crucial for guiding time-sensitive treatments. For instance, clot-dissolving medications must be administered within a limited window after a stroke begins. MRI provides confidence in the diagnosis, helping clinicians determine if a patient qualifies for these reperfusion therapies. While CT is faster and more widely available, the diagnostic accuracy of an MRI in the hyperacute phase often makes it the preferred test when immediately accessible.
Key MRI Techniques for Identifying Stroke Damage
The stroke imaging protocol relies on specialized sequences that measure different properties of brain tissue. Diffusion-Weighted Imaging (DWI) is the most sensitive sequence for detecting acute ischemic stroke. Within minutes of a blockage, brain cells swell due to a failure in their energy-dependent pumps, causing water molecules to become restricted in their movement.
This restricted water movement causes the affected area to appear bright, or hyperintense, on the DWI scan. To confirm that this brightness represents true restricted diffusion, the image is compared with the Apparent Diffusion Coefficient (ADC) map. The ADC map displays the calculated rate of water molecule movement, and in a true acute stroke, the corresponding area will appear dark, or hypointense, confirming the diagnosis.
The Fluid-Attenuated Inversion Recovery (FLAIR) sequence is used as a complementary tool, primarily to filter out the bright signal from cerebrospinal fluid (CSF) that can obscure lesions. In an acute stroke, the DWI sequence will show the lesion immediately, but the FLAIR sequence generally remains normal for the first few hours. This difference in signal intensity between DWI and FLAIR is an indicator of the stroke’s age.
Differentiating Ischemic and Hemorrhagic Strokes
The primary goal of initial stroke imaging is to distinguish an ischemic stroke (a blockage) from a hemorrhagic stroke (a bleed), as the treatments for each are drastically different. Ischemic strokes are identified by the classic pattern of restricted diffusion—bright on DWI and dark on the ADC map—within the affected vascular territory. In contrast, hemorrhagic strokes involve blood leaking into the brain tissue, which is best visualized using sequences sensitive to magnetic susceptibility.
Susceptibility-Weighted Imaging (SWI) or T2-weighted imaging is highly effective for detecting blood products. Hemoglobin in the blood contains iron, which is strongly paramagnetic, meaning it distorts the local magnetic field. This distortion causes a significant loss of signal, making the blood products appear as dark, or hypointense, areas on the SWI scan, a phenomenon known as blooming.
While an ischemic stroke shows tissue that appears bright on DWI, a hemorrhagic stroke shows a dark, “blooming” lesion on the SWI sequence. This clear difference in appearance guides the immediate clinical decision-making, such as whether to administer clot-busting drugs, which are safe for ischemia but potentially dangerous for a hemorrhage.
Interpreting the Age of the Stroke
The MRI images provide a biological clock to estimate how long ago a stroke occurred, which is important for determining treatment eligibility. The relationship between the DWI and FLAIR signals changes predictably over time. A DWI-FLAIR mismatch—where the lesion is bright on DWI but normal on FLAIR—is a strong indicator that the stroke is hyperacute, often suggesting the event occurred within the last four and a half hours.
As the stroke progresses into the subacute phase, typically over four to six hours, the signal on the FLAIR sequence will also begin to brighten. A lesion that is bright on both DWI and FLAIR suggests the stroke is beyond the most acute window, likely occurring several hours prior. Over the next one to two weeks, the DWI signal will gradually fade, a process known as “pseudonormalization,” while the FLAIR signal remains bright.
For a chronic stroke, which is weeks to years old, the damaged area shows significant volume loss, or atrophy, on both T1- and T2-weighted images. The final appearance is often an area of gliosis or a fluid-filled cavity called encephalomalacia, which signal permanent tissue destruction. The ability to stage the stroke allows physicians to tailor interventions for maximum effectiveness and safety.