Magnetic Resonance Imaging (MRI) provides detailed images of the brain, allowing medical professionals to identify the presence, location, and extent of brain tissue damage caused by a stroke. This imaging technique aids in stroke diagnosis and treatment planning.
How MRI Detects Stroke
MRI technology relies on strong magnetic fields and radio waves to create detailed images of the brain. The body’s water molecules, which are abundant in brain tissue, align with the magnetic field. Radiofrequency pulses then temporarily knock these aligned molecules out of alignment. When the pulses are turned off, the water molecules realign, releasing energy signals that are detected by the MRI scanner. These signals are then processed by a computer to generate cross-sectional images of the brain.
Different MRI sequences are used to highlight specific changes in brain tissue indicative of a stroke. Diffusion-Weighted Imaging (DWI) is particularly sensitive to acute ischemic strokes, often detecting changes within minutes of onset. In an ischemic stroke, brain cells deprived of oxygen undergo swelling due to the failure of their energy-dependent pumps, trapping water inside. This restricted movement of water molecules, known as restricted diffusion, appears as a bright signal on DWI and a corresponding dark signal on an Apparent Diffusion Coefficient (ADC) map.
Fluid-Attenuated Inversion Recovery (FLAIR) is another sequence. FLAIR images are similar to T2-weighted images but suppress the signal from cerebrospinal fluid, making abnormalities near fluid-filled spaces more visible. While DWI is effective for early detection, FLAIR sequences can show changes, such as increased fluid accumulation (edema), in infarcted tissue after 6-12 hours, becoming more prominent in the acute phase. The combination of DWI and FLAIR helps differentiate acute stroke from older lesions or other conditions.
Types of Stroke Revealed by MRI
MRI distinguishes between the two main types of stroke: ischemic and hemorrhagic. Ischemic strokes, which result from a blocked blood vessel interrupting blood flow to the brain, are primarily identified by restricted diffusion on DWI and ADC maps. This signature helps confirm acute ischemic damage.
Hemorrhagic strokes, caused by bleeding into the brain, appear differently on MRI. Sequences like Susceptibility-Weighted Imaging (SWI) or Gradient-Echo (GRE) are sensitive to blood products due to their magnetic properties, making them useful for detecting acute intracranial hemorrhage. These sequences show hypointense (dark) areas where blood has accumulated. MRI can also help determine the “age” of a stroke. The appearance of the stroke on various MRI sequences changes over time, providing clues about its timeline, which guides treatment decisions.
Why MRI is Preferred for Stroke Diagnosis
MRI offers advantages over other imaging techniques, such as Computed Tomography (CT) scans, for stroke diagnosis. It provides better soft tissue contrast and is more sensitive in detecting acute ischemic strokes, particularly in the early hours after symptom onset. While CT scans are faster and more widely available, MRI can detect ischemic changes within minutes, whereas CT may not show them for several hours. This early detection is important for patients who might benefit from time-sensitive clot-busting therapies.
MRI is also more effective at identifying smaller lesions or strokes located in areas difficult for CT to image, such as the brainstem or posterior fossa, due to surrounding bone artifacts. Unlike CT, MRI does not use ionizing radiation, which is a consideration for patients requiring multiple scans or those sensitive to radiation exposure. Despite MRI taking longer to perform, its imaging capabilities and ability to differentiate between stroke types and ages make it a preferred modality for comprehensive stroke evaluation when time permits.
What Happens During an MRI Scan
Undergoing an MRI scan for stroke involves steps to ensure patient comfort and image quality. Before the scan, patients are asked to remove all metallic objects, including jewelry, watches, hearing aids, and clothing with metal fasteners. This is because the MRI machine uses a powerful magnet, which can attract ferromagnetic materials, posing a safety risk or causing image distortion. Patients with certain implants, such as pacemakers or aneurysm clips, must inform the medical staff as these may be contraindications for the scan.
During the procedure, the patient lies on a movable table that slides into the large, tube-shaped MRI machine. The scan can be noisy, producing loud knocking or buzzing sounds, so earplugs or headphones are provided to protect hearing. The patient must remain still throughout the scan to ensure clear images. A brain MRI takes between 30 to 60 minutes. Medical staff can communicate with the patient via an intercom system throughout the process.