A stroke occurs when blood flow to a part of the brain is interrupted, either by a blockage or bleeding, leading to the death of brain cells. Magnetic Resonance Imaging (MRI) is a powerful tool in neurology that helps diagnose strokes. While pinpointing the exact moment a stroke occurred is challenging, MRI can provide valuable information about its approximate age.
The Science of Stroke Evolution on MRI
Following an ischemic stroke, the brain undergoes biological changes detectable by MRI over time. Initially, within minutes to hours, brain cells deprived of oxygen and nutrients begin to die. This early phase involves cellular swelling, known as cytotoxic edema, as water shifts into the cells.
As the stroke progresses, inflammation and tissue breakdown occur. Within days to weeks, the affected area may show signs of tissue repair and gliosis, the formation of scar tissue by glial cells. These evolving processes create distinct appearances on MRI scans, which helps estimate the stroke’s age.
In the acute phase (hours up to seven days), specific changes reflecting fresh tissue damage are visible. The subacute phase (one to three weeks) shows continued tissue reorganization. Beyond three weeks, the stroke enters the chronic phase, where damaged brain tissue may appear as a fluid-filled cavity or a shrunken area.
Why Stroke Timing Matters for Treatment
Knowing the approximate age of a stroke is important for guiding acute treatment. Certain therapies, such as intravenous thrombolysis with tissue plasminogen activator (tPA), are time-sensitive. This clot-dissolving medication is most effective and safest when administered within a narrow “therapeutic window,” typically within 3 to 4.5 hours of symptom onset.
Beyond this initial window, tPA benefits decrease, and risks, such as bleeding in the brain, increase. For some patients with large vessel blockages, mechanical thrombectomy, a procedure to physically remove the clot, can be performed. This intervention generally has a longer therapeutic window, often up to six hours, and in selected cases, can be extended to 16 or even 24 hours based on imaging.
For “wake-up strokes,” where symptoms are noticed upon waking, or strokes of unknown onset, MRI becomes important. Since the exact time of symptom onset is unclear, imaging helps determine if the patient might still be within a treatment window. Imaging can identify salvageable brain tissue, guiding decisions on whether to proceed with time-sensitive treatments.
Common MRI Sequences for Stroke Imaging
Several MRI sequences are used to evaluate and date strokes, providing information about brain tissue. Diffusion-Weighted Imaging (DWI) is sensitive for detecting acute ischemic stroke, often showing changes within minutes to hours of onset. It measures the movement of water molecules in brain tissue; restricted movement, indicating acute injury, appears bright on DWI.
Fluid-Attenuated Inversion Recovery (FLAIR) sequences are useful for differentiating acute from subacute strokes, particularly in wake-up stroke scenarios. While DWI shows acute changes quickly, abnormalities on FLAIR typically become visible later, after 4.5 to 6 hours. A mismatch between positive DWI and negative FLAIR can indicate a stroke that occurred within this earlier timeframe.
T1-weighted and T2-weighted imaging sequences are also used. T2-weighted images can show initial changes like loss of normal arterial signal or hyperintense lesions within hours, while T1-weighted images are less sensitive for early changes but can show cortical laminar necrosis in chronic stages. Susceptibility-Weighted Imaging (SWI) detects hemorrhage or blood products, which is important for distinguishing between ischemic and hemorrhagic strokes and identifying complications. Neurologists combine findings from these different MRI sequences to estimate the stroke’s age.
Challenges in Precisely Dating a Stroke
Despite MRI’s advanced capabilities, precisely pinpointing stroke onset can be challenging. MRI provides an estimate, classifying a stroke into acute, subacute, or chronic phases, but does not always offer exact timing. Factors such as individual patient variability in brain response can influence how quickly changes appear on scans.
The size and location of the stroke also play a role; smaller or differently located strokes might evolve differently on imaging. Pre-existing conditions or atypical stroke presentations can further complicate dating. While MRI provides information for stroke evaluation, it is one component of a larger clinical assessment.