Introduction:
MRI plays a crucial role in the early detection of acute ischemic stroke, especially in the hyperacute window (<6 hours) when CT may appear normal. It is the most sensitive modality for detecting early parenchymal changes, infarct core, and penumbra.
Essential MRI Sequences for Early Stroke Detection:
1. Diffusion-Weighted Imaging (DWI)
- Most sensitive sequence for early ischemia (detects within minutes).
- Infarcted tissue shows hyperintensity (bright) due to restricted diffusion.
- Reflects cytotoxic edema from cellular ATP failure and Na-K pump dysfunction.
- Helps identify the core infarct.
2. Apparent Diffusion Coefficient (ADC) Map
- Complementary to DWI.
- Hypointense (dark) in acute infarct regions.
- Confirms true restriction (rules out T2 shine-through).
- Useful to differentiate acute from chronic infarcts.
3. T2-Weighted and FLAIR (Fluid-Attenuated Inversion Recovery)
- T2W: Hyperintensity in infarcted area due to edema, appears later (~6 hrs).
- FLAIR: Suppresses CSF signal, improves lesion conspicuity in cortical sulci and periventricular region.
- Helps determine lesion age.
- In DWI positive/FLAIR negative cases, infarct is <4.5 hours old → eligibility for thrombolysis.
4. Gradient Echo (GRE) or Susceptibility Weighted Imaging (SWI)
- Detects hemorrhage or microbleeds.
- Useful to exclude hemorrhagic transformation before thrombolysis.
- GRE/SWI: Blooming effect due to paramagnetic substances (deoxyhemoglobin, hemosiderin).
5. Time-of-Flight MR Angiography (TOF MRA)
- Non-contrast angiography to assess intracranial arterial occlusion, stenosis, or dissection.
- Visualizes large vessel occlusion (LVO).
- Can guide endovascular thrombectomy decision-making.
Additional Useful Sequences:
- T1W pre and post contrast (if indicated): To rule out mimics (e.g., tumors, demyelination).
- 3D DWI or multi-b-value DWI: May help in subtle infarcts.
- Arterial spin labeling (ASL): Non-contrast perfusion method in centers without contrast availability.
Advantages of MRI in Early Stroke:
- Detects ischemia within minutes, before CT.
- Differentiates acute, subacute, and chronic infarcts.
- Provides vascular and parenchymal information in a single session.
- Helps assess thrombolysis window (based on DWI/FLAIR mismatch).
Conclusion:
MRI with DWI, ADC, FLAIR, GRE/SWI, and MRA forms the cornerstone of early stroke detection and management. It enables precise localization, estimation of infarct age, and guides treatment decisions such as thrombolysis and thrombectomy.
