On DWI-MRI, a 65-year-old woman with acute left hemiplegia shows a wedge-shaped area of restricted diffusion in the right MCA territory involving cortex and subcortical white matter. The ADC map shows a corresponding dark (low ADC) area. What is the earliest MRI sequence to show ischemic infarction, and what is the mechanism of restricted diffusion?
- A DWI; cytotoxic edema causes water movement intracellularly, reducing free molecular motion ✓
- B T2-weighted imaging; restricted diffusion due to increased extracellular water
- C FLAIR; restricted diffusion due to vasogenic edema expanding extracellular space
- D T1 post-contrast; breakdown of blood-brain barrier allows gadolinium entry into infarcted tissue
Explanation
DWI is the earliest and most sensitive MRI sequence for detecting acute ischemia — it becomes positive within 30–60 minutes of ischemia onset. Cytotoxic edema (cell swelling) occurs as Na-K-ATPase fails due to energy depletion; intracellular water accumulation restricts random Brownian motion of water molecules, producing high DWI signal and low ADC (apparent diffusion coefficient). FLAIR becomes positive only after 4–6 hours. T2 changes take 6–12 hours. Gadolinium enhancement (blood-brain barrier breakdown) occurs from 24–72 hours. This DWI-FLAIR mismatch (DWI positive + FLAIR negative) helps identify strokes within the thrombolysis window.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.