On DWI (diffusion-weighted imaging), an acute ischaemic infarct appears bright (restricted diffusion) due to which mechanism?
- A Cytotoxic oedema causing failure of Na-K ATPase, leading to intracellular water accumulation and reduced Brownian motion ✓
- B Vasogenic oedema causing increased extracellular water with high ADC
- C Blood-brain barrier breakdown leading to contrast extravasation
- D Increased cerebral blood flow and hyperaemia in the penumbra
Explanation
Acute ischaemic stroke causes cytotoxic oedema within 30 minutes: energy failure leads to Na-K ATPase pump failure, intracellular sodium and water accumulation, cell swelling, and reduced extracellular space. This restricts the random Brownian motion (diffusion) of water molecules, producing high signal on DWI and low apparent diffusion coefficient (ADC) values. Vasogenic oedema (BBB breakdown) increases extracellular water, resulting in T2 shine-through on DWI but high ADC — the opposite pattern. DWI is positive in minutes to hours and 'pseudo-normalises' after 7–10 days.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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