DWI-MRI of a patient with acute right-sided weakness shows diffusion restriction in the left posterior limb of internal capsule with corresponding ADC map hypointensity. At what time window after symptom onset is DWI most sensitive for ischaemic infarction?

• A Within 30 minutes (ultra-acute)
• B 24–48 hours — peak DWI signal
• C Only after 72 hours when T2 changes appear
• D 1–6 hours after onset ✓

Explanation

DWI is the most sensitive and specific sequence for acute cerebral ischaemia, becoming positive within 30–60 minutes of onset in most cases, with sensitivity increasing to >95% by 3–6 hours. The restricted diffusion (cytotoxic oedema — water trapped in swollen cells) is detected as high DWI signal and low ADC values. The DWI signal normalises (pseudo-normalisation) at around 7–10 days as vasogenic oedema replaces cytotoxic oedema. T2-weighted FLAIR changes lag behind, appearing at 6–12 hours. Ultra-acute (<30 min) DWI may occasionally be negative due to very early onset.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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