## Analysis of Imaging Modalities in Acute Ischemic Stroke **Key Point:** Conventional CT has LOW sensitivity for acute ischemic stroke in the first 6 hours. Its primary role is to EXCLUDE hemorrhage, not to detect ischemia. MRI, particularly DWI, is SUPERIOR for detecting acute ischemic changes. ### Why Option B (Correct Answer) is Wrong Conventional CT is **insensitive** in the hyperacute phase (0–6 hours). Early ischemic changes (loss of gray-white differentiation, subtle hypodensity) are often missed on CT. MRI with DWI detects cytotoxic edema within **minutes** of stroke onset, making it far more sensitive and specific for acute ischemia. **High-Yield:** In clinical practice, CT is used to rule out hemorrhage before thrombolysis; MRI (especially DWI/PWI) is the gold standard for confirming acute ischemia and assessing penumbra. ### Why the Other Options are Correct | Feature | Details | |---------|----------| | **DWI hyperintensity (Option A)** | Appears within minutes; represents restricted water diffusion due to cytotoxic edema in acute ischemia | | **PWI hypoperfusion (Option C)** | Shows areas of reduced perfusion; the mismatch between PWI (larger) and DWI (smaller) defines the ischemic penumbra — tissue at risk of infarction | | **CTA (Option D)** | Identifies large vessel occlusions (LVO) in internal carotid, middle cerebral, or basilar arteries; critical for thrombectomy candidacy | **Clinical Pearl:** The **DWI/PWI mismatch** is the imaging hallmark of salvageable tissue and guides decisions for late window reperfusion (up to 24 hours in selected patients). **Mnemonic — SWIFT for acute stroke imaging:** - **S**ensitivity: DWI > conventional CT - **W**hite matter: Gray-white differentiation loss is subtle on early CT - **I**schemic penumbra: Seen on PWI/CTA - **F**ast MRI: DWI/PWI in minutes - **T**hrombus: CTA shows vessel occlusion
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