## Acute Ischemic Stroke Imaging Sensitivity **Key Point:** Diffusion-weighted imaging (DWI) MRI is the gold standard for detecting acute ischemic stroke and can identify cytotoxic edema within minutes of symptom onset, making it far more sensitive than conventional CT in the hyperacute phase. ### Imaging Modality Comparison | Modality | Sensitivity (Acute) | Timing | Purpose | |----------|---------------------|--------|----------| | Conventional CT | ~10–20% | Hyperacute | Rule out hemorrhage | | DWI-MRI | ~90–95% | Hyperacute (minutes) | Detect cytotoxic edema | | CT perfusion | ~70–80% | Hyperacute | Assess penumbra | | CTA | Not for ischemia | Hyperacute | Assess vessel patency | ### Why DWI is Superior 1. **Cytotoxic edema detection:** DWI shows restricted water diffusion in ischemic tissue within minutes, before conventional imaging changes appear. 2. **Timing independence:** DWI remains positive for days to weeks after stroke onset, unlike conventional CT which may appear normal for 6–12 hours. 3. **Clinical correlation:** DWI lesions correlate with clinical deficits and infarct volume better than any other modality. **Clinical Pearl:** Conventional CT is performed first in acute stroke to exclude hemorrhage (contraindication to thrombolysis), but MRI-DWI is the most sensitive for confirming acute ischemia. **High-Yield:** DWI-MRI sensitivity for acute stroke is ~90–95%; conventional CT sensitivity is only ~10–20% in the first 6 hours. This is a high-yield fact for NEET PG. 
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