## Distinguishing Acute Ischemic Stroke from TIA: The Role of DWI-MRI ### Key Imaging Principle **Key Point:** Diffusion-weighted imaging (DWI) detects cytotoxic edema within minutes of ischemic stroke onset, making it the most sensitive and specific tool to differentiate acute stroke from TIA. ### Pathophysiology of Restricted Diffusion In acute ischemic stroke, cytotoxic edema develops within **minutes** due to failure of Na⁺/K⁺-ATPase pumps. Water molecules become restricted in their movement, producing: - **High signal intensity on DWI** - **Low apparent diffusion coefficient (ADC) values** (typically <600 × 10⁻⁶ mm²/s) In TIA, there is **no cytotoxic edema** because collateral flow prevents infarction; therefore, DWI remains normal. ### Comparison Table: Acute Stroke vs. TIA on MRI | Feature | Acute Ischemic Stroke | TIA | |---------|----------------------|-----| | **DWI signal** | Hyperintense (restricted diffusion) | Normal | | **ADC values** | Low (<600) | Normal | | **Onset of DWI change** | Minutes (before CT hypodensity) | No change | | **T2/FLAIR** | May be normal in first 6–8 hours | Normal | | **Clinical outcome** | Persistent neurological deficit | Complete resolution (by definition) | ### High-Yield Timing Insight **High-Yield:** DWI is **positive within 30 minutes** of stroke onset, whereas conventional CT may appear normal for 6–24 hours. This makes DWI the gold standard for early diagnosis and thrombolytic eligibility assessment. ### Clinical Pearl **Clinical Pearl:** A patient with TIA-like symptoms but **positive DWI** has had a **stroke** (by definition), even if symptoms resolve. This is sometimes called a "DWI-positive TIA" and carries higher recurrence risk. ### Why ADC Matters - **Low ADC + high DWI** = cytotoxic edema (acute infarction) - **Normal ADC + normal DWI** = no infarction (TIA, or mimics like migraine, Todd's paralysis) 
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