## Acute Ischemic Stroke Detection on MRI **Key Point:** Diffusion-weighted imaging (DWI) is the gold standard for detecting acute ischemic stroke and shows hyperintense signal within minutes of symptom onset, even before conventional T2 or FLAIR sequences become abnormal. ### Mechanism of DWI Sensitivity DWI detects restricted water diffusion in cytotoxic edema: 1. Acute ischemia causes failure of Na⁺/K⁺-ATPase pumps 2. Intracellular sodium and water accumulation (cytotoxic edema) 3. Restricted water molecule movement in the intracellular space 4. Reduced apparent diffusion coefficient (ADC) values 5. Hyperintense signal on DWI with corresponding low ADC values ("restricted diffusion") ### Timeline of MRI Findings in Acute Stroke | Sequence | Timing | Sensitivity | |----------|--------|-------------| | **DWI** | **Minutes (0–30 min)** | **95–100%** | | Conventional T2 | Hours (6–8 h) | 60–70% | | FLAIR | Hours (6–8 h) | 70–80% | | PWI | Minutes (shows perfusion deficit) | High, but nonspecific | **High-Yield:** DWI + ADC map combination is pathognomonic for acute ischemia. Restricted diffusion (bright DWI + dark ADC) = acute stroke. Unrestricted diffusion (bright DWI + bright ADC) = chronic or vasogenic edema. **Clinical Pearl:** PWI may show perfusion deficits even before DWI becomes positive, but DWI is more specific for acute infarction. The DWI-PWI mismatch (PWI abnormality larger than DWI abnormality) identifies the ischemic penumbra — tissue at risk of infarction. **Warning:** Do not confuse DWI hyperintensity with T2 hyperintensity. DWI is T2-weighted but also includes diffusion weighting; true restricted diffusion requires correlation with low ADC values. ### Why DWI is Superior in Hyperacute Phase - Cytotoxic edema (intracellular) appears first → DWI positive - Vasogenic edema (extracellular) appears later → T2/FLAIR positive - Acute stroke is primarily cytotoxic in the first hours 
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