## Imaging Findings Interpretation **Key Point:** The combination of DWI hyperintensity with ADC hypointensity is pathognomonic for acute ischemic stroke. This represents restricted diffusion due to cytotoxic edema — the hallmark of acute infarction. ## Why DWI-ADC Mismatch Confirms Acute Ischemia | Imaging Sequence | Acute Ischemia | Chronic Infarction | Vasogenic Edema | |---|---|---|---| | **DWI** | Hyperintense (bright) | Hyperintense | Hypointense | | **ADC** | Hypointense (dark) | Hyperintense | Hyperintense | | **T2/FLAIR** | May be normal early | Hyperintense | Hyperintense | | **Timing** | Minutes to hours | Days to weeks | Variable | **High-Yield:** DWI becomes positive within 30 minutes of stroke onset; ADC changes follow within hours. This DWI-ADC mismatch is the earliest and most specific sign of acute ischemia and appears before conventional T2 changes. ## Clinical Decision Point ```mermaid flowchart TD A[Acute stroke symptoms < 4.5 hrs]:::outcome --> B[Non-contrast CT normal]:::outcome B --> C[MRI DWI/ADC]:::action C --> D{DWI+ with ADC-?}:::decision D -->|Yes: Acute ischemia confirmed| E[Check MRA for LVO]:::action E --> F{LVO present?}:::decision F -->|No| G[IV alteplase]:::action F -->|Yes| H[Thrombectomy ± alteplase]:::action D -->|No: Not acute ischemia| I[Consider other diagnosis]:::outcome ``` **Clinical Pearl:** The patient is at 6 hours from onset — still within the extended thrombolytic window (up to 4.5 hours) if symptom onset is precisely documented, or within the thrombectomy window (up to 24 hours in selected cases with penumbra). MRA shows no LVO, so mechanical thrombectomy is not indicated. Intravenous alteplase is the appropriate next step. **Warning:** Do not confuse DWI hyperintensity with T2 hyperintensity. Early acute ischemia may appear normal on T2/FLAIR but bright on DWI — this is the key diagnostic advantage of DWI. [cite:Harrison 21e Ch 379] 
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