A 58-year-old man with hypertension and diabetes presents with acute left hemiparesis. Non-contrast CT head is normal. MRI is performed within 6 hours of symptom onset. Which imaging finding best distinguishes acute ischemic stroke from transient ischemic attack (TIA) on diffusion-weighted imaging (DWI)?

Explanation

## Distinguishing Acute Ischemic Stroke from TIA on MRI ### Key Imaging Principle **Key Point:** The hallmark of acute ischemic stroke is restricted diffusion — hyperintense signal on DWI with corresponding hypointense (low) signal on ADC maps. This reflects cytotoxic edema and represents irreversible tissue injury. ### DWI/ADC Interpretation | Finding | DWI Signal | ADC Signal | Pathology | Clinical Significance | |---------|-----------|-----------|-----------|----------------------| | **Acute ischemia** | Hyperintense (bright) | Hypointense (dark) | Cytotoxic edema | Irreversible injury; stroke | | **TIA** | Normal | Normal | No tissue injury | Complete recovery; no infarction | | **Chronic infarct** | Hyperintense | Hyperintense | Vasogenic edema | Old lesion | | **T2 shine-through** | Hyperintense | Hyperintense | Free water (CSF/edema) | Not true restriction | ### Clinical Pearl **Clinical Pearl:** Up to 30% of patients with clinical TIA have DWI-positive lesions on MRI, indicating silent infarction. However, the presence of restricted diffusion (low ADC) confirms acute ischemia, whereas TIA by definition shows no DWI abnormality or only T2 shine-through without ADC restriction. ### High-Yield Distinction **High-Yield:** DWI positivity with ADC restriction is the **gold standard** for confirming acute ischemic stroke within the first 7–10 days. This finding: - Persists for 7–10 days (peak at 3–5 days) - Correlates with clinical deficit severity - Guides thrombolytic therapy decisions - Distinguishes stroke from stroke mimics ### Why This Matters **Mnemonic:** **DWI-ADC Mismatch = Stroke** - **D** = DWI hyperintense (bright) - **W** = Water restricted (low ADC) - **I** = Irreversible injury In TIA, the brain recovers completely; therefore, no permanent diffusion restriction occurs. MRI obtained during or shortly after TIA may show normal DWI/ADC or only T2 hyperintensity without ADC restriction. [cite:Harrison 21e Ch 369]