In acute ischemic stroke, what does a hypointense signal on the ADC (apparent diffusion coefficient) map indicate?

Explanation

## ADC Maps in Acute Ischemic Stroke **Key Point:** A **hypointense (dark) ADC signal** confirms **restricted diffusion**, which is pathognomonic for acute cytotoxic edema in ischemic stroke. ### Understanding DWI and ADC Diffusion-weighted imaging (DWI) and ADC maps are complementary: | Finding | DWI Signal | ADC Signal | Interpretation | |---|---|---|---| | **Acute ischemia** | Hyperintense (bright) | Hypointense (dark) | Restricted diffusion = cytotoxic edema ✓ | | **T2 shine-through** | Hyperintense | Hyperintense | Vasogenic edema (chronic, not acute) | | **Normal tissue** | Isointense | Isointense | Free water diffusion | | **Chronic infarct** | Hyperintense | Hyperintense | Gliosis and encephalomalacia | **High-Yield:** The **combination** of hyperintense DWI + hypointense ADC is the hallmark of acute ischemic stroke and distinguishes it from other causes of T2 hyperintensity. ### Why ADC Matters 1. **Excludes T2 shine-through:** Chronic lesions (gliosis, vasogenic edema) appear bright on both DWI and ADC 2. **Confirms cytotoxic edema:** Hypointense ADC = water molecules trapped inside cells (restricted motion) 3. **Timing:** Hypointense ADC appears within **minutes** of stroke onset 4. **Prognostic value:** Extent of restricted diffusion correlates with final infarct size **Clinical Pearl:** If DWI is hyperintense but ADC is also hyperintense (T2 shine-through), the lesion is **not acute ischemia**—it is chronic infarction, gliosis, or vasogenic edema. Always check the ADC map! **Mnemonic: ADC DARK = Acute Diffusion Crisis** — Hypointense (dark) ADC = acute cytotoxic edema from ischemia.