A 58-year-old woman with atrial fibrillation presents with acute right-sided hemiparesis and facial droop of 90 minutes duration. Non-contrast CT head is normal. Which imaging finding on DWI-MRI would indicate an acute infarct in the left middle cerebral artery (MCA) territory?

Explanation

## DWI-MRI Signal Characteristics in Acute Ischemic Stroke **Key Point:** Acute ischemia produces **restricted diffusion** — hyperintensity on DWI with corresponding hypointensity on the ADC (apparent diffusion coefficient) map. This pattern is pathognomonic for acute cytotoxic edema. ### Understanding Diffusion-Weighted Imaging **High-Yield:** DWI is sensitive to water molecule motion (diffusion). In acute ischemia: - **Cytotoxic edema** develops due to ATP depletion and failure of Na^+^/K^+^ ATPase - Water shifts from extracellular to intracellular compartment - Intracellular water is **restricted** (cannot move freely) - Restricted diffusion appears **bright (hyperintense)** on DWI ### The DWI-ADC Interpretation Matrix | DWI Signal | ADC Signal | Interpretation | Timing | |------------|------------|-----------------|--------| | **Hyperintense** | **Hypointense** | **Acute ischemia** (restricted diffusion) | **Minutes to days** | | Hyperintense | Hyperintense | Vasogenic edema (tumor, infection, inflammation) | Subacute/chronic | | Hypointense | Hypointense | Chronic infarct (encephalomalacia) | Weeks to months | | Hypointense | Hyperintense | Normal brain (free diffusion) | — | **Clinical Pearl:** The **DWI-ADC mismatch** is critical: - If DWI hyperintense but ADC normal/hyperintense → likely vasogenic edema (not acute ischemia) - If DWI hyperintense AND ADC hypointense → acute cytotoxic edema (acute ischemia) ✓ ### Mechanism: Why Acute Ischemia Shows This Pattern ```mermaid flowchart TD A[Arterial Occlusion]:::outcome --> B[Cessation of Blood Flow]:::action B --> C[ATP Depletion]:::action C --> D[Na+/K+ ATPase Failure]:::action D --> E[Water Shifts Intracellularly]:::action E --> F[Cytotoxic Edema]:::outcome F --> G[Restricted Water Diffusion]:::outcome G --> H[DWI Hyperintense + ADC Hypointense]:::action ``` **Mnemonic:** **DWI-ADC = Acute Ischemia = Diffusion Impaired (bright DWI, dark ADC)** ### Timeline of MRI Changes in Acute Stroke 1. **0–30 minutes:** DWI becomes abnormal (most sensitive); conventional MRI (T1, T2, FLAIR) still normal 2. **Hours:** T2/FLAIR hyperintensity appears 3. **Days–weeks:** Vasogenic edema develops; ADC normalizes (T2 shine-through) 4. **Weeks–months:** Chronic infarct with encephalomalacia; DWI normalizes, ADC hyperintense **High-Yield:** In this patient with 90 minutes of symptoms and normal CT, DWI-MRI will show acute infarction in the left MCA territory with the hyperintense DWI + hypointense ADC pattern, confirming acute ischemia and guiding thrombolytic therapy. [cite:Harrison 21e Ch 370]