A 72-year-old woman with atrial fibrillation presents with acute right-sided weakness and aphasia. CT head is performed emergently and shows no acute hyperdensity. MRI brain with perfusion-weighted imaging (PWI) and diffusion-weighted imaging (DWI) is obtained 4 hours after symptom onset. Which imaging pattern best distinguishes a salvageable penumbra from established infarction?

Explanation

## Penumbra vs. Infarct Core: The DWI-PWI Mismatch Concept ### Tissue Compartments in Acute Ischemia Acute ischemic stroke creates two distinct tissue zones: 1. **Ischemic Core** (irreversibly infarcted) - Cytotoxic edema present from onset - Shows **restricted diffusion** (high DWI signal, low ADC) - Metabolically dead; cannot be salvaged 2. **Penumbra** (at-risk but viable) - Hypoperfused but not yet infarcted - Shows **PWI abnormality** (perfusion delay or hypoperfusion) - **DWI remains normal** (no cytotoxic edema yet) - Can be salvaged with timely reperfusion (thrombolysis, thrombectomy) ### The Mismatch Concept: PWI-DWI Mismatch = Penumbra **High-Yield:** A **PWI-DWI mismatch** (abnormal perfusion but normal diffusion) represents the **salvageable penumbra**. This is the tissue that benefits most from urgent reperfusion therapy. ```mermaid flowchart TD A[Acute Ischemic Stroke]:::outcome --> B{DWI-PWI Pattern?}:::decision B -->|DWI+ PWI+<br/>Same area| C[DWI-PWI Match]:::outcome C --> D[Core infarction<br/>+ some penumbra]:::outcome D --> E[Limited salvage potential]:::action B -->|DWI- PWI+<br/>Different areas| F[PWI-DWI Mismatch]:::outcome F --> G[Penumbra only<br/>Core minimal]:::outcome G --> H[Maximal salvage potential]:::action B -->|DWI+ PWI-<br/>Rare| I[Luxury perfusion<br/>or reperfused]:::outcome ``` ### Comparison Table: DWI-PWI Patterns | Pattern | DWI | PWI | Tissue Status | Salvage Potential | Clinical Implication | |---------|-----|-----|----------------|------------------|----------------------| | **PWI-DWI Mismatch** | Normal | Abnormal | Penumbra (viable) | **High** | Urgent reperfusion indicated | | **DWI-PWI Match** | Abnormal | Abnormal | Core + penumbra | Moderate | Some salvage possible | | **DWI+ PWI−** | Abnormal | Normal | Reperfused or luxury perfusion | Low | Late presentation or prior recanalization | | **DWI− PWI−** | Normal | Normal | No ischemia | None | TIA or stroke mimic | ### Clinical Pearl: Time Window Implications **Clinical Pearl:** In the **4–24 hour window** (late presentation), PWI-DWI mismatch identifies patients who may still benefit from thrombectomy, even beyond the traditional 4.5-hour thrombolysis window. This is the basis for **extended window thrombectomy** selection in recent trials (DAWN, DEFUSE 3). ### Why This Matters **Key Point:** The **volume of mismatch** (PWI volume minus DWI volume) predicts the amount of salvageable tissue. A large mismatch (e.g., PWI 100 mL, DWI 20 mL) suggests substantial penumbra; a small or absent mismatch suggests limited salvage potential.