A 62-year-old woman with atrial fibrillation presents with acute right-sided weakness. CT head at 2 hours is normal. MRI obtained at 4 hours shows a focal area of restricted diffusion in the left middle cerebral artery (MCA) territory. Which imaging feature on perfusion-weighted imaging (PWI) best distinguishes a salvageable penumbra from established infarct core?

Explanation

## PWI-DWI Mismatch: The Ischemic Penumbra ### Core Concept: Tissue Viability Window **Key Point:** The ischemic penumbra is tissue that is hypoperfused but not yet infarcted. It is identified on MRI as a **PWI-DWI mismatch** — a region of perfusion abnormality (PWI positive) that exceeds the diffusion abnormality (DWI positive). This mismatch represents salvageable brain tissue. ### Pathophysiology of Ischemic Stroke Zones | Zone | DWI | PWI | Pathology | Prognosis | |------|-----|-----|-----------|----------| | **Infarct Core** | Abnormal | Abnormal | Irreversible cytotoxic edema | Lost tissue | | **Penumbra** | Normal | Abnormal | Hypoperfused but viable | **Salvageable** | | **Oligemia** | Normal | Normal | Mild hypoperfusion | Recovers spontaneously | ### Clinical Significance of Mismatch **High-Yield:** PWI-DWI mismatch is the **imaging marker of the therapeutic window**: - Identifies patients who may benefit from thrombolysis or thrombectomy beyond standard time windows - Larger mismatch = larger penumbra = greater potential for recovery - Absence of mismatch suggests completed infarction; intervention unlikely to help - Used in clinical trials (e.g., DEFUSE, DAWN) to select late-window candidates ### Perfusion-Weighted Imaging Parameters **Clinical Pearl:** PWI detects hypoperfusion by measuring: - **Mean transit time (MTT)** — most sensitive for ischemia - **Cerebral blood flow (CBF)** — reduced in ischemia - **Cerebral blood volume (CBV)** — preserved in penumbra, reduced in core The **core** shows reduced CBF + reduced CBV. The **penumbra** shows reduced CBF + relatively preserved CBV. ### Mnemonic for Mismatch **Mnemonic:** **PWI > DWI = Penumbra (Salvageable)** - **P** = Perfusion defect (larger) - **W** = Wider than diffusion - **I** = Infarct core (smaller) - **Penumbra** = The tissue between them ### Timing and Clinical Decision-Making **Tip:** In acute stroke: 1. **DWI abnormality** = infarct core (irreversible) 2. **PWI abnormality** = total ischemic tissue (core + penumbra) 3. **Mismatch volume** = penumbra volume (potentially recoverable) Large mismatch in a patient 6–24 hours from symptom onset may qualify for mechanical thrombectomy even outside the standard 4.5-hour thrombolysis window. [cite:Harrison 21e Ch 369; Robbins 10e Ch 28]