Introduction
Stroke is a medical emergency where early diagnosis and treatment are essential. In acute ischemic stroke, blood supply to a part of the brain is interrupted, leading to tissue damage. Not all brain tissue is equally affected — some areas are irreversibly damaged (infarct core), while others are hypoperfused but salvageable (penumbra).
Perfusion imaging helps identify this penumbra, guiding treatment decisions such as thrombolysis or mechanical thrombectomy. It shifts stroke management from a time-based approach to a tissue-based approach, especially useful in patients presenting beyond the traditional treatment window.
Physiological Basis
Perfusion imaging evaluates:
- Cerebral Blood Flow (CBF): Volume of blood passing through a unit of brain tissue per minute.
- Cerebral Blood Volume (CBV): Total volume of blood in a given brain tissue volume.
- Mean Transit Time (MTT): Time taken for blood to pass through a specific brain region.
- Time to Peak (TTP): Time from contrast injection to peak enhancement.
These parameters help determine:
- Infarct core (permanently damaged tissue)
- Penumbra (at-risk but viable tissue)
Imaging Modalities for Perfusion
1. CT Perfusion (CTP)
- Performed by injecting iodinated contrast and rapidly scanning the brain.
- Generates color-coded maps of CBF, CBV, MTT, and TTP.
- Interpretation:
- Core infarct: ↓CBF, ↓CBV, ↑MTT, ↑TTP
- Penumbra: ↓CBF, normal/increased CBV, ↑MTT, ↑TTP
2. MR Perfusion (MRP)
- Uses gadolinium contrast to measure perfusion parameters (same as CT).
- Often combined with Diffusion Weighted Imaging (DWI).
- Mismatch between perfusion and diffusion maps helps define:
- DWI abnormality = infarct core
- Perfusion abnormality > DWI = penumbra
3. Arterial Spin Labeling (ASL)
- Contrast-free MR perfusion technique using magnetic labeling of blood.
- Useful in patients with renal dysfunction or contrast allergy.
- Limited availability and longer acquisition times.
Clinical Applications
- Treatment Decision-Making:
- Identifies patients with salvageable brain tissue (penumbra).
- Helps extend treatment window for thrombectomy up to 24 hours, guided by perfusion mismatch.
- Used in major trials like DEFUSE-3 and DAWN.
- Avoids Harmful Recanalization:
- Recanalizing a brain already fully infarcted may cause hemorrhage or edema.
- Perfusion imaging helps exclude such patients.
- Prognostication:
- Extent of infarct and penumbra predicts recovery potential.
- Guides family counseling and rehabilitation planning.
Advantages
- Enables personalized treatment beyond time limits.
- Helps reduce futile thrombolysis or thrombectomy.
Limitations
- CTP uses radiation and contrast (risk in renal impairment).
- MRP is time-consuming and less available in emergencies.
- Requires post-processing software and trained personnel.
- Susceptible to motion artifacts and technical errors.
Conclusion
Perfusion imaging has become a cornerstone of acute stroke imaging, especially for patients arriving beyond the conventional 4.5-hour window. By identifying salvageable brain tissue, it helps make timely decisions that can significantly improve outcomes.
