Lung perfusion scan

A Lung Perfusion Scan, also known as a Pulmonary Perfusion Study, is a type of nuclear medicine imaging that evaluates the blood flow distribution within the lungs. It specifically assesses how well blood is circulating through the pulmonary arteries, which are the vessels that carry blood from the heart to the lungs for oxygenation.This test is simple, non-invasive, and highly useful in diagnosing or ruling out several serious lung and cardiovascular conditions. One of the most important clinical applications of the lung perfusion scan is in the diagnosis of pulmonary embolism (PE)—a potentially life-threatening condition caused by a blood clot blocking a pulmonary artery.

Apart from pulmonary embolism, the lung perfusion scan also plays a vital role in pre-operative evaluations before procedures such as lung cancer surgery, lung volume reduction surgery, and lung transplantation. It helps quantify the regional pulmonary function, offering insights into which parts of the lungs are functioning normally and which are compromised.

What is a Lung Perfusion Scan?

A lung perfusion scan is a specialized test that evaluates the blood supply to the lungs using a small amount of radioactive material. This material is injected into a vein and travels through the bloodstream to the lungs. A gamma camera then captures detailed images of how blood is distributed throughout the lungs.

The study provides essential information about:

• The pattern of blood flow in the lungs
• Any areas of reduced or absent perfusion
• The extent of lung damage or blockage
• The functional contribution of different lung segments

Clinical Indications for Lung Perfusion Scan

The lung perfusion scan is indicated in a wide variety of clinical scenarios, especially when information about pulmonary blood flow is crucial for diagnosis or surgical planning:

• Pulmonary embolism (PE) – To detect or rule out the presence of clots in the pulmonary arteries
• Pre-operative evaluation – To quantify regional lung function before lung cancer resection, volume reduction surgery, or lung transplant
• Assessment of chronic lung diseases such as COPD (Chronic Obstructive Pulmonary Disease) or interstitial lung disease.
• Differentiation of vascular vs non-vascular causes of pulmonary abnormalities seen on X-ray or CT
• Evaluation of pulmonary hypertension or vascular anomalies
• Post-operative monitoring after lung surgery
• Unexplained shortness of breath or decreased oxygen levels
  Evaluation of lung perfusion in congenital heart diseases, especially in pediatric patients

How is the Lung Perfusion Scan Performed?

1. The patient lies on a scanning table, and a small amount of radiopharmaceutical (commonly 99mTechnetium-labeled macroaggregated albumin, or 99mTc-MAA) is injected into a vein, typically in the arm.
2. The injected tracer travels to the lungs via the bloodstream, lodging in the capillaries proportional to the blood flow.
3. A gamma camera takes multiple images of the lungs from different angles to evaluate blood distribution patterns.
4. The entire procedure usually takes about 20–30 minutes, and the patient can go home shortly afterward.

Note: In certain cases, a lung ventilation scan may also be performed alongside to complete a V/Q (Ventilation-Perfusion) Scan, which compares air flow and blood flow in the lungs.

What Does the Scan Reveal?

A lung perfusion scan can provide critical insights into the lung’s blood supply, such as:

• Normal perfusion – Uniform distribution of tracer across both lungs
• Segmental or lobar defects – May indicate pulmonary embolism or tumor obstruction
• Reduced perfusion in specific lung regions – Helps in assessing the viability of lung tissue before surgery
• Mismatch with ventilation scan – Suggestive of PE (Perfusion defects without ventilation defect)
• Matched defects (both ventilation and perfusion decreased) – Indicates chronic diseases like COPD or fibrosis

Why is Lung Perfusion Imaging Important Before Lung Surgery?

In oncological settings, such as lung cancer surgery, or procedures like lung volume reduction, surgeons need to know:

• Which parts of the lungs are functioning well
• Which regions are damaged or poorly perfused
• Whether removing a lobe or segment will leave enough functioning lung for the patient to breathe effectively

By quantifying relative perfusion, this scan guides the surgical approach and helps minimize post-operative complications.

Comparison with Other Imaging Techniques

While CT and MRI scans provide structural details, a lung perfusion scan offers functional information that is often crucial for diagnosis and planning:

• CT Pulmonary Angiography (CTPA) is the gold standard for detecting pulmonary embolism, but it may not always be feasible, especially in patients with kidney dysfunction or contrast allergies.
• Lung perfusion scans are an excellent non-contrast alternative, particularly helpful in pregnant women, children, or those with renal insufficiency.
• MRI has limited use in lung perfusion due to technical limitations in air-filled spaces.
• Lung perfusion scans can detect chronic embolic disease, which may be missed on CTPA.

Benefits of Lung Perfusion Scan

• Non-invasive and painless procedure
• Quick and accurate assessment of lung blood flow
• Helps in early detection of pulmonary embolism
• Provides quantitative data for surgical planning
• Useful in high-risk patients who cannot undergo CT
• Minimal radiation exposure compared to CT angiography
• Effective in pediatric and pregnant patients
• Can guide therapy and prognosis in chronic lung conditions

Limitations of the Scan

1. May not detect very small emboli
2. Functional changes may not always correlate with anatomical imaging
3. Interpretation can be affected by underlying lung diseases like emphysema or fibrosis
4. Inconclusive results may require additional imaging such as CT or V/Q scan

Patient Preparation and Safety

• There is no specific preparation required; fasting is not necessary.
• Patients should inform the doctor if they are pregnant or breastfeeding.
• The radiotracer used is safe, with minimal risk of allergic reaction.
• Patients can resume normal activities immediately after the test.
• Drink plenty of water post-scan to help flush the tracer from the body.

After the Test

The results are interpreted by a nuclear medicine physician and are typically available within 24 hours. These results are sent to the referring doctor, who will correlate them with other tests and clinical findings to plan appropriate treatment or surgical intervention.

Conclusion

The Lung Perfusion Scan is a powerful, non-invasive tool for evaluating pulmonary blood flow and plays an essential role in diagnosing pulmonary embolism, assessing lung function, and planning surgical treatments. Its ability to quantify regional perfusion makes it especially valuable in pre-operative evaluations and managing chronic lung diseases. Safe, quick, and highly informative, the lung perfusion study remains a cornerstone in functional pulmonary imaging—particularly when structural imaging alone doesn't tell the full story.

If you are experiencing unexplained shortness of breath, have risk factors for blood clots, or are scheduled for lung surgery, your doctor may recommend this test to ensure your lungs are functioning properly and to guide your care accordingly.