Lung VQ scan

The Lung VQ scan, also known as Ventilation-Perfusion Scintigraphy, is a specialized nuclear medicine imaging test used to evaluate the airflow (ventilation) and blood flow (perfusion) in the lungs. It is most commonly performed to detect or rule out pulmonary embolism (PE), a potentially life-threatening condition caused by blood clots that block the arteries in the lungs. This diagnostic scan is also used in a range of other clinical situations where understanding regional lung function is essential—such as before lung surgery, lung transplantation, or in cases of chronic lung diseases like COPD or interstitial lung disease.

The lung VQ scan is non-invasive, quick, and safe, making it a preferred diagnostic tool in certain patient groups such as pregnant women, patients allergic to contrast agents, or those with kidney disease who cannot undergo CT scans with contrast.

What is a Lung VQ Scan?

A Lung VQ scan comprises two parts:

1. Ventilation Scan: This portion evaluates how air moves throughout the lungs. The patient inhales a radioactive gas or aerosol, which disperses into the lungs. A gamma camera captures the distribution of the inhaled material to assess the ventilation pattern.
2. Perfusion Scan: This part assesses blood flow to different regions of the lungs. A radioactive tracer, typically 99mTechnetium-labeled macroaggregated albumin (99mTc-MAA), is injected into a vein and travels through the pulmonary arteries. The gamma camera records how blood is distributed across the lung fields.

Together, these two scans allow the physician to compare areas of ventilation and perfusion, revealing whether any blood flow mismatch may be due to blood clots (emboli) or other vascular issues.

Why is the Lung VQ Scan Performed?

The primary and most critical reason for performing a Lung VQ scan is to detect pulmonary embolism (PE), but several other conditions and clinical requirements also warrant this test.

Common Indications Include:

• Diagnosis or exclusion of pulmonary embolism (PE)

Assessment of differential lung function before:

• Lung resection in lung cancer surgery
• Lung volume reduction procedures
• Lung transplantation
• Evaluation of regional lung function in patients with:
• Chronic obstructive pulmonary disease (COPD)
• Interstitial lung disease
• Pulmonary hypertension
• Investigation of unexplained shortness of breath or oxygen desaturation
• Post-operative monitoring in patients with lung surgery or trauma
• Assessment of congenital heart or lung defects, especially in children

Understanding Pulmonary Embolism (PE)

Pulmonary embolism is a serious medical condition where a blood clot (usually from the leg or pelvis) travels to the lungs and becomes lodged in a pulmonary artery, blocking blood flow. The severity depends on:

• The size of the clot
• The number of clots
• The extent of lung involvement

PE can lead to reduced oxygen levels, lung tissue damage, or even death if not promptly diagnosed and treated. The Lung VQ scan is especially useful when:

• CT Pulmonary Angiography (CTPA) is contraindicated due to contrast allergies or renal impairment
• Pregnancy—to avoid high doses of ionizing radiation
• Repeated clots are suspected in patients with chronic or recurrent PE

How is the Lung VQ Scan Performed?

The procedure is simple and painless, typically completed in less than an hour.

1. Ventilation Phase:

• The patient inhales a radioactive gas (e.g., Xenon-133) or aerosolized radiopharmaceutical (e.g., 99mTc-DTPA) via a mask or mouthpiece.
• The gamma camera captures multiple images of the lungs, recording the distribution of air within the airways and alveoli.
• This phase evaluates airflow and helps detect airway obstruction or diseases like asthma or emphysema.

2. Perfusion Phase:

• After the ventilation scan, the patient receives an intravenous injection of 99mTc-MAA.
• This tracer travels through the bloodstream to the pulmonary vasculature.
• The gamma camera records perfusion images from various angles, showing the pattern of blood flow in the lungs.

The images from both phases are compared to identify matched or mismatched defects, which help differentiate between various diseases.

How Are the Results Interpreted?

A nuclear medicine physician interprets the images by comparing the ventilation and perfusion patterns.

Possible Findings:

• Normal VQ scan – Uniform airflow and blood flow throughout both lungs
• Mismatched defect – Area with normal ventilation but reduced or absent perfusion, highly suggestive of pulmonary embolism
• Matched defect – Both ventilation and perfusion are reduced in the same region, often indicating non-vascular lung disease (e.g., COPD, fibrosis, consolidation)
• Triple matched defect – Correlated defect in ventilation, perfusion, and chest X-ray, often considered low probability for PE

Advantages of Lung VQ Scan

• Non-invasive and painless
• Low radiation exposure, particularly in comparison to CT scans
• Does not require iodinated contrast agents
• Safe for patients with kidney issues or allergies to contrast media
• Can be performed in pregnant women with minimal fetal exposure
• Useful in pre-surgical planning for lung resection or transplant
• Provides real-time functional evaluation of both airflow and blood supply
• Helpful in pediatric cases and in patients with chronic lung disorders

Limitations of Lung VQ Scan

1. May produce non-diagnostic or indeterminate results, especially if ventilation is not well visualized
2. Interpretation may be difficult in severe underlying lung diseases
3. Less anatomic detail compared to CT angiography
4. May not detect small or subsegmental emboli

Preparation and Safety

• Minimal preparation is required
• Patients should inform the physician about pregnancy, breastfeeding, or recent respiratory infections
• Usually no need to stop medications, but it should be confirmed with the referring physician
• The radioactive tracers used are very low dose and generally well-tolerated
• Patients can return to normal activities immediately after the scan
• Drinking extra water post-scan helps eliminate the tracer from the body

Comparison with Other Imaging Modalities

• CT Pulmonary Angiography (CTPA) is typically the first-line test for pulmonary embolism, offering excellent anatomical detail.
• However, a Lung VQ scan is equally sensitive in detecting PE in select patient populations.
• VQ scan is the preferred choice in pregnancy, renal failure, or contrast allergy, where CT is contraindicated.
• In chronic lung disease, VQ scans may offer more functional insights, especially when surgery is being considered.

Conclusion

The Lung VQ Scan (Ventilation-Perfusion Scintigraphy) is an essential diagnostic test that evaluates how well air and blood circulate within the lungs. While it is most commonly used to detect or rule out pulmonary embolism, it also plays a key role in evaluating lung function before major surgeries, in patients with chronic respiratory diseases, or those unsuitable for CT imaging. This scan’s unique ability to assess both ventilation and perfusion offers a comprehensive picture of pulmonary health, often providing critical guidance in life-saving medical decisions. Safe, reliable, and non-invasive, the Lung VQ scan remains a cornerstone of pulmonary imaging in modern medicine.