V/Q Scan - Indications, Symptoms, Procedure, Preparation, Interpretation, and Complications

Introduction:

V/Q scan, or pulmonary ventilation (V) and perfusion (Q) scan, is a nuclear medicine imaging device used to determine the airflow (ventilation) and blood flow (perfusion) to the lungs.V/Q scan is commonly used for the diagnosis of pulmonary embolism. Pulmonary embolism is a lung disorder in which blood vessels supplying the lungs are blocked by the formation of blood clots.

What Are the Indications of a Pulmonary Ventilation and Perfusion Scan?

The doctor will advise a V/Q scan to determine the following conditions:

• Find out the blood clots in the lungs (pulmonary embolism).

• Assess the lung function before surgery or lung transplant.

• Assess the lung function in patients having lung diseases such as COPD (chronic obstructive pulmonary disease).

Sometimes, patients are asked to take a V/Q scan when they have the symptoms or risk factors of pulmonary embolism.

What Are the Symptoms of Pulmonary Embolism?

• Increased heart rate.

• Chest pain.

• Difficulty in breathing.

• Hemoptysis (coughing up blood).

What Are the Risk Factors of Pulmonary Embolism?

• Clotting disorder.

• Cancer.

• Heart disorders.

• COPD (a lung disease that obstructs the airway and makes breathing difficult).

• Family history of pulmonary embolism.

• Obesity.

• Smoking.

• Older people.

• Inactivity for longer periods (bed rest or long car travel).

What Are the Radiotracers Used in This Scan?

Ventilation Scan:

• 99Tc- DTPA is the most commonly used radiotracer in ventilation scans at a dosage of 900-1300 MBq with a photopeak of 140KeV (kilo-electron volt).

• 81mKr at a dosage of 40 to 400 MBq (megabecquerel) with a photopeak of 81KeV.

• 133Xe at a dosage of 200 to 750 MBq with a photopeak of 190 KeV.

Perfusion Scan:

• 99mTc-MAA at a dosage of 40 to 150 MBq.

• 133Xe is recommended for patients with obstructive (blockage) airway disorder.

How Do Patients Prepare for the V/Q Scan?

• Before the V/Q scan, the health provider will ask the patients to sign a consent form.

• Patients are asked to have a chest X-ray 24 to 48 hours before the scan.

• Patients should inform their doctor about their medical conditions and regular medicines.

• The health provider will reduce the dosage of radiotracers in pregnancy as babies in the womb are more sensitive to radioactive tracers.

• Breastfeeding women are instructed to avoid feeding for about 48 hours after the scan, as the radioactive materials may pass through the breast milk.

• Patients should tell their doctor if they had allergic reactions to radiotracers from their previous scan.

• Patients may be asked to remove any jewelry or objects over the target area before the scan, as they affect the accuracy of the result.

How Is the V/Q Scan Taken?

V/Q scan is taken as a ventilation scan and perfusion scan alone or both at the same time. The whole scan usually takes about 30 to 45 minutes to complete.

• Ventilation Scan Procedure: For a ventilation scan, patients are asked to lie on the imaging table. Patients are given a disposable nebulizer containing a mouthpiece through which they are asked to breathe in gas (containing radiotracers). The images of the lungs are taken with a gamma camera at different angles while breathing in the gas. The radiotracers commonly used in the scan are xenon or technetium. Patients are instructed to hold their breath for a few seconds to get better images. Finally, the mouthpiece is removed. Gas in their lungs will eventually come out while breathing normally.

• Perfusion Scan Procedure: For a perfusion scan, patients may be asked to cough and take deep breaths before receiving a radiotracer injection. The doctor may inject the radioactive tracer into the vein of the patient's arm through an intravenous (IV) line. The radiotracer passes through the blood vessels and reaches the lungs. A gamma camera is used in the scanner to take the images while the radiotracer passes through blood vessels and collects in the lungs. Patients may be asked to change their body position during the scan to get the required images at different angles. The radiotracer used in perfusion scan is usually technetium.

What Are the Post-scan Instructions Given to the Patients?

• Patients can continue their regular activities after the scan.

• Limit contact with pregnant women and infants.

• Drink plenty of water which helps to flush out the tracers from your body. Usually, it takes about two to three days to come out completely.

• Patients should inform the doctor if they face any discomfort or swelling at the injection site.

What Are the Complications of the V/Q Scan?

The risks of a V/Q scan are very less. They are:

• Bleeding or swelling at the injection site.

• Infection at the injection site.

• Allergic reaction to radiotracers.

• Radiation exposure.

What Are the Interpretations of the V/Q Scan?

The results of the ventilation and perfusion scan are combined for the interpretation of the V/Q scan.

1. Normal Result: The radiotracer is found to be homogeneous (evenly spread out). No perfusion or ventilation defects or perfusion exactly outlines the shape of the lungs on the chest X-ray.

2. Abnormal Results: Defect in V/Q scan is classified into three types:

• Matched - Both ventilation and perfusion defects are present.

• Mismatched - Defect in perfusion with normal or almost normal ventilation found in pulmonary embolism, radiation therapy, tumor blocking the artery.

• Reverse matched - Defect in ventilation with normal or less defective perfusion.

The size of the defect is determined as:

• Large (greater than 75 percent of the segment.)

• Moderate (25 to 75 percent of the segment.)

• Small (less than 25 percent of the segment.)

What Are the Criteria Used in the V/Q Scan?

For the interpretation of the V/Q scan, different criteria are used. They are:

• Original PIOPED (prospective investigation of pulmonary embolism diagnosis) criteria.

• Modified PIOPED ii criteria.

• Perfusion only modified PIOPED criteria.

• Perfusion only PISAPED (prospective investigative study of acute pulmonary embolism diagnosis) criteria.

Modified PIOPED ii criteria are commonly used for V/Q scan interpretation. It classifies the results of the V/Q scan as follows:

High Probability:

• Two or more large (greater than 75 percent of a segment) mismatched segments are present.

• Equivalent segmental defects.

Very Low Probability:

• One to three small defective segments.

• Two or more matched defects with a normal chest X-ray.

• Smaller perfusion defect than lesion in the chest X-ray.

• Single matched defect.

Non-diagnostic:

• All the other findings.

What Are the Factors Affecting the Results of the V/Q Scan?

• Obstructive lung disorder affects the accuracy of the result. In such patients, bronchodilators are used to dilate the airway prior to the scan.

• Heart failure.

• Position change from ventilation scan to perfusion scan.

• Blood clot formation during administration of radiotracer as an injection.

Conclusion:

V/Q scan is a noninvasive imaging technique used to diagnose lung disorders. This scan can be recommended in pregnancy except in the first trimester by reducing the dosage of radiotracers only when needed. Though V/Q scans have radiation exposure, the radiation exposure is very minimal. This scan is a quick diagnostic tool that does not require hospitalization.