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A ventilation/perfusion lung scan, also called a V/Q lung scan, is a type of medical imaging used to evaluate the circulation of air and blood within a patient's lungs. The ventilation part of the test looks at the ability of air to reach all parts of the lungs, while the perfusion part evaluates how well the blood circulates within the lungs. This test is most commonly done in order to check for the presence of a blood clot or abnormal blood flow inside the lungs (pulmonary embolism or PE), although computed tomography with radiocontrast is now more commonly used for this purpose. A V/Q lung scan may also be performed in the case of serious lung disorders such as COPD or pneumonia as well as a lung performance quantification tool pre and post lung lobectomy surgery.
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Procedure
The ventilation and perfusion phases of a V/Q lung scan are performed together and may include a chest x-ray for comparison or to look for other causes of lung disease. A defect in the perfusion images requires a mismatched ventilation defect to be indicative of pulmonary embolism.
In the ventilation phase of the test, a gaseous radionuclide xenon or technetium DTPA in an aerosol form is inhaled by the patient through a mouthpiece or mask that covers the nose and mouth. The perfusion phase of the test involves the intravenous injection of radioactive technetium macro aggregated albumin (Tc99m-MAA). A gamma camera acquires the images for both phases of the study.
Significance of results
Decreased uptake of the inhaled radioisotope may indicate the impaired ability to breathe, airway obstruction, or possible pneumonia.
Decreased circulation of the injected MAA indicates a problem with blood flow into or within the lungs. A localized area of decreased uptake, usually in a wedge shaped (or pie) configuration with normal ventilation images (mismatched defect) suggests a pulmonary embolus or blood clot in the lungs.
Risks
Although this test uses radioactive materials, the total amount of radiation exposure is low. In order to decrease the radiation exposure in pregnant patients, the total radioactive dose may be decreased or the ventilation phase omitted. Computed tomography with radiocontrast can alternatively be performed. If breastfeeding, patient must be counselled to refrain from this activity for approximately 24 hours.
See also
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