A ventilation/perfusion lung scan, also called a V/Q lung scan, is a type of medical imaging using scintigraphy and medical isotopes to evaluate the circulation of air and blood within a patient's lungs, in order to determine the ventilation/perfusion ratio. 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 blood circulates within the lungs. As Q in physiology is the letter used to describe bloodflow the term V/Q scan emerged.
Ventilation-perfusion scintigraphy in a woman taking oral contraceptives and valdecoxib with a pulmonary embolism. (A) After inhalation of 20 mCi of Xenon-133 gas, scintigraphic images were obtained in the posterior projection, showing uniform ventilation to lungs.. (B) After intravenous injection of 4.1 mCi of Technetium-99m-labeled macroaggregated albumin, scintigraphic images were obtained, shown here in the posterior projection. This and other views showed decreased activity in the following regions: apical segment of right upper lobe, anterior segment of right upper lobe, superior segment of right lower lobe, posterior basal segment of right lower lobe, anteromedial basal segment of left lower lobe, and lateral basal segment of left lower lobe.
V/Q Scan Interpretation
No perfusion deficit
Excludes pulmonary thromboembolism
Perfusion deficit with matched ventilation deficit
< 20% probability of PE
Perfusion deficit that corresponds to parenchymal abnormality on chest x-ray
20% - 80% probability of PE
Multiple segmental perfusion deficits with normal ventilation
> 80% probability of PE
Decreased uptake of the inhaled radioisotope may indicate an 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 shaped) configuration with normal ventilation images (mismatched defect) suggests a pulmonary embolus or blood clot in the lungs, which leads to reduced perfusion beyond the obstruction.
Although this test uses radioactive materials, the total amount of radiation exposure is low. Typical effective doses for a V/Q scan range from 1-5 mSv. In order to decrease the radiation exposure in pregnant patients, the total radioactivity administered may be decreased or the ventilation phase omitted. A CT pulmonary angiogram (CTPA) with radiocontrast can alternatively be performed, although this can result in a greater radiation dose to the patient.
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 indicate pulmonary embolism.
It is also possible to perform the scan with positron emission tomography (PET) rather than conventional gamma camera scintigraphy. This has been performed with Gallium-68 labelled carbon nanoparticles (Galligas) using a conventional Technegas machine for ventilation images, and with Gallium-68 labelled MAA (Ga68-MAA) for perfusion images. PET has multiple potential advantages including superior resolution, speed and quantification.
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