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A ventilation/perfusion scan or V/Q scan is a radiological procedure used to study the circulation of gasses in the lungs. It is usually performed to look for a pulmonary embolism, clot or other abnormal blood flow. It has largely been replaced by the use of a CT scan with radiocontrast. However, when radiocontrast is inappropriate, such as in cases of kidney failure, or there is a concern about radiation exposure a V/Q scan is a safer alternative with a lower radiation dose that can be more carefully controlled.

The ventilation procedure is performed by having the patient inhale an inert gas which contains a radioisotope. For perfusion, an isotope containing technetium, a manmade radioactive element, is injected intravenously.

For both parts of the procedure, the isotopes will emit a gamma ray when it decays. A gamma camera or PET scan detects the gamma rays, which create a pixilated image.

In a normal patient, the density of the images will be consistent and will slowly change from the center of each lung outward. However, if there is an obstruction, it will appear light against a much darker background

Ventilation/perfusion scan at Wikipedia