How does a ventilation-perfusion scan diagnose pulmonary embolism?

A ventilation-perfusion scan is one way to diagnose pulmonary embolism by testing the flow of air and blood through your lungs. For the ventilation portion of the scan, you inhale harmless radioactive material, which travels to the air sacs in the lungs. Then a scanner maps the air flow in your lungs. The perfusion portion of the scan tests blood flow through your lungs. Some harmless radioactive material is injected into one of your veins, and as the material travels through your lungs in the bloodstream, it shows the blood flow through your lungs. Abnormal airflow and blood flow can indicate pulmonary embolism. Ventilation-perfusion scans have been largely supplanted by helical CT scans for diagnosing PE. These require the use of a contrast dye, however, and may not be appropriate for patients with kidney problems or those with an allergy to the dye.

A Ventilation-Perfusion (V/Q) scan is typically a procedure performed in the Nuclear Medicine section of the Radiology Department of a hospital. There are 2 parts of this test: one to look at ventilation and the other to evaluate the perfusion (blood flow) of the lungs. Each of them uses a very small amount of radioactive material (radiopharmaceutical) which is detected by a special camera called a gamma camera. The first part of the study requires the patient to breathe in the first radiopharmaceutical from a mask and takes ~5 minutes while pictures are obtained from the gamma camera. A different radiopharmaceutical is injected in to an arm vein and the same set of images of the lungs is obtained to compare against the ventilation images. The entire procedure takes ~1 hour. Areas that ventilate but don't perfuse (aka V/Q mismatch) are suspicious for pulmonary embolus (PE). V/Q studies are assigned a risk for PE by the interpreting Radiologist which the following 4 categories: normal, low, intermediate or indeterminate, and high risk for PE. V/Q scanning for PE has largely been replaced by lung CT scanning due to higher resolution and ability to see the actual clots in the arterial branches of the lungs.