Ventilation-Perfusion (V/Q) Scan

A V/Q scan is most commonly used to evaluate for a pulmonary embolism, a blood clot that has traveled to the lungs, by comparing patterns of airflow and blood flow. It is also the key screening test for chronic thromboembolic pulmonary hypertension (CTEPH), a distinct form of pulmonary hypertension that can develop when clot material in the lungs does not fully resolve.

Both tests are used to evaluate for a pulmonary embolism, but they work differently. CTPA uses a CT scanner and injected iodinated contrast dye to directly visualize a clot within the pulmonary arteries, and it is the standard, default first-choice test in most situations. A V/Q scan is a nuclear medicine test that compares patterns of airflow and blood flow rather than directly visualizing a clot, does not require iodinated contrast, and involves substantially less radiation. Each test has situations where it is the better choice, and your physician determines which is appropriate for you.

A V/Q scan is often preferred over CTPA specifically for patients with significant kidney impairment, since it avoids iodinated contrast dye that can affect kidney function, and for patients with a prior contrast allergy or reaction. It is also generally preferred during pregnancy, since it involves a substantially lower radiation dose, including to the fetus, than CTPA. Other situations include a normal or only mildly abnormal chest X-ray in a patient with ongoing clinical suspicion, or when a prior CTPA result was inconclusive.

The test has two parts. For the ventilation portion, you inhale a small amount of a radioactive tracer gas or aerosol while images are taken to show how air is distributed throughout your lungs. For the perfusion portion, a different radioactive tracer is injected into a vein, and images show how blood flows through the lungs. A physician compares the two sets of images together, looking for areas with normal airflow but reduced blood flow, a pattern that suggests a blood clot is blocking that area's blood supply.

A V/Q scan is generally considered a reasonable and often preferred imaging option during pregnancy when pulmonary embolism is a genuine clinical concern, since it involves a substantially lower radiation dose, including to the fetus, than CT pulmonary angiography. The specific protocol used may be adjusted for pregnancy. As with any imaging decision during pregnancy, this is a discussion between you and your treating physicians, weighing the real risk of an undiagnosed pulmonary embolism against the radiation exposure of any imaging option.

A V/Q scan uses small amounts of radioactive tracers (a nuclear medicine technique) rather than the X-ray-based technology a CT scanner uses, and overall radiation exposure from a V/Q scan is generally lower than from CT pulmonary angiography. Both are considered safe and medically justified when used for their intended purpose.

V/Q scan results are typically reported using a probability classification that reflects how consistent the imaging pattern is with pulmonary embolism, rather than a simple yes-or-no answer. Your physician interprets this result together with your clinical probability of having a pulmonary embolism, based on your symptoms, history, and exam, to decide whether the result is sufficient to confirm or exclude the diagnosis, or whether further testing is needed.

V/Q SPECT is a more detailed three-dimensional version of ventilation-perfusion imaging that can offer greater sensitivity for smaller perfusion defects than standard two-dimensional (planar) imaging. It is used more selectively in the United States than in some other countries, where it is more commonly the default approach. Both techniques are considered to have strong diagnostic performance for pulmonary embolism when used appropriately.

A V/Q scan is the key screening test for CTEPH and is generally considered more sensitive for this purpose than a standard chest CT, since it can reveal the mismatched perfusion defects characteristic of chronic, unresolved clot material. A V/Q scan alone does not complete a CTEPH diagnosis; right heart catheterization and CT pulmonary angiography are also used to confirm and further characterize the condition. More detail is available on the CTEPH page.