VQ Scan

A VQ scan is a two-part test that providers most commonly use to diagnose a blood clot in your lungs (pulmonary embolism). It measures the airflow (ventilation) and blood flow (perfusion) in your lungs. You breathe in and are injected with radioactive material while a provider takes pictures of your lungs.


What is a VQ scan?

A VQ scan is a two-part, noninvasive test. It takes pictures of your lungs to measure their airflow and blood flow. Providers usually use it to diagnose a blood clot in your lungs.

The “V” stands for ventilation, or airflow in and out of your lungs. The “Q” stands for quantity of perfusion, or blood flow to the small blood vessels in your lungs. You might also hear it called a ventilation-perfusion scan, a lung scan or lung scintigraphy.


Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

What is a VQ scan used for?

Providers most often use a VQ scan to diagnose a blood clot in your lungs (pulmonary embolism, or PE). A pulmonary embolism can be fatal if left untreated. It’s also used to diagnose chronic thromboembolic pulmonary hypertension (CTEPH), a cause of high blood pressure in the blood vessels in your lungs.

Providers can also use a VQ scan to check your lung function:

Can a VQ scan detect lung disease?

Providers use VQ scans to check lung function. They usually use other imaging (like CT scans or chest X-rays) to identify lung disease.

How does a VQ scan work?

A healthcare provider performs a VQ scan in two parts, one right after another on the same day. For the first part (ventilation), they’ll have you breathe in a small amount of radioactive particles (tracer). Think of the tracer like bright tags attached to parts of the air. Your provider uses a special camera — similar to an X-ray or a CT camera — to take pictures of your lungs. The bright tags of the tracer make the air flowing into your lungs show up on the images.

For the second part (perfusion), your provider will give you an injection of a similar tracer. This time, the bright tags flow through your blood vessels. They’ll use the same camera to get pictures of the blood flow in your lungs.

What is the difference between a CT scan and a VQ scan?

CT scans and VQ scans both take pictures of your lungs to help diagnose issues like blood clots. A CT scan uses a camera that takes pictures of a contrast dye in your body. A provider looks at the pictures to identify issues.

A VQ scan is a two-part test. It uses a camera that takes pictures of radioactive particles in your body. A provider can look at one set of pictures or compare the two to understand how your lungs are working.

Procedure Details

How do I prepare for a VQ scan?

You don’t need to do anything specific to prepare for a VQ scan. You can eat and take medications as you normally would. Tell your provider:

  • If you’re pregnant or think you might be. Your provider might suggest a different form of imaging to reduce the risk of radiation exposure to the fetus.
  • If you’re breastfeeding/chestfeeding. Your provider might give you instructions on how to feed your baby on the day of the test or following the procedure. This reduces the risk of exposing your baby to radiation in your milk.
  • If you don’t think you can sit or lay still for several minutes at a time. Movement can blur the images and your provider won’t be able to use them.


How long does a VQ scan take?

A VQ scan takes between 30 and 60 minutes. Each test takes about 15 minutes. Yours may take more or less time depending on prep time and whether your provider’s getting the images they need.

What happens before a VQ scan?

Before you get a VQ scan, you’ll get a chest X-ray. This can be anywhere from one to 24 hours before your scan. This gives your provider more information to help them understand the VQ scan results.


What happens during this procedure?

A VQ scan has two parts: the ventilation test and the perfusion test. Some people will only do one, but most people will do the ventilation test followed immediately by the perfusion test in the same day. A provider will be there to guide you through both parts of the scan. They may ask you to change into a gown for the procedures.

Ventilation scan

During the scan:

  1. Your provider will help you put on a device that you’ll breathe from. This might be a mouthpiece (your provider will also put a clip over your nose to make sure you only breathe through your mouth) or a mask that fits closely over your mouth and nose.
  2. Your provider will attach the mouthpiece or mask to a machine with a hose. They’ll add the tracer to the hose.
  3. You’ll either lie on a table or sit in a chair to take pictures. If you’re lying on a table, it’ll slide into a machine that looks like a cylinder. If you’re in a chair, your provider will position you in front of the camera.
  4. You’ll breathe in through the mask or mouthpiece while the camera takes pictures.
  5. Your provider will reposition your body during the test to get the pictures they need.
  6. Your provider will have you take the mask off or mouthpiece out. They may take pictures for another minute or two after that.

Perfusion scan

In most cases, you’ll move to the perfusion scan immediately after the ventilation scan. The perfusion scan is very similar to the ventilation scan. Instead of breathing the radioactive tracer in through a mask or mouthpiece, your provider will inject the tracer into your vein using an IV.

What happens after a VQ scan?

After a VQ scan, the tracer will leave your lungs as you breathe in fresh air. The injected tracer will lose its radioactivity and leave the rest of your body through your pee and poop in a few hours or days. The provider who ordered the scan will let you know the results.

When will I know the results of my VQ scan?

You’ll usually know the results of a VQ scan within 24 hours.

What do the results of a VQ scan mean?

If the results of your VQ scan are abnormal, it means something is preventing your lungs from working properly. If the sets of pictures from the two parts of the scan don’t match, it could mean you have a pulmonary embolism (PE).

If your images show you have normal air and blood flow in your lungs, or that your chance of a blood clot is low, you might see one of these terms on your scan report:

  • Normal.
  • Low probability.
  • PE absent.
  • No evidence of PE.

If there’s a possibility that you have a blood clot, but it couldn’t be determined based on these images. You might need more tests if you see one of these terms on your scan report:

  • Intermediate probability.
  • Indeterminate probability.
  • Nondiagnostic.

Based on the following scan results, you likely have a blood clot. Your provider will talk to you about your treatment options.

  • High probability.
  • PE present.
  • PE-positive.

Risks / Benefits

What are the advantages of a VQ scan?

The advantages of a VQ scan include:

  • It exposes you to less radiation than a CT scan.
  • It’s safe for people whose kidneys don’t work properly (renal insufficiency). Contrast dye used in CT scans can cause kidney damage in people with renal insufficiency.
  • It’s safe for people who are allergic to contrast dye used in CT scans.

What are the risks or side effects of a VQ scan?

The risks of having a VQ scan are minimal. They include:

  • Exposure to radiation. VQ scans expose you to a small amount of radiation. This leaves your body within a few hours to days of the scan.
  • Inability to shield from radiation. While VQ scans only expose you to a small amount of radiation, there’s no way to shield a fetus from it. If you’re pregnant, your provider may recommend a CT scan instead. A provider can use a shield during a CT scan to reduce the fetus’s exposure to radiation.
  • Allergic reaction. These are rare and usually mild.
  • Discomfort. The needle stick to inject the tracer or staying still for several minutes may be uncomfortable.

Recovery and Outlook

What is the recovery time?

After a VQ scan, you can return to normal activities unless your provider tells you otherwise. If you’re breastfeeding/chestfeeding, check with your provider about how to safely feed your baby for the day or two following the scan.

When To Call the Doctor

When should I see my healthcare provider?

Call your provider or go to the nearest emergency room if you have symptoms of a pulmonary embolism, including:

A note from Cleveland Clinic

Providers most commonly perform VQ scans to diagnose blood clots, or pulmonary embolisms. They check for airflow and blood flow in your lungs. A VQ scan is low-risk and noninvasive. It takes less than an hour to help your provider find a life-threatening blood clot. But let your provider know if you’re pregnant or think you’re pregnant. They may recommend a different type of test to look for lung issues.

Medically Reviewed

Last reviewed on 01/19/2023.

Learn more about our editorial process.

Appointments 216.444.6503