A VQ scan (ventilation-perfusion scan) is a two-part imaging test that checks how well air and blood are flowing through your lungs. It’s most commonly used to detect blood clots in the lungs, a potentially life-threatening condition called pulmonary embolism. The test uses small amounts of radioactive material to create images of airflow and blood flow separately, then compares the two to spot problems.
How the Two Parts Work
The “V” stands for ventilation (airflow) and the “Q” stands for perfusion (blood flow). These are actually two separate scans done back to back, each producing its own set of images.
During the ventilation scan, you breathe in a radioactive gas or aerosol through a mask while sitting or lying under a scanner. This lets the camera track where air goes inside your lungs. During the perfusion scan, a technician injects a small amount of radioactive albumin (a protein) into a vein in your arm. As that protein travels through your bloodstream and into the tiny blood vessels of your lungs, the scanner maps where blood is flowing.
The key diagnostic insight comes from comparing these two images. In a healthy lung, air and blood reach the same areas. If a region of your lung is getting air but no blood, that mismatch suggests something is blocking blood flow, most likely a clot.
Why Your Doctor Might Order One
The primary reason for a VQ scan is to check for a pulmonary embolism. This is a blood clot that has traveled to the lungs and lodged in an artery, cutting off blood supply to part of the lung tissue. Symptoms that typically prompt the test include sudden shortness of breath, sharp chest pain, coughing (sometimes with blood), a racing heartbeat, or skin that looks pale, clammy, or bluish.
Beyond pulmonary embolism, doctors also use VQ scans to:
- Diagnose chronic thromboembolic pulmonary hypertension (CTEPH), a condition where old blood clots cause persistently high pressure in the lung’s blood vessels
- Evaluate lung function before surgery, particularly when a surgeon needs to remove all or part of a lung
- Assess transplant outcomes, checking how well a transplanted lung is working
- Monitor chronic lung conditions like COPD
VQ Scan vs. CT Pulmonary Angiogram
The other main test for detecting lung blood clots is a CT pulmonary angiogram (CTPA), which uses a CT scanner and an injected contrast dye to directly image the blood vessels in your lungs. In many hospitals, CTPA is the first-line test because it produces detailed images quickly and is widely available.
A VQ scan becomes the better choice for specific groups of patients. If you have a kidney problem, the iodine-based contrast dye used in CTPA can cause further damage to your kidneys. If you’re allergic to contrast dye, a VQ scan avoids that risk entirely. Pregnant patients are sometimes steered toward a VQ scan as well, since it delivers less radiation overall.
The radiation difference is notable. A VQ scan typically delivers about 2.6 to 3.2 millisieverts (mSv) of radiation. A CTPA ranges from 4.2 to nearly 20 mSv depending on the equipment and protocol. For context, the average person absorbs about 3 mSv per year from natural background radiation, so a VQ scan is roughly equivalent to one year of everyday exposure.
How Accurate Is It?
A VQ scan is very good at ruling out pulmonary embolism and quite reliable at confirming it, though its accuracy depends on the technology used. Newer versions called SPECT VQ scans (which capture 3D images rather than flat ones) achieve a specificity of about 88%, meaning they correctly identify people who don’t have a clot roughly 88% of the time. When combined with a low-dose CT, that specificity climbs to around 93%.
Results are reported in probability categories rather than a simple yes or no. Your report will typically describe the likelihood of a pulmonary embolism as normal, very low probability, low probability, intermediate probability, or high probability. Your doctor combines this result with their clinical assessment of your symptoms and risk factors. When a VQ scan shows very low probability and your clinical risk is also low, the chance of actually having a clot drops to about 3%, which is reliable enough to rule it out without further testing.
Intermediate-probability results are the main limitation. They don’t clearly point in either direction, and you may need additional testing to get a definitive answer.
What to Expect During the Procedure
No special preparation is usually required. You don’t need to fast, and you can take your regular medications. When you arrive, you’ll likely do the ventilation portion first. A technician will place a mask or mouthpiece over your nose and mouth and ask you to breathe normally while a radioactive aerosol or gas flows through. You’ll need to stay still while the scanner captures images from multiple angles.
For the perfusion portion, the technician places an IV line and injects the radioactive tracer into your bloodstream. You then lie or sit under the scanner again while it captures a second set of images. The entire process, both parts combined, generally takes 30 to 60 minutes. Neither portion is painful beyond the brief needle stick for the IV.
Safety and Radiation Concerns
The radioactive tracers used in a VQ scan break down quickly. The most commonly used materials have a half-life of about six hours, meaning half the radioactivity is gone within that window. Your body clears the rest through urine over the following day.
After the scan, you don’t need to avoid contact with other people, including children and pregnant women. It’s safe to hug, kiss, and be physically close to others immediately. The one recommendation is to drink plenty of water for the rest of the day to help flush the tracer from your system faster.
VQ Scans During Pregnancy and Breastfeeding
The American College of Obstetricians and Gynecologists states that the radiation dose from imaging tests like VQ scans falls well below the threshold associated with harm to a developing baby. If a VQ scan is the best available test for the situation, it should not be withheld from a pregnant patient. Pulmonary embolism is a serious enough condition that the risk of missing it outweighs the minimal radiation exposure.
For breastfeeding mothers, the situation requires a bit more caution. Some radioactive materials can be excreted into breast milk. Guidelines recommend discussing the specific tracer being used with the nuclear medicine team so they can advise whether you need to pump and discard milk for a period after the scan or whether you can continue breastfeeding without interruption.