Imaging
VQ Scan
A VQ scan is a nuclear medicine test that compares air flow and blood flow in the lungs, used mainly to look for a blood clot (pulmonary embolism).
Quick answer
VQ Scan: what it is and what the results mean
A ventilation-perfusion (VQ) scan uses small amounts of radioactive material to image two things: how air moves into the lungs (ventilation) and how blood flows through them (perfusion). Areas with blood flow but no ventilation problem can indicate a clot.
- Why it is done: It is used to diagnose pulmonary embolism, especially in people who cannot have a CTPA — for example in pregnancy, contrast allergy or kidney problems — and sometimes to assess lung function before surgery.
- Understanding results: The scan is reported as low, intermediate or high probability of pulmonary embolism based on mismatches between air and blood flow.
What it is
A ventilation-perfusion (VQ) scan uses small amounts of radioactive material to image two things: how air moves into the lungs (ventilation) and how blood flows through them (perfusion). Areas with blood flow but no ventilation problem can indicate a clot.
Why it is done
It is used to diagnose pulmonary embolism, especially in people who cannot have a CTPA — for example in pregnancy, contrast allergy or kidney problems — and sometimes to assess lung function before surgery.
What to expect
You breathe in a radioactive gas or mist for the ventilation part and have a small radioactive injection into an arm vein for the perfusion part. A special (gamma) camera takes pictures while you lie still. The test takes about 30–60 minutes.
Understanding the results
The scan is reported as low, intermediate or high probability of pulmonary embolism based on mismatches between air and blood flow. Results are interpreted with your symptoms and other tests to decide on treatment.
Good to know
Risks and limitations
The radiation dose is low and the radioactive material clears quickly. It can be less clear than a CTPA if you have other lung disease, and results are sometimes “intermediate”, needing further tests.
Education and reference only. This explains the test in general terms and is not medical advice. Always follow the specific instructions from the team arranging your test, and discuss your results with your clinician.
Answers
VQ Scan: frequently asked questions
Is the radioactive injection dangerous?
The amount used is very small and passes out of your body quickly, mainly in urine. Drinking fluids afterwards helps clear it. The dose is considered low-risk.
Why have a VQ scan instead of a CTPA?
A VQ scan avoids iodine contrast and gives a lower radiation dose to the chest, so it is often preferred in pregnancy, contrast allergy or kidney problems, especially when a chest X-ray is normal.
Related tests
Other imaging investigations
Sources
Where this is drawn from
- NHS — Tests and treatments
- NICE — diagnostic guidance
- Royal College of Radiologists / relevant professional body
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