V/Q Scan (Lung Ventilation-Perfusion Scan)

A V/Q scan — formally a Ventilation-Perfusion lung scan — is a nuclear-medicine study that compares two things: the air going into the lungs ("V" for ventilation) and the blood reaching the lungs ("Q" for perfusion). When these two are matched, the lungs are working normally. When there is normal ventilation but reduced perfusion in the same area, that mismatch is the signature finding of a pulmonary embolism — a blood clot in the lung arteries.

While CT pulmonary angiogram (CTPA) is the most commonly used test for pulmonary embolism in adults, V/Q scans remain important — particularly in pregnancy, severe contrast allergy, or significant kidney disease, where the radiation pattern or contrast load of CTPA is less desirable.

Common Indications for a V/Q Scan

Your doctor may request a V/Q scan to:

• Diagnose suspected pulmonary embolism, especially in:
  • Pregnant women (V/Q delivers lower radiation to the breast than CTPA).
  • Patients with a documented severe iodine-contrast allergy.
  • Patients with significant kidney disease where contrast is risky.
  • Younger patients where reducing breast radiation matters.
• Assess regional lung function before lung surgery (such as lung cancer resection).
• Evaluate chronic thromboembolic pulmonary hypertension.
• Investigate longstanding shortness of breath when other tests are inconclusive.

What exactly does a V/Q scan show?

A normal lung shows matched ventilation and perfusion — wherever air reaches, blood follows. The scan distinguishes between several patterns:

• Matched defects — both ventilation and perfusion are reduced in the same area (often pneumonia, COPD, or an area of consolidation). Usually not from pulmonary embolism.
• Mismatched defects — perfusion is reduced but ventilation is preserved. This is the classic finding of pulmonary embolism.
• Normal study — strongly argues against pulmonary embolism.

The result is reported using a probability scale (high, intermediate, low, very low, or normal) based on the pattern and number of mismatches.

What happens during the procedure?

The scan has two parts, performed on the same visit:

Ventilation phase:

• You inhale a radioactive gas or an aerosolised radioactive substance (technegas or Tc-99m DTPA aerosol) through a mouthpiece for a few minutes.
• The gamma camera takes images of where the radiotracer reached in the lungs.

Perfusion phase:

• A small amount of radioactive tracer (Tc-99m MAA — macroaggregated albumin) is injected into a vein in your arm. The tracer travels through the bloodstream and lodges briefly in the smallest blood vessels of the lungs.
• The gamma camera then takes images of where blood is reaching in the lungs.

The radiologist compares the two image sets. The entire visit usually takes 1 to 2 hours.

Do I need to prepare?

• No fasting is required.
• Wear comfortable clothing without metal in the chest area.
• Bring previous chest imaging if available — a recent chest X-ray is helpful for the interpretation.
• Tell the team if you have severe asthma or are pregnant.
• Tell the team if you are breastfeeding (you may be asked to pump and discard breast milk for a brief period).

Will the test be painful?

The scan itself is painless. The only mildly uncomfortable parts are:

• A small needle prick when the perfusion tracer is injected.
• Breathing through a mouthpiece during the ventilation phase, which some patients find slightly awkward.

There is no contrast injection like in a CT scan.

How long do the results take?

A nuclear medicine physician usually reviews the study soon after it is completed. A formal written report typically reaches your doctor within 24 to 72 hours, sometimes faster for urgent cases.

How does V/Q compare to CTPA?

Is the radiation dose high?

The total radiation dose for a combined V/Q scan is similar to or lower than a CTPA — typically equivalent to 1 to 2 years of natural background radiation. In pregnancy specifically, perfusion-only scans (no ventilation phase) deliver an even smaller dose to both mother and foetus.

Can I have a V/Q scan if I am pregnant?

Yes. V/Q scans (often perfusion-only in early pregnancy) are commonly used when pulmonary embolism is suspected during pregnancy. The dose to the foetus is small and well below thresholds known to cause harm. The risk of an undiagnosed pulmonary embolism is far higher than the risk of the scan.

Can I breastfeed after the scan?

You may be advised to pump and discard breast milk for a brief period (typically a few hours to 24 hours, depending on the tracer used). Your nuclear medicine team will give specific instructions.

What if the scan is "indeterminate" or "intermediate probability"?

A V/Q scan reported as "indeterminate" or "intermediate probability" does not give a clear yes-or-no answer. Your doctor will combine the result with your clinical likelihood (from history, examination, and tests like D-dimer) to decide whether to:

• Proceed to CTPA or formal pulmonary angiography.
• Begin empirical anticoagulation.
• Use serial leg venous Doppler scans to look for clots in the legs.
• Reassess after a period of observation.

Conclusion

A V/Q scan is a focused nuclear-medicine test that compares lung airflow and blood flow to detect pulmonary embolism and similar conditions. It is the preferred test in pregnancy, in severe contrast allergy, and in significant kidney disease, and remains a key alternative to CTPA. The visit takes 1 to 2 hours, is painless, and the result — when clear — answers a high-stakes question quickly.