V/Q Scan (Lung Ventilation-Perfusion Scan)

A V/Q scan — formally a ventilation-perfusion lung scan — compares air reaching the lungs (ventilation) with blood flow (perfusion). Certain mismatched perfusion defects raise suspicion for pulmonary embolism, but the pattern is interpreted with the chest radiograph, clinical probability, and the acquisition method.

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:
  • Pregnancy, when the clinical team judges V/Q preferable after considering the chest radiograph, local expertise, and maternal and fetal dose.
  • 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; several lung conditions can produce this pattern.
• Mismatched defects — perfusion is reduced but ventilation is preserved. This is the classic finding of pulmonary embolism.
• Normal study — strongly argues against pulmonary embolism.

Reporting varies by technique and local criteria. Some reports use probability categories, while SPECT-based protocols may report the study as positive, negative, or nondiagnostic.

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 so it can provide tracer-specific written advice; interruption is not the same for every protocol.

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?

Radiation dose depends on the radiopharmaceutical activities, whether ventilation is performed, and the CTPA technique used for comparison. In pregnancy, the team selects the test and protocol after considering both maternal and fetal exposure as well as diagnostic reliability.

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

A V/Q scan may be appropriate in pregnancy when pulmonary embolism is suspected. The clinical and imaging teams should choose between V/Q and CTPA promptly using the chest radiograph, local protocol, and an individual benefit-risk assessment; suspected pulmonary embolism should not be left uninvestigated because of pregnancy alone.

Can I breastfeed after the scan?

Breastfeeding advice depends on the exact ventilation and perfusion agents and administered activities. Ask the nuclear-medicine team for written instructions before the test rather than following a generic interruption time.

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.

What are the important limitations and safety checks?

Nuclear medicine shows physiology and tracer uptake, but uptake is not always specific to one disease and spatial detail may be lower than CT or MRI. Inflammation, infection, treatment effects, medicines, blood glucose, recent imaging, movement, and the timing of images can change a result. A normal scan does not exclude every abnormality, and an abnormal focus may need correlation with CT, MRI, ultrasound, laboratory tests, biopsy, or follow-up imaging.

Preparation and radiation precautions are radiopharmaceutical- and protocol-specific. Tell the department before the tracer is given if you are or may be pregnant, are breastfeeding, care for a young child, or recently had another nuclear-medicine test. Do not stop medicines, fast, interrupt breastfeeding, or follow a fixed distancing period based only on a general webpage; obtain written instructions for the exact tracer, activity, and examination from the nuclear-medicine team.

Sources and further reading

• RadiologyInfo.org: General nuclear medicine
• RadiologyInfo.org: Preparing for a nuclear-medicine examination
• IAEA: Basics of quality management for nuclear medicine practices

Conclusion

A V/Q scan is an important test for pulmonary embolism and regional lung function, particularly when iodinated contrast is unsuitable. The best test in pregnancy or kidney disease is individualized, and a nondiagnostic result is not a negative result—it requires prompt clinical follow-up.