V/Q scan
Summary
V/Q scan (ventilation-perfusion scan) is a nuclear medicine imaging study that evaluates pulmonary ventilation and perfusion to diagnose pulmonary embolism. It compares airflow (ventilation) to blood flow (perfusion) in the lungs using radioactive tracers. A high-probability V/Q scan shows perfusion defects with normal ventilation (V/Q mismatch).
Detail
The V/Q scan consists of two phases: ventilation imaging uses inhaled radioactive xenon-133 or technetium-99m aerosol to assess airflow distribution, while perfusion imaging uses IV technetium-99m macroaggregated albumin (MAA) particles that lodge in pulmonary capillaries to evaluate blood flow. Results are interpreted as normal, low probability, intermediate probability, or high probability for pulmonary embolism. Classic PE finding is a perfusion defect with normal ventilation (V/Q mismatch). However, CTPA (CT pulmonary angiography) has largely replaced V/Q scans as first-line imaging due to higher specificity and ability to identify alternative diagnoses. V/Q scans remain useful in patients with contrast allergies, renal dysfunction, or pregnancy where radiation exposure should be minimized. Other applications include evaluating chronic thromboembolic pulmonary hypertension and assessing regional lung function before lung resection.
Sources
- Harrison's Principles of Internal Medicine
- Robbins Basic Pathology
- First Aid for the USMLE Step 1
- UpToDate
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