Ventilation-Perfusion (V/Q) Scan: Diagnostic Imaging for Pulmonary Vascular Disease
A ventilation-perfusion (V/Q) scan is a nuclear medicine imaging test that evaluates both airflow (ventilation) and blood flow (perfusion) in the lungs to diagnose conditions like pulmonary embolism, particularly when CT pulmonary angiography is contraindicated. 1
How V/Q Scans Work
Perfusion Component
- Involves intravenous injection of technetium-99m labeled macroaggregated albumin (Tc-99m MAA) particles
- These particles temporarily block a small fraction of pulmonary capillaries
- Areas with blocked blood flow (such as from pulmonary embolism) appear as "cold" spots or perfusion defects 1
- Typically performed with the patient supine to reduce gravitational effects 1
Ventilation Component
- Uses radioactive gases or aerosols to assess airflow in the lungs
- Common tracers include:
- Xenon-133 gas
- Krypton-81m gas
- Technetium-99m labeled aerosols
- Technetium-99m labeled carbon microparticles (Technegas) 1
- Ventilation imaging can be performed before or after perfusion scanning 1
Clinical Applications
Primary Use: Pulmonary Embolism (PE) Diagnosis
- Based on the principle that in PE, affected areas will show reduced perfusion but normal ventilation (mismatch) 1
- A normal V/Q scan effectively excludes PE with 90-100% sensitivity and 94-100% specificity 1
- V/Q findings for PE are typically categorized as:
- Normal (excludes PE)
- High probability (diagnostic of PE)
- Non-diagnostic/indeterminate 1
Other Applications
- Chronic thromboembolic pulmonary hypertension (CTEPH) assessment 1
- Evaluation of regional lung function before lung surgery or radiation therapy
- Assessment of congenital heart and lung abnormalities
- Evaluation of childhood respiratory diseases (cystic fibrosis, asthma) 2
- Identification of COVID-19 related ventilation and perfusion defects 3
Advantages of V/Q Scans
- Lower radiation exposure (1.1 mSv) compared to CT pulmonary angiography (3-10 mSv) 1
- No iodinated contrast required, making it suitable for patients with:
- Contrast allergies
- Renal impairment
- Pregnancy
- Hyperthyroidism 1
- Relatively inexpensive 1
- Few contraindications 1
Limitations
- Not as widely available as CT angiography
- Requires a recent chest X-ray for proper interpretation 4
- Higher rate of inconclusive results (up to 50% with planar imaging) 1
- Cannot provide alternative diagnoses if PE is excluded 1
- Interobserver variability in interpretation 1
- Less accurate with underlying lung disease (COPD, fibrosis) 1
Modern Developments
- Single-photon emission computed tomography (SPECT) V/Q imaging:
- Provides 3D images rather than traditional planar views
- Significantly reduces non-diagnostic results (<3% vs 50%)
- Allows binary interpretation ("PE" vs "no PE")
- Improves sensitivity and specificity compared to planar imaging 1
Clinical Pearls
- A normal chest X-ray increases the diagnostic yield of V/Q scanning 4
- In pregnancy, the recommended sequence is: chest X-ray first, then V/Q scan if X-ray is normal 4
- A normal perfusion scan can obviate the need for ventilation imaging, reducing radiation exposure 1
- V/Q scanning remains valuable despite the widespread use of CT pulmonary angiography, particularly for patients with contraindications to CT 1
- When interpreting results, clinical probability assessment should be integrated with imaging findings 1
Radiation Considerations
- V/Q scan delivers approximately 1.1-2 mSv effective dose
- Significantly lower radiation than CT pulmonary angiography (3-10 mSv)
- Particularly important consideration for young patients and pregnant women 1
V/Q scanning remains a valuable diagnostic tool in the evaluation of pulmonary vascular disease, offering complementary information to other imaging modalities while minimizing radiation exposure and avoiding contrast-related complications.