Is a chest X-ray (CXR) required when getting a ventilation/perfusion (V/Q) scan?

Chest X-ray Requirements Before V/Q Scan

A chest X-ray (CXR) is required before performing a ventilation/perfusion (V/Q) scan to ensure proper interpretation of the results and improve diagnostic accuracy.

Rationale for CXR Before V/Q Scan

Guidelines Support CXR Requirement

• The American College of Radiology (ACR) guidelines indicate that CXR is often the first imaging test performed in the evaluation of pulmonary vascular disease and is essential for proper interpretation of V/Q scans 1.
• The European Society of Cardiology (ESC) and European Respiratory Society (ERS) guidelines recommend CXR as part of the initial evaluation before proceeding to V/Q scanning 1.
• The American Thoracic Society/Society of Thoracic Radiology clinical practice guideline specifically recommends performance of CXR as the first radiation-associated procedure in the diagnostic algorithm for suspected pulmonary embolism 1.

Clinical Importance

• CXR helps identify alternative diagnoses that may explain symptoms and avoid unnecessary further testing.
• CXR findings influence the selection of subsequent imaging studies and improve the diagnostic yield of V/Q scans.
• CXR abnormalities can affect the interpretation of V/Q scan results by identifying potential causes of perfusion defects.

Impact of CXR on V/Q Scan Interpretation

Normal CXR

• When the CXR is normal, V/Q scan is more likely to provide diagnostic results:
  • Studies show significantly higher diagnostic yield (94% vs. 70%) with V/Q scan compared to CT pulmonary angiography in patients with normal CXR 1.
  • In patients with a normal CXR, any perfusion defect can be considered a mismatch, potentially eliminating the need for ventilation imaging in some cases 1, 2.

Abnormal CXR

• When the CXR is abnormal:
  • V/Q scans have a higher rate of non-diagnostic results (40% vs. 16.4%) compared to CT pulmonary angiography 1.
  • However, research suggests that CXR abnormalities should not be considered an absolute contraindication to V/Q scanning 3.
  • Triple-matched defects (matched ventilation, perfusion, and CXR abnormalities) have varying positive predictive values for pulmonary embolism depending on their location and size 4.

Clinical Applications and Considerations

Diagnostic Algorithm

1. Obtain CXR as the first radiation-associated procedure
2. If CXR is normal, proceed with V/Q scan as the preferred test
3. If CXR is abnormal, consider the nature of abnormalities:
   • For suspected pulmonary embolism, V/Q scan may still be appropriate
   • For other conditions, CT pulmonary angiography may be preferred

Special Populations

• In pregnant women, the recommended sequence is:
  • CXR first
  • V/Q scan as the preferred test if CXR is normal
  • CT pulmonary angiography if V/Q scan is non-diagnostic 1

Potential Pitfalls

• Relying solely on CXR to exclude pulmonary vascular disease is inadequate as CXR can be normal in mild pulmonary hypertension 1.
• Failure to obtain a recent CXR before V/Q scan may lead to misinterpretation of perfusion defects.
• Over-reliance on CXR abnormalities to avoid V/Q scanning may unnecessarily increase radiation exposure through alternative imaging.

Conclusion

CXR is an essential prerequisite before performing a V/Q scan. It helps in selecting the appropriate subsequent imaging study, improves interpretation of V/Q scan results, and may identify alternative diagnoses. The diagnostic approach should be tailored based on CXR findings, with V/Q scan being particularly valuable when the CXR is normal.