What is the recommended protocol for a Ventilation-Perfusion (VQ) scan in patients with suspected pulmonary embolism?

Recommended Protocol for Ventilation-Perfusion (VQ) Scan in Suspected Pulmonary Embolism

V/Q scintigraphy remains a valid diagnostic option for patients with suspected pulmonary embolism, particularly when there are contraindications to CT pulmonary angiography such as renal failure or contrast allergy, or in younger patients to reduce radiation exposure. 1

Indications for V/Q Scan

• V/Q scanning is recommended as an alternative to CT pulmonary angiography in patients with:
  • Contraindications to CT contrast (allergy to iodine contrast dye or renal failure) 1
  • Young patients, particularly women, to reduce radiation exposure (CT may increase lifetime risk of breast cancer) 1
  • Patients with a normal chest X-ray (higher proportion of diagnostic V/Q scans) 1

Protocol Components

Patient Preparation

• Current good quality chest radiograph should be available for comparison 1
• Clinical information including Wells score or other validated clinical probability assessment should be provided 1

Technical Aspects

• Both ventilation and perfusion components should be performed for optimal diagnostic accuracy 1
  • However, perfusion scanning alone may be acceptable in centers where ventilation scanning is unavailable 1
• Multiple views should be obtained to reduce the proportion of indeterminate scan reports 1

Interpretation Criteria

• V/Q scans should be reported using validated criteria such as:
  • Modified PIOPED criteria 1
  • PISA-PED reporting method (based on whether wedge-shaped perfusion defects are present) 1
• Reports should categorize findings as:
  • Normal (effectively rules out PE) 1
  • High probability (confirms PE with 86-92% accuracy) 1
  • Non-diagnostic/indeterminate (requires further testing) 1

Diagnostic Algorithm

1. Assess clinical probability of PE using validated prediction rule 1
2. Measure D-dimer (except in high clinical probability patients) 1
3. If D-dimer is elevated or clinical probability is high, proceed with imaging 1
4. Perform V/Q scan with both ventilation and perfusion components when possible 1
5. Interpret results in conjunction with clinical probability 1
   • Normal V/Q scan excludes PE 1
   • High probability V/Q scan confirms PE 1
   • Non-diagnostic scan requires further evaluation:
     • Consider lower limb compression ultrasonography (CUS) 1
     • A non-diagnostic scan with low clinical probability and negative CUS can safely exclude PE 1

Clinical Interpretation Guidelines

• A normal perfusion scan effectively rules out PE (high negative predictive value) 1
• A high probability V/Q scan in a patient with high clinical probability confirms PE 1
• For non-diagnostic scans:
  • Combine with clinical probability assessment 1
  • Consider additional testing with lower limb CUS 1
  • Patients with non-diagnostic scans and low clinical probability have low prevalence of PE 1

Pitfalls and Limitations

• V/Q scan interpretation may be difficult or misleading in patients with:
  • Previous pulmonary embolism 1
  • Left heart failure 1
  • Chronic obstructive airways disease 1
  • Lung fibrosis 1
  • Proximal lung cancer 1
• Approximately 30-50% of V/Q scans are diagnostic (normal or high probability), while the remainder may be non-diagnostic and require additional testing 1
• Misinterpretation of indeterminate scan results is common among clinicians and may lead to inappropriate management 2

Comparison with CT Pulmonary Angiography

• CT pulmonary angiography has become the main thoracic imaging test for suspected PE in most centers 1
• V/Q scanning remains a valid alternative with comparable safety in diagnostic algorithms 3, 4
• V/Q scanning exposes patients to significantly less radiation than CT, particularly important for young women 1, 3