What is the recommended imaging test for suspected pulmonary embolism in a patient who cannot receive intravenous (IV) contrast?

Ventilation-Perfusion (V/Q) Scan is the Recommended Imaging Test for Suspected Pulmonary Embolism in Patients Who Cannot Receive IV Contrast

For patients with suspected pulmonary embolism who cannot receive intravenous contrast, a ventilation-perfusion (V/Q) scan is the recommended first-line imaging test. 1

Diagnostic Algorithm for Suspected PE in Patients Who Cannot Receive IV Contrast

Initial Assessment

• Begin with clinical probability assessment and D-dimer testing 1
• If low clinical probability and negative D-dimer: PE can be safely excluded without imaging 1
• If intermediate/high probability OR positive D-dimer: proceed to imaging 1

Recommended Imaging Options

1. V/Q Scan (First Choice)

   • Highly effective when IV contrast is contraindicated 1
   • Normal V/Q scan has high negative predictive value confirmed by outcome studies 1
   • Perfusion imaging alone can sometimes be performed in certain protocols 1
   • V/Q SPECT (3D technique) offers improved sensitivity (97%) and specificity (88%) compared to planar imaging 2

2. V/Q SPECT with Low-Dose CT (If Available)

   • Combines functional and anatomical information 2
   • Maintains high sensitivity (97%) while improving specificity to 100% 2
   • Does not require IV contrast enhancement 2
   • Should be considered first-line when available 2

3. MR Angiography Without Contrast

   • Limited evidence for non-contrast MRA in PE diagnosis 1
   • May be considered in specialized centers with expertise 1

Interpretation of V/Q Scan Results

• Normal/Near-Normal Scan: Safely excludes PE 1, 3
• High Probability Scan (wedge-shaped perfusion defects): Strong indicator of PE 3
• Intermediate/Non-Diagnostic Scan: May require additional testing 4

Clinical Considerations and Caveats

• Strengths of V/Q scanning:

  • Does not require nephrotoxic contrast 1
  • Validated diagnostic accuracy in multiple studies 3
  • Lower radiation dose than CT angiography 5

• Limitations of V/Q scanning:

  • Higher proportion of non-diagnostic results compared to CTPA 1
  • Limited ability to provide alternative diagnoses 1
  • Interpretation may be challenging in patients with underlying lung disease 3

• Important clinical pitfalls to avoid:

  • Do not rely solely on perfusion imaging without clinical assessment 3
  • Combining clinical probability with V/Q results improves diagnostic accuracy 3
  • Non-diagnostic scans may require additional testing in high-risk patients 4

Special Populations

• Pregnancy: V/Q scan may be preferred due to lower radiation to breast tissue 6
• Chronic kidney disease: V/Q scan avoids contrast-induced nephropathy risk 1
• Contrast allergy: V/Q scan is the primary alternative to CTPA 1

When CTPA cannot be performed due to contraindications to IV contrast, V/Q scanning remains a well-established and effective diagnostic tool for suspected pulmonary embolism, with excellent diagnostic performance, particularly when combined with clinical assessment 1, 3.