What are the benefits and risks of CT (Computed Tomography) pulmonary embolism (PE) in pregnancy?

Benefits and Risks of CT Pulmonary Embolism Imaging in Pregnancy

For pregnant women with suspected pulmonary embolism (PE) and a normal chest X-ray, lung scintigraphy (V/Q scan) is strongly recommended as the first-line imaging test rather than CT pulmonary angiography (CTPA) due to significantly lower maternal breast radiation exposure. 1, 2

Diagnostic Algorithm for Suspected PE in Pregnancy

1. Initial Assessment

   • Begin with chest X-ray as the first radiation-associated procedure 1
   • Consider compression ultrasound of lower extremities if signs/symptoms of DVT are present 1

2. Based on Chest X-ray Results:

   • Normal chest X-ray: Proceed with V/Q scan 1, 2
   • Abnormal chest X-ray: Proceed with CTPA 1, 3

3. If V/Q scan is non-diagnostic:

   • Proceed to CTPA rather than clinical management alone 1

Benefits of Imaging

• Diagnostic accuracy: Both CTPA and V/Q scans have high negative predictive values (median 100% for both) for excluding PE 4
• Mortality prevention: PE is a leading cause of pregnancy-related mortality (20% of maternal deaths in the US) 1
• Alternative diagnoses: CTPA can identify alternative causes of symptoms when PE is not present 1, 3
• Treatment guidance: Accurate diagnosis prevents unnecessary anticoagulation and guides appropriate management 2

Radiation Risks

Maternal Risks

• Breast tissue exposure:
  • CTPA: 10-60 mGy (older techniques) or 3-4 mGy (modern techniques) 1, 2, 5
  • V/Q scan: 0.28-1.07 mGy 1, 2, 5
  • The lifetime cancer risk increase from a single CTPA is minimal (factor of 1.0003-1.0007) 1, 2

Fetal Risks

• Fetal radiation exposure:
  • CTPA: 0.1-0.46 mGy 2, 5
  • V/Q scan: 0.25-0.40 mGy 2, 5
  • Both are well below the threshold for fetal harm (50-100 mGy) 1, 2

Practical Considerations

• Image quality and inconclusive results:

  • CTPA: Median 5.9% inconclusive results (range 0.9-36%) 4
  • V/Q scan: Median 4.0% inconclusive results (range 0-23%) 4
  • Image quality for CTPA may be affected by physiologic changes of pregnancy (increased cardiac output, blood volume) 6

• Availability and expertise:

  • CTPA is more widely available and increasingly used in all trimesters 3
  • V/Q scan techniques can be modified to reduce radiation (half-dose protocols) 2

Strategies to Minimize Radiation Exposure

• Use half the usual administered activity of Technetium-99m for perfusion scans 2
• Consider omitting ventilation component of V/Q scan when possible 2
• Use technical modifications for CTPA: iterative reconstruction, reduced kilovoltage, shorter scan length 2
• Encourage hydration and frequent urination to reduce fetal exposure 2

Common Pitfalls and Caveats

1. Overreliance on D-dimer testing: D-dimer levels physiologically increase throughout pregnancy, limiting utility 2

2. Failure to start with chest X-ray: A normal chest X-ray should guide toward V/Q scan, while abnormal findings favor CTPA 1

3. Excessive concern about radiation: The risk of missing a potentially fatal PE diagnosis far outweighs the minimal radiation risks 2

4. Inadequate technique: Technical modifications are essential to minimize radiation while maintaining diagnostic quality 2

5. Delaying diagnosis: Empiric anticoagulation should be considered while awaiting imaging if clinical suspicion is high and bleeding risk is low 2

The choice between CTPA and V/Q scan should be guided by chest X-ray findings, local expertise, and availability, with V/Q scan preferred when chest X-ray is normal due to lower maternal breast radiation exposure.