Is it safe to get a computed tomography (CT) pulmonary angiogram at 34 weeks gestation?

CT Pulmonary Angiography Safety at 34 Weeks Gestation

Yes, a CT pulmonary angiogram (CTPA) can be safely performed at 34 weeks gestation, but ventilation-perfusion (V/Q) scanning is preferred when available due to lower maternal breast radiation exposure. 1, 2

Diagnostic Approach for Suspected PE in Pregnancy

First-Line Imaging Recommendation

• V/Q scanning is the preferred first-line imaging when available due to:
  • Lower maternal breast radiation exposure (3-4 mGy vs 7 mGy with CTPA) 1, 2
  • Similar diagnostic accuracy to CTPA in pregnancy 1
  • Comparable rates of false-negative results (0.5% for V/Q vs 0.4% for CTPA) 1

When CTPA is Appropriate

CTPA is an acceptable alternative in the following situations:

• When V/Q scanning is not readily available 1
• When the chest X-ray is abnormal 1, 2
• In hemodynamically unstable patients requiring immediate diagnosis 2
• At centers with expertise in low-dose CTPA techniques 3

Radiation Safety Considerations

Fetal Radiation Exposure

• Fetal radiation dose from CTPA at 34 weeks is approximately 0.1-0.3 mGy 3, 4
• This is well below the threshold associated with fetal complications (50-100 mSv) 1
• The risk of childhood cancer from this exposure is extremely small 1

Maternal Radiation Exposure

• Primary concern with CTPA is breast tissue exposure (2.9-7 mGy) 1, 3
• Modern CTPA techniques have reduced this exposure significantly 1, 3
• The lifetime cancer risk increase from a single CTPA is minimal (factor of 1.0003-1.0007) 1

Technical Considerations for CTPA in Late Pregnancy

Optimizing Image Quality

• Late pregnancy (34 weeks) presents greater technical challenges for CTPA 5
• Higher rates of suboptimal scans occur in late pregnancy (33.3%) compared to early pregnancy (11.1%) 5
• Technical modifications to improve image quality include:
  • Automated bolus triggering
  • High iodine flux (flow rate 4.5-6 ml/s)
  • High iodine concentration (350-400 mg I/ml)
  • Clear breathing instructions to minimize Valsalva effects 1

Radiation Reduction Strategies

• Shorter scan length (can reduce fetal dose by 56%) 4
• mA modulation (can reduce dose by 10%) 4
• Iterative reconstruction techniques 1
• Reducing kilovoltage 1

Clinical Decision-Making Algorithm

1. Initial assessment:

   • Evaluate clinical probability of PE
   • Consider D-dimer testing (though less useful in pregnancy)
   • Perform compression ultrasound if signs of DVT present

2. Imaging selection:

   • If chest X-ray normal and V/Q scan available → Proceed with V/Q scan
   • If chest X-ray abnormal OR V/Q not available → Proceed with CTPA
   • If CTPA selected, ensure low-dose protocol is used

3. If CTPA is performed:

   • Document radiation dose in medical records
   • Ensure proper technique to minimize maternal and fetal exposure
   • Have images reviewed by experienced radiologist

Important Caveats

• The risk of missing a potentially fatal PE diagnosis far outweighs the minimal radiation risk 2
• Empiric anticoagulation should be considered while awaiting imaging results if clinical suspicion is high and bleeding risk is low 1
• Repeat CTPA should be avoided unless technical factors can be improved 1
• Recent evidence from the OPTICA study confirms the safety of optimized low-dose CTPA protocols in pregnancy 3

Remember that while V/Q scanning is generally preferred, the most important consideration is obtaining a timely and accurate diagnosis using whichever modality is readily available at your institution.