Risks of CTPA During the 3rd Trimester of Pregnancy
Modern CTPA techniques during the third trimester of pregnancy pose minimal risks to both mother and fetus, with fetal radiation doses well below the threshold for complications and negligible lifetime cancer risk increase for the mother.1
Radiation Exposure Risks
Fetal Radiation Exposure
- Fetal radiation doses from properly performed CTPA are well below the threshold associated with fetal radiation complications (which is 50-100 mSv) 1
- The mean absorbed fetal/uterine dose from CTPA is approximately 0.1 ± 0.2 mGy, which is extremely low 2
- Shortening the scan length can reduce fetal absorbed dose by up to 70% compared to standard protocols, while still maintaining diagnostic quality 3
Maternal Radiation Exposure
- In the past, CTPA was reported to cause high radiation exposure to the maternal breast, but CT technology has evolved significantly 1
- Modern CTPA imaging techniques expose the maternal breast to median doses as low as 3-4 mGy 1
- The mean absorbed breast dose with optimized protocols is approximately 2.9 ± 2.1 mGy 2
- The maternal effective dose is approximately 1.4 ± 0.9 mSv with optimized protocols 2
- The effect on maternal cancer risk with modern CTPA techniques is negligible (lifetime cancer risk reportedly increased by a factor of 1.0003-1.0007) 1
Technical Challenges in the 3rd Trimester
Physiological Changes Affecting Image Quality
- Pregnancy-related physiological changes can affect CTPA quality, particularly in the third trimester 4
- Hyperdynamic circulation in pregnancy reduces average enhancement of the pulmonary vasculature 4
- Increased risk of Valsalva maneuver during breath-holding can negatively affect pulmonary vascular enhancement 4
- Inconclusive results can be more common in late pregnancy (43% of investigations) 1
Protocol Optimization for 3rd Trimester
- CTPA protocols should be specifically adapted for pregnancy to account for physiological changes 1, 4
- Optimization techniques include:
- Automated bolus triggering with short start delays 4
- High flow rates (4.5-6 ml/s) or high iodine concentration (350-400 mg I/ml) 1, 4
- Preferential use of fast CT systems 4
- Low kVp CT techniques 4
- Avoiding deep inspiration during acquisition (shallow respiration may be better than breath-holding) 4
- Reducing anatomical coverage of the scan 1
- Using iterative reconstructive techniques 1
- Reducing the contrast-monitoring component 1
Clinical Considerations
Diagnostic Accuracy and Safety
- A recent prospective study (OPTICA) validated that optimized low-dose CTPA protocols safely exclude PE in pregnant women with zero incidence of recurrent symptomatic VTE at 3-month follow-up 2
- CTPA is increasingly used as the primary diagnostic modality for PE across all trimesters of pregnancy, with usage increasing linearly from first trimester to postpartum period 5
- In the third trimester specifically, CTPA is used in approximately 70% of confirmed PE cases 5
Alternative Approaches
- If chest X-ray is normal, perfusion lung scan may be considered as an alternative to CTPA 6
- If chest X-ray is abnormal, CTPA is the recommended next imaging test 6
- Compression ultrasonography of lower extremities should be considered first if signs/symptoms of DVT are present 6
Common Pitfalls and Avoidance Strategies
Pitfall: Using standard non-pregnancy CTPA protocols
Pitfall: Unnecessary abdominal shielding
- Solution: Avoid patient shielding as it may interfere with automatic exposure control and potentially increase both maternal and fetal radiation dose 3
Pitfall: Relying on D-dimer testing in the third trimester
Pitfall: Repeated imaging due to non-diagnostic studies
- Solution: Optimize the first CTPA study with pregnancy-specific protocols to avoid the need for repeat imaging 7