Is CT (Computed Tomography) pulmonary angiography (CTPA) safe for diagnosing pulmonary embolism (PE) in a pregnant woman?

CT Pulmonary Angiography for PE in Pregnancy

CTPA is safe and appropriate for diagnosing pulmonary embolism in pregnant women, with fetal radiation exposure well below the threshold for complications (50-100 mSv) and negligible maternal cancer risk, though the optimal diagnostic approach depends on chest X-ray findings. 1, 2

Radiation Safety Profile

Both CTPA and V/Q scanning expose the fetus to radiation doses far below safety thresholds, making either modality safe when properly performed. 1, 3

• Modern CTPA techniques result in fetal radiation doses of approximately 0.10 mGy (unshielded standard protocol) or 0.03 mGy (shortened scan), both well below the 50-100 mSv threshold associated with fetal complications 4, 1
• Maternal breast tissue receives 3-4 mGy with optimized CTPA protocols, resulting in a lifetime cancer risk increase of only 1.0003-1.0007 (essentially negligible) 1, 2
• V/Q perfusion scanning delivers even lower radiation: effective dose of 1.04 mSv to mother, 0.28 mGy to breast tissue, and 0.25 mGy to fetus, though fetal dose is slightly higher than CTPA 5

Evidence-Based Diagnostic Algorithm

The American Thoracic Society provides a structured approach based on chest X-ray findings and DVT symptoms: 6, 1

Step 1: Assess for DVT Symptoms

• If signs/symptoms of deep vein thrombosis are present, perform bilateral compression ultrasound (CUS) of lower extremities first 6, 3
• If CUS is positive for proximal DVT, initiate therapeutic anticoagulation immediately without further imaging 1
• If no DVT symptoms are present, proceed directly to pulmonary vascular imaging (more cost-effective than CUS) 1

Step 2: Obtain Chest X-Ray

• Chest X-ray should be the first radiation-based imaging study 6, 1, 3

Step 3: Choose Pulmonary Imaging Based on CXR Results

If chest X-ray is normal:

• Perform V/Q (perfusion) scintigraphy as the next imaging test rather than CTPA (strong recommendation from American Thoracic Society) 6, 1, 3
• V/Q scanning has 100% negative predictive value and delivers lower maternal radiation exposure 7

If chest X-ray is abnormal:

• Perform CTPA as the next imaging test rather than V/Q scanning 6, 1, 3
• Abnormal chest X-rays reduce the diagnostic accuracy of V/Q scans, making CTPA the preferred modality 6

Step 4: Manage Non-Diagnostic Results

• If V/Q scan is non-diagnostic, proceed to CTPA rather than clinical management alone 6, 1
• If CTPA is indeterminate and clinical suspicion remains high with low bleeding risk, initiate empiric therapeutic anticoagulation while pursuing additional testing 1

Critical Protocol Optimization for CTPA

CTPA protocols must be specifically adapted for pregnancy to minimize non-diagnostic studies, which occur in 43% of late pregnancy investigations when standard protocols are used. 2, 8

Technical Modifications:

• Use bolus triggering with short start delays to account for hyperdynamic circulation in pregnancy 8
• Employ high flow rates or high contrast medium concentration 8
• Reduce anatomical coverage along the z-axis (shortened scan reduces fetal dose by 70% compared to standard protocol: 0.03 mGy vs 0.10 mGy) 4, 2
• Use iterative reconstructive techniques 2
• Avoid deep inspiration during acquisition; consider shallow respiration instead to prevent Valsalva maneuver that devastates pulmonary vascular enhancement 8
• Preferentially use fast CT systems 8

Important Caveats and Pitfalls

Do NOT use abdominal shielding during CTPA in pregnancy - it paradoxically increases both maternal effective dose by 47% and fetal absorbed dose (0.12 mGy vs 0.10 mGy) by interfering with automatic exposure control 4

Do NOT rely on D-dimer testing to exclude PE in pregnant women, particularly in the third trimester where only 4.2% of women with non-high pre-test probability have negative results 1, 2, 3. The American Thoracic Society recommends against D-dimer use, prioritizing avoidance of false negatives over reducing unnecessary testing 1

Third trimester considerations: Inconclusive imaging results occur more frequently in late pregnancy (43% of investigations), making protocol optimization even more critical 2

Clinical Context

PE accounts for 20% of maternal deaths in the United States, making accurate diagnosis essential despite radiation concerns 6, 3. Common presenting features include dyspnea (62%), pleuritic chest pain (55%), cough (24%), and tachycardia, though no validated clinical prediction rules exist for pregnant populations 6. Normal pregnancy symptoms overlap with PE symptoms, requiring high clinical suspicion 6, 3.