Which is safer for a pregnant female, a CT (Computed Tomography) angiogram or a VQ (Ventilation-Perfusion) scan?

V/Q Scan is Safer for Pregnant Women than CT Angiogram

V/Q scanning should be favored over CT pulmonary angiography for pregnant women with suspected pulmonary embolism due to lower maternal breast radiation exposure and similar diagnostic accuracy. 1, 2

Diagnostic Accuracy Comparison

• Both V/Q scanning and CT pulmonary angiography have similar and low false-negative rates when used to evaluate suspected pulmonary embolism during pregnancy (0.5% vs 0.4% respectively) 1
• For every 1000 pregnant women scanned, approximately 5 pulmonary emboli would be incorrectly ruled out with V/Q scanning and 4 with CT pulmonary angiography 1
• Unlike in non-pregnant populations, the rate of inconclusive or non-diagnostic scans is not appreciably higher with V/Q scanning compared to CT pulmonary angiography in pregnant women 1
• Recent research confirms similar diagnostic yield between both modalities in pregnant women 3

Radiation Exposure Considerations

• V/Q scanning exposes the mother to significantly lower breast-absorbed radiation dose compared to CT pulmonary angiography 1, 3
• Breast-absorbed radiation dose is approximately 8.0 mGy with CTPA versus 0.3 mGy with V/Q scanning, representing a substantial difference that could impact future breast cancer risk 3
• While fetal radiation dose may be slightly greater with V/Q scanning than with CT pulmonary angiography (depending on protocol and gestational age), both are far below the accepted maximal threshold for fetal radiation exposure 1
• The potential absolute increase in risk of childhood cancer from either imaging modality is likely to be very small 1

Special Considerations and Exceptions

• CT pulmonary angiography might be preferred in patients with abnormal chest radiographs, as V/Q scans are less accurate in this scenario 1
• If V/Q scanning is not readily available, CT pulmonary angiography is an acceptable alternative 1, 2
• Modern CT scanners with dose reduction protocols can achieve lower radiation doses than historically described, though V/Q still maintains the advantage for breast dose reduction 3, 4
• For pregnant patients with indeterminate CT results, further diagnostic testing with V/Q scanning should be considered, especially with normal chest X-ray 2

Clinical Implementation

• A chest X-ray should be obtained prior to deciding between V/Q scan and CT angiography, as abnormal findings would favor CT angiography 1, 2
• Pregnant patients should be informed about the risks associated with pulmonary embolism and radiation exposure to facilitate informed decision-making 5
• When explaining radiation risks, doses should be presented in comparison with known radiation thresholds for fetal harm to provide context 5
• If high clinical suspicion for PE exists and bleeding risk is low, anticoagulation should be initiated while awaiting imaging results 2

Recent Advances and Considerations

• Protocol optimization for CT angiography in pregnant patients (including automated bolus triggering, high iodine flux, and high iodine concentration) can improve image quality while reducing radiation exposure 2, 4
• Combined reduction of kilovoltage and milliampere-second settings and z-axis coverage in CT protocols can substantially reduce radiation dose while maintaining diagnostic imaging quality 4
• The positive scan rate for pulmonary embolism in pregnancy is relatively low (3.5%), highlighting the importance of appropriate patient selection for imaging 6

In conclusion, while both imaging modalities are safe and effective for diagnosing pulmonary embolism in pregnancy, V/Q scanning should be preferred when available due to the lower maternal breast radiation exposure and comparable diagnostic accuracy.