Pregnancy and Pulmonary Embolism: Which Imaging Study Should Be Used?

Robert D. Glatter, MD, FAAEM

Disclosures

June 09, 2008

Question

Which imaging study should be used for ruling out pulmonary embolism during pregnancy?


Response From the Expert

 

Robert D. Glatter, MD, FAAEM
Attending Physician, Department of Emergency Medicine, Lenox Hill Hospital, New York, NY

 

Pulmonary embolism (PE) is a leading cause of maternal mortality during pregnancy and up to 6 weeks postpartum.[1] Compared with nonpregnant women, women who are pregnant have a 5-fold increased risk for venous thromboembolism (VTE).[2]

An evaluation to rule out PE during pregnancy is especially challenging due to concerns regarding fetal radiation exposure. Clinical examinations are unreliable, and imaging the patient is essential. In these cases, ventilation-perfusion scans (V/Q) and helical computed axial tomographic pulmonary angiography (HCTPA) are the 2 most common imaging modalities available.

Radiation from HCTPA is dose-dependent, with radiation exposures greater than 0.05 Gy (5-rad) potentially causing fetal damage. The most vulnerable period for radiation-induced central nervous system damage is 8-15 weeks after conception. Radiation dose to the uterus is generally a good approximation of the radiation dose to the fetus in early pregnancy.[2] Exposure to less than 0.05 Gy (5-rad) has not been shown to be associated with adverse genetic outcomes when compared with a control population.[3]

V/Q scanning has traditionally been associated with less radiation exposure to the fetus than HCTPA. However, Winer-Muram and colleagues[4] report that HCTPA is actually associated with a lower average fetal radiation dose than V/Q scanning during all trimesters. The mean fetal dose with HCPTA was less than 6 mrad (0.00006 Gy) for 20 of 23 patients in their study. Comparatively, the mean fetal dose from V/Q scanning was approximately 10-37 mrad (0.0001-0.00037 Gy).[5,6] The fetal dose for both techniques is well below the 5 rad limit considered safe for fetal exposure. Nijkeuter and colleagues[2] showed similar results, consistently demonstrating lower radiation doses in both single and multidetector HCTPA compared with V/Q scans.

One drawback, however, of HCTPA is that it exposes maternal breast tissue to relatively high doses of radiation -- up to 35 mGy per breast.[7] While the full effects of this radiation are uncertain, the lifetime risk for breast cancer has been reported to increase after 10 mGy of radiation to the breast in women less than 35 years of age.[8]

Physicians may be hesitant to order additional imaging studies in pregnant patients following an indeterminate V/Q scan, but it is important to recognize that HCTPA is associated with lower radiation doses when compared with V/Q scanning during all trimesters of pregnancy. HCTPA also provides greater accuracy for the diagnosis of PE in main, lobar, and segmental pulmonary arteries. Making the diagnosis of PE after an indeterminate V/Q scan is essential to minimize risks to both mother and fetus.

Of note, magnetic resonance imaging (MRI) is another alternative to V/Q scanning and HCTPA. MRI is advantageous because the fetus is not exposed to ionizing radiation or intravenous contrast material. Additionally, the sensitivity and specificity of MRI have been reported in ranges comparable to HCTPA for the diagnosis of PE.[9] The disadvantages of MRI include long acquisition times, with the need for respiratory and cardiac gating. Spatial resolution can still be inadequate with use of these techniques. MRI is still not widely available for routine use in this setting.

Conclusion

The greater accuracy of HCTPA, along with findings that the average fetal radiation dose is consistently lower than V/Q scanning for all 3 trimesters, illustrates that HCTPA is more appropriate for evaluating a pregnant patient in whom you suspect acute PE.

The issue of increased radiation exposure to breast tissue from HCTPA should be discussed with each patient. If there is still a concern regarding radiation risk, a half-dose perfusion scan[10] or MRI (if available) may be suitable alternatives.

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