Imaging of Pregnant Women Is Not a No-Go

Due to confusions about the different modalities safety characteristics, radiologic imaging of pregnant women is often avoided unnecessarily.

Presenter Charis Bourgioti from Athens, Greece, discussed the safety aspects of the different imaging modalities with regard to the effects on the human fetus.

Sonography – the Workhorse

Ultrasound is usually the modality of choice, when it comes to examining pregnant women. It is widely regarded safe; adverse fetal effects have not been reported yet. Even the risk for a temperature increase can be avoided by adjusting the imaging settings, Bourgioti explained. These facts also apply to color Doppler imaging (CDI), still, the presenter recommended its cautious use, especially during the first trimester.

MR Imaging – the Second Choice

Magnetic resonance imaging (MRI) is regarded an alternative to sonography. Pregnant patients can be imaged safely with field strengths of up to 3.0 Tesla. However, radiologists should ensure that the specific absorption rate (SAR) is set to less than 2 watt/kg. The guidelines of the American College of Radiology (ACR) do not ask for any other adjustments for examining women throughout the different trimesters. Still, Bourgioti recommended using MRI only when absolutely necessary. Radiologists should always ask themselves if waiting is an option until the women has given birth.

Both modalities, for example, can be used for the different steps of assessing cancer disease. Ultrasound is the technique for detecting the tumor and MRI for staging purposes.

Ionizing Modalities – Do they Need to Be Avoided?

Due to the ionizing radiation, radiologists need to be more cautious with conventional X-ray and computed tomography (CT). When scanning a pregnant patient, it is important not to exceed the fetal dose of five Millisievert (mSv). In most cases, the average dose for the fetus is far below this maximal dose, ranging around 0.5-1 mSv. Having these facts in mind, Bourgioti criticized that doctors often get too nervous when ionizing radiation is applied to a pregnant women. A Canadian study (Ratnapalan CMAJ 2008), for instance, showed that family practitioners recommended unnecessary abortions after an X-ray examination in one percent of all cases and after CT even in six percent of the cases. “Let’s just calm down, before you make such a decision,” Bourgioti advised.

For understanding the effects of radiation on the fetus, one has to differentiate between the teratogenic deterministic effects and the carcinogenic stochastic effects. Teratogenesis depends on gestational age and dose:

  • < 4 weeks: doses of more than 50-100 mGy may lead to spontaneous abortion, if damage has occurred
  • 8-15 weeks: doses of more than 100-200 mGy may lead to intrauterine growth retardation or defects of the central nervous system (CNS)
  • > 15 weeks, >100 mGy: fetus becomes less sensitive to CNS effects
  • > 27 weeks: no malformations at diagnostic doses expected

In summary: a fetal dose of less than 100 mGy should not be considered a reason to terminate pregnancy. But the risk of fetal malformation is significantly increasing at fetal doses of more than 150-200 mGy and fetal damage is definitely occurring at exposures of more than 500 mGy. As almost all imaging assessments do not exceed 50 mGy, Bourgioti saw small risk for fetal malformations. However, radiologists also need to keep the cumulative fetal dose in mind.

In addition to the teratogenic effects of ionizing radiation, radiologists have to consider the carcinogenic stochastic effects. But the data regarding cancer and radiation exposure in fetuses are still inconsistent, Bourgioti explained. The International Commission of Radiological Protection (ICRP) estimates an occurrence of one cancer per 500 fetuses exposed to 30 mGy, or 0.2%. ACR comes up with higher risk: “A dose of 20 mGy represents an additional projected lifetime risk of about 40 additional cancers or less per 5,000 babies, or about 0.8 percent.”

So, CT should only be performed after an extensive risk-benefit assessment and according to the ALARA principle (As Low As Reasonable Achievable). Bourgioti recommended using abdominal shielding, although she has a contradictory opinion about it: “It provides the mother comfort. But I don’t think it helps, as most of the fetal radiation dose comes from internal scatter.”

CT examinations are, for example, necessary in patients with acute trauma, when ultrasound provided evidence of intraperitoneal free fluids.

CT angiography is the technique of choice for examining pulmonary emboli.

Contrast Agents – Hardly Any Studies Available

The use of contrast agents should generally be limited, as their effects on the fetus are not completely understood yet. It is known that contrast agents cross the blood-placental barrier, enter the fetal circulation, get excreted into the amniotic fluid and are swallowed by the fetus eventually reentering the fetuses circulation. Iodinated agents, however, did not show any mutagenic or teratogenic effects in animal tests and do not seem to affect neonatal thyrotropin (TSH). Bourgioti advised to be more cautious with gadolinium-based agents as animal studies revealed teratogenic effects at high and repeated doses. Guidelines therefore recommend administering only gadolinium-based contrast agents when a significant benefit is expected that outweighs possible unknown risks. If such a necessity appears, only macrocyclic agents and the lowest doses (0.1 mmol/kg) should be injected. An extremely low dose of the gadolinium-based agents is finally found in the breast milk, so that mothers do not necessarily need to stop breast-feeding.  

Conclusion

Practically all mentioned imaging modalities can be performed in pregnant women if really necessary for the woman’s health. The American College of Obstetricians and Gynecologists (ACOG) additionally reminds that “fear of fetal radiation exposure should not delay imaging studies that may help identify underlying maternal pathologic conditions.”

Presentation Title: Safety issues in pregnancy: what radiologists need to know 
Speaker: Charis Bourgioti, Athens, Greece
Date: Saturday, March 2nd, 2019
Session Code: RC 1307 – A 0809