CT in Pregnancy

Risks and Benefits

Claudia T. Sadro MD, FABR, FRCPC; Theodore J. Dubinsky MD

Disclosures

Appl Radiol. 2013;42(10):6-16. 

In This Article

Diagnostic Imaging Tests

The safest imaging test in pregnancy is ultrasound. It has no known adverse effects on the mother or fetus. It is the first-line imaging test in pregnant patients where applicable.

Plain films result in negligible dose to the fetus when the fetus is not in the field of view. It is recommended that a radiation shield be applied over the gravid uterus for these examinations. Studies through the gravid uterus, such as radiographs of the abdomen and lumbar spine, result in a fetal dose of 1 mGy to 3.5 mGy.[5,6] The background radiation dose to the fetus for 9 months of pregnancy is 0.5 to 1 mGy[6] (Table 1).

CT focuses a collimated beam of x-rays on the body part of interest. When the fetus is not in the field of view, the fetal radiation dose is negligible (Table 1). The fetal dose for a CT head is 0 mGy and for a CT chest is 0.2 mGy or less depending on the trimester of pregnancy. Shielding the gravid uterus does little to reduce the radiation dose to the fetus for CT, but it is often done for reassurance.[6] Concern about the harmful effects of x-rays to the fetus from CT is only considered for those examinations where the gravid uterus is in the field of view. The typical fetal radiation dose for a routine CT of the abdomen and pelvis is 25 mGy.[6] With modern CT scanners that use automated exposure control the dose is about 13 mGy.[7,8] Low-dose CT KUB protocols can result in doses on the order of 10 mGy to 11 mGy[6] (Table 1). Intravenous iodinated and oral contrast may be given in pregnancy.[1,9] Intravenous iodinated contrast is a Food and Drug Administration (FDA) class B agent. Very little iodinated contrast crosses the placenta and enters the fetal circulation. Animal studies have shown no adverse effects to animal pregnancies and no adverse effects have been observed in humans. Thyroid dysfunction in the newborn has not been observed when iodinated contrast has been administered to the pregnant mother intravenously.[1,3] As with all drugs in pregnancy, intravenous iodinated contrast should be administered only if necessary and after informed consent has been obtained.

X-rays are a form of ionizing radiation that has potentially harmful effects on biologically living tissue. Deterministic effects are due to cell death. In the mother, deterministic effects from ionizing radiation include skin erythema, hair loss, and cataracts. They do not occur below a threshold of 1000 mGy[10,11] and are not observed for diagnostic CT scans. In the fetus, deterministic effects include teratogenic effects, such as prenatal death, small head size, mental retardation, intrauterine growth restriction, and organ malformations. These teratogenic effects do not occur below a threshold of 50 mGy to 100 mGy and only occur in the first 15 weeks postconception[5,6] (Table 2). Above 100 to 150 mGy, the risk is serious enough that one may discuss therapeutic abortion with the mother[5] (Table 3). Since the fetal dose from a CT scan through the abdomen and pelvis is < 50 mGy, teratogenic effects are not a concern to the fetus when exposed to a single exam. They become a concern when multiple exams or multiphase exams are performed.

Stochastic effects are due to DNA damage. In both the mother and fetus, this is carcinogenic. The probability of the effect rather than the severity increases with increasing dose, and there is no threshold below which there is no risk. According to the BEIR VII that studies the survivors of Hiroshima and Nagasaki, the background risk of cancer over the course of an individual's lifetime is 42%. Exposure to 100 mSv of ionizing radiation increases lifetime attributable risk of cancer by 1%. This risk is greater for women and children. It assumes a linear no-threshold risk model and extrapolates back for lower exposures from diagnostic imaging studies.[12] Exposure to 10 mSv ionizing radiation, such as from a CT chest or a CT abdomen, increases one's lifetime attributable risk of cancer by 0.1 %, and this risk is cumulative with serial CT scans performed over one's lifetime. Radiation-induced cancers in those exposed after birth include breast, lung, thyroid, stomach, and colon cancer, as well as leukemia.[2,11,12] This must be taken into account when performing CT scans in pregnant women as in all young individuals. The fetus is more sensitive to carcinogenesis than children and adults and also has a longer life expectancy in which cancers may manifest. Studies of the survivors of Hiroshima and Nagasaki as well as human studies of in utero exposure to diagnostic radiation have demonstrated an increased risk of cancer in children when the fetus is exposed in utero to doses as low as 10 mGy.[5,12,13] The main radiation-induced cancer in childhood is leukemia, but in utero exposure also increases the risk for solid organ cancers.[12,13]

The BEIR VII interpretation of the Oxford Survey of Childhood Cancer indicates that the background risk of cancer death in children is 0.14 % and that exposure to 10 mGy in utero increases the risk of childhood cancer death by 0.06%.[13,14] In other words, there is a 40% increase in childhood cancer with in utero exposures of 10 mGy (relative risk of 1.4).[12] Childhood cancer is often defined as cancer occurring before 15 years of age. According to the ACR Practice Guideline for Imaging Pregnant or Potentially Pregnant Adolescents and Women with Ionizing Radiation from 2008, exposure of the newborn child to 10 mGy of ionizing radiation increases the absolute lifetime risk of developing cancer by 0.4 % and exposure to 50 mGy increases the absolute lifetime risk of developing cancer by 2%, assuming a linear no-threshold model. It is also assumed that this reflects the potential risk to the conceptus in utero for the second and third trimesters and part of the first trimester, although the authors acknowledge that there are uncertainties with this estimate.[15] Nonetheless, the consensus is that for a single CT scan through the abdomen and pelvis that exposes the fetus to 25 mGy of ionizing radiation or less, the risk is low (on the order of 1%) and CT may be performed in patients who have a medically necessary indication.

Organizations involved in radiation concerns in pregnancy have issued key policy statements. In 1977, the National Council of Radiation Protection and Measurements issued the following statement: "The risk of abnormality (to the fetus) is considered to be negligible at 50 mGy or less when compared to other risks of pregnancy, and the risk of malformations is significantly increased above control levels only at doses above 150 mGy."[16] In 2004, the American Council of Obstetricians and Gynecologists issued this statement: "Women should be counseled that x-ray exposure from a single diagnostic procedure does not result in harmful fetal effects. Specifically, exposure to < 5 rad (50 mGy) has not been associated with an increase in fetal anomalies or pregnancy loss."[17] These statements are supported by the American College of Radiology (ACR). The ACR Appropriateness Criteria for imaging acute pelvic pain in the reproductive age group from 2008 states: "CT should not be withheld in the pregnant patient if deemed clinically necessary to avoid significant delay in diagnosing life threatening conditions, such as appendicitis, bowel perforation, or sepsis." It acknowledges that CT demonstrates the best diagnostic performance in identifying gastrointestinal and urinary tract causes of acute pelvic pain.[18]

Magnetic resonance imaging (MRI) at 1.5 Tesla or less is safe in all trimesters of pregnancy. The safety of MRI at 3.0 T has not been proved and pregnant patients should be imaged at 1.5 T or less.[1,9] Intravenous gadolinium for MRI is an FDA class C agent and should not be given in pregnancy. It crosses the placenta into the fetal circulation and circulates indefinitely.[1,9] Although no adverse effects have been observed in humans, animal studies have shown congenital anomalies in animal offspring exposed in utero. Noncontrast MRI at 1.5 T has a specific role in pregnancy. Of course, it should be used for indications where it is superior to CT. This includes certain neurologic and musculoskeletal indications. It also includes hepatobiliary indications, such as MR cholangiopancreatography (MRCP). The use of MRI in place of CT for conditions usually diagnosed with CT will be discussed in this review.

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