Ms. Wang, who was 5 months pregnant, consulted the doctor because she had a fever and cough for 3 days. During the flu season, it was difficult to tell whether it was influenza or pneumonia. In such a situation, should imaging be done or not? Should we do a CT or a radiograph? Dr. Li, who received the case, was torn. Similarly, 22-year-old Ms. Liu and her family are also torn. Because Ms. Liu had an X-ray taken 10 days ago due to wrist trauma, and later realized that she was nearly 1 month pregnant. will the X-ray harm the fetus? Will the X-rays harm the fetus and is it possible to have the baby? Ms. Liu’s family felt very anxious and uneasy. Due to the deep-rooted fear and lack of awareness of the dangers of X-ray ionization, many patients and medical professionals have many concerns and questions about imaging examinations for pregnant women. In addition, there is confusion and concern about the safety of ultrasound, magnetic resonance imaging (MRI), and contrast media for pregnant and breastfeeding women. In response to these concerns, the American College of Obstetricians and Gynecologists (ACOG), in conjunction with the American College of Radiology and the American Society of Ultrasound Medicine, released the latest edition of its Guidelines for Diagnostic Imaging in Pregnancy and Lactation in February 2016. X-rays and radiation are not as scary as one might think 1. X-rays and radiation X-rays are electromagnetic waves with extremely short wavelengths and high energies, and their main hazard is that they can produce an ionizing effect on the tissues of the body. Because embryos and fetuses are more sensitive to X-ray exposure, there is a deep-seated fear and concern about X-rays in pregnant women. According to studies, the risk of fetal exposure to X-rays correlates with gestational age at the time of exposure and radiation dose. For early developing embryos, very high doses (>1 Gy) of X-ray exposure can result in embryonic death. In addition, high-dose X-ray exposure can cause fetal growth restriction, microcephaly, and mental retardation. However, in actual diagnostic imaging, even multiple X-rays rarely reach the threshold doses that can cause these hazards. Figure 1 X-ray hazards as a function of week of gestation and radiation dose Note: *Data derived from animal studies, studies of Japanese atomic bomb survivors, and people treated with radiation for medical reasons (e.g., radiotherapy for uterine-related cancers) Based on data from studies of atomic bomb survivors, the greatest effects on the central nervous system of the foetus are associated with exposure to X-rays between the eighth and fifteenth weeks of gestation. Although it has been suggested that the minimum X-ray radiation threshold for mental retardation ranges from 60 to 310 mGy (1 Gy = 1000 mg), the lowest clinically documented exposure for such patients is 610 mGy or more. The new guidelines recognize that CT scans are also X-ray examinations and should not be avoided if there is a clear indication for the examination, but the risk/benefit ratio should be carefully evaluated.The radiation exposure from CT examinations varies significantly depending on the number of layers scanned, the location, and the parameters of the exposure (Figure 2). For example, the radiation dose for CT pelvic measurements can be as high as 50 mGy, but can be reduced to 2.5 mGy when using a low-exposure scanning technique. Therefore, it is important to communicate fully with the technician at the time of the examination in order to minimize the dose of radiation without compromising the effectiveness of the examination. In addition, the radiation dose to the fetus from spiral CT is usually comparable to that from conventional CT. Figure 2 Fetal radiation dose during common radiologic examinations 2. Safety of X-ray contrast agents Oral contrast agents used for X-ray examinations are not absorbed by the body and therefore pose no real or theoretical risk of harm. Iodine contrast agents commonly used in CT scanning, although they can pass through the placenta into the fetal circulation or amniotic fluid, have not been shown to have teratogenic or mutagenic effects in animal studies. Its potential adverse effects on the fetal thyroid gland have also not been demonstrated in human studies. Therefore, the risk of its use may not be significant, but it is prudent to note that the new guidelines still recommend that X-ray contrast agents be used only when they can give diagnostic information to the fetus or the pregnant woman that could definitely affect treatment. 3. Safety of Nuclear Medicine Imaging The basic principle of nuclear medicine imaging is to determine the physiologic function or dysfunction of an organ using tracers produced by radioactive isotopes. These isotopes can emit alpha, beta, or gamma rays and produce an ionizing effect similar to that of X-ray radiation. Fetal exposure in nuclear medicine imaging during pregnancy depends on the physical and biochemical characteristics of the radioisotopes used. Among these, the embryonic or fetal radiation exposure dose for lung ventilation perfusion scans using 99mTc, the most commonly used, is less than 5 mGy, and all the facts support that 99 Tcm examinations of less than 5 mGy are safe and harmless for pregnant women. However, radioactive iodine (131 I) can easily cross the placenta and has a half-life of up to 8 days, which may have adverse effects on the fetal thyroid gland (especially when used after 10 to 12 weeks of gestation), and is therefore contraindicated in pregnant women. 4. 4. The new guidelines state that: ① With few exceptions, the radiation exposure from diagnostic X-rays, CT, and nuclear medicine imaging is well below the dose that would be harmful to the fetus. Their use should not be denied or avoided if the examination is necessary. In rare cases where the exposure is higher than the usual dose, the patient should be informed of the risks and individualized prenatal imaging and diagnosis should be made for fetal structural abnormalities and growth restriction. (ii) Considering the all-or-none effect of prenatal X-ray exposure, women who have been exposed to X-rays within 2 weeks of fertilization and have had a successful pregnancy need not be overly concerned about their fetal safety. (iii) Whether intrauterine exposure to ionizing radiation is carcinogenic remains unclear, but it is currently thought that the risk of cancer is probably minimal. Some studies have shown that the natural probability of a fetus developing leukemia is 1/3000, and fetal X-ray exposure of 10-20 mGy may only increase the risk of leukemia by 1.5-2.0 times. Therefore, a pregnant woman should not be advised to terminate her pregnancy simply because she has received X-rays. ④ If a pregnant woman requires multiple imaging examinations with ionizing radiation effects, it would be prudent to consult a radiation physicist to calculate the total radiation dose that the fetus will receive, to assess the risk/benefit ratio, and to decide on the subsequent management. Safety of ultrasonography Unlike X-ray and CT examinations, there are no conclusive studies on the adverse effects of diagnostic ultrasonography (including Doppler ultrasonography) on the fetus. There have been concerns that the thermal effects of ultrasound during ultrasonography may have adverse effects on the fetus. However, the new guideline concludes that the thermal effect of ultrasound, even when obstetric ultrasonography (including ultrasound Doppler ultrasonography) is performed, does not increase the risk to the fetus or to the pregnancy when limiting the ultrasound transducer to a spatial peak time-averaged sound intensity of <720 mW/cm2. However, in view of the potential risk of hypothermia from ultrasound, the new guidelines still recommend that healthcare professionals should also be vigilant about the indications for ultrasound in their patients. Safety of MRI The basic principle of MRI is that it is a non-ionizing radiation imaging technique that uses the resonance of atomic nuclei in the body with an applied radiofrequency magnetic field to produce images. There are no clear contraindications for pregnant women. Although there are theoretical concerns about fetal malformations, tissue thermal damage, and hearing loss, based on the available data, the American College of Radiology does not have specific recommendations for MRI in pregnant women or women during the first trimester of pregnancy. Safety of Contrast Agents Associated with MRI Examinations MRI examinations sometimes require the use of contrast agents, such as gadolinium agents and superparamagnetic iron oxide. The use of gadolinium agents in pregnancy is currently controversial. The main concern is that water-soluble gadolinium agents can enter the fetal circulation and amniotic fluid and become toxic or teratogenic. Although the only prospective study confirmed that 26 patients who received gadolinium agents during early pregnancy (first trimester) did not experience adverse perinatal or neonatal endpoints, based on theoretical concerns and data from animal studies, the new version of the guidelines still recommends that gadolinium contrast agents be used only when the benefits clearly outweigh their potential risks. To date, there have been no animal or human fetal trials evaluating the safety of superparamagnetic iron oxide contrast agents, and there is no information on their use during pregnancy and lactation. Therefore, gadolinium agents are currently recommended when contrast agents are needed. Recommendations for the use of imaging in breastfeeding women ① Exogenous ionizing radiation (diagnostic X-rays) has no effect on breast milk and the subject does not need to discontinue breastfeeding. Radionuclide compounds can be secreted into breast milk at different concentrations and for different periods of time, and the amount of the same compound secreted into breast milk varies from person to person. Since some radionuclides can have harmful effects when secreted into breast milk, it is recommended that breastfeeding women discuss the use of these compounds with breastfeeding and nuclear medicine specialists. (iii) Conventional wisdom is that breastfeeding women should stop breastfeeding for 24 hours after receiving intravenous iodinated contrast media. However, since iodine contrast agent is water-soluble, the amount secreted in breast milk is less than 1%, and the dose absorbed by the infant through the gastrointestinal tract is less than 1% of the breast milk, there is no need to stop breastfeeding after the administration of iodine contrast agent. ④ The amount of gadolinium contrast agent excreted into breast milk is less than 0.04% of the intravascular dose used, and less than 1% of the latter may be absorbed by the infant's gastrointestinal tract. Therefore, breastfeeding does not need to be interrupted after the use of gadolinium contrast agents, and there are no reports of harm to the infant's health. Key Points: Key Recommendations for Imaging in Pregnant and Breastfeeding Women ① Ultrasound and MRI are not risky, but should be used with caution and are recommended only to answer clinically relevant questions or when beneficial to the patient. With few exceptions, diagnostic X-rays, CT scans, and nuclear medicine imaging examinations deliver radiation doses that are far below the dose that would be harmful to the fetus. If it is necessary to use them in combination with ultrasound or MRI, it is recommended that they not be rejected or avoided. (iii) The use of gadolinium contrast agents in MRI examinations should be limited, and it is recommended that they be used only when they can significantly improve diagnostic accuracy or are expected to improve the prognosis for the pregnant woman and the fetus. (iv) Breastfeeding does not need to be interrupted after the use of gadolinium contrast contrast agents.