First, the ALARA (as low as reasonably achievable) principle is taken into account at every radiological investigation. If it is possible, pregnant patients should wait until the end of pregnancy and after that, they should undergo radiological studies. Nevertheless, some pathologies are more common during pregnancy which require deeper and immediate examinations

Firstly we will discuss those radiological investigations that don’t require ionizing radiation (ultrasound and MRI). Secondly, we will discuss those that use ionizing radiation (radiographs, CT, nuclear medicine).

1. Ultrasound

            The U.S. Food and Drug Administration set a max limit for ultrasound exams: 720 mW/cm2. From this number the fetus's temperature may rise by 2 C, but it is controversial. B-mode has the lowest risk of temperature rise. Color and spectral Doppler have the highest risk. Obstetric ultrasounds don't produce high temperatures. However, color Doppler will produce a higher temperature when used correctly for obstetric indications and won't affect the fetus or pregnancy  [3].

2. MRI

            Many theories about MRI caused fetal harm exist, which include heat deposition into the fetus, altered cell migration and proliferation in the 1^st trimester, and damage to developing auditory nerves due to high acoustic noise [4]. However, heat deposition is likely clinically insignificant on magnets that are 3T or lower in field strength, and the risk of altered migration remains theoretical. Acoustic noise causing permanent damage is unlikely because of the brief exposure to MRI as well as maternal body attenuation of at least 30dB insulating and protecting the fetus [5] .

            Bear in mind that gadolinium contrast agent shouldn’t be used, because its use is associated with a higher incidence of rheumatologic, inflammatory and infiltrative skin conditions, stillbirth and even death.

3. Radiography

            Ionizing radiation greatly affects the conceptus. It has two main effects: stochastic and deterministic (Figure 1).  Deterministic effect is threshold-dependent and short-term and includes: intrauterine death, malformation, mental retardation, and intrauterine growth restriction. The most common deterministic effects are: growth restriction, microcephaly, and intellectual disability (Table 1). The risk of intellectual disability is the highest at 8-15 weeks of exposure, as data from atomic bomb survivors show. The minimal threshold for this adverse effect may be in the range of 60-310 mGy. However, the lowest clinically documented dose to cause severe mental retardation is 610 mGy [3] Stochastic effect is a long-term and threshold-independent mutagenic effect. Because of its single cell damage, stochastic effect includes cancer and hereditary disease. Although cancer, especially leukemia and lymphoma [6] has to be given further consideration, for the most of radiological procedures the risk of ionizing radiation required for childhood cancer is very low (below 1 in 10,000). Compared to the natural risk (around 1 in 500), this number is acceptable  [7]. Hereditary diseases are negligible. Its risk is over ten times lower than ionizing radiation-induced childhood cancer. Moreover, the naturally appearing congenital risk frequency is much higher (1-6%) than radiation ionization-exposed hereditary defects [7].

           Many articles firmly confirm that adverse effects are negligible at radiation doses < 50 mGy. X-rays use low-dose ionizing radiation (Table 2).  Nevertheless, fetal doses below 100 mGy should not be considered a reason to terminate a pregnancy based on radiation risk [6]. One study performed a subgroup analysis of patients exposed to a relatively low dose (<1 mGy) versus a higher dose (>1.1 mGy). No abnormality was observed in the high-dose group. The live birth rate was significantly higher in the low-dose group, probably because of the high miscarriage rate [2]. Even dental radiography does not pose a risk of fetal effects, including fetal growth retardation. Therefore, in indicated cases, a plain radiograph doesn't change the physiological development of the conceptus.

            There isn’t any link between pre-conception irradiation of either parent’s gonads and fetus malformations or childhood cancer [8]. However, some authors suggest to wait for 2 spermatic cycles and 3 menstrual cycles [1]. When a pregnant woman is irradiated, the irradiated zone has to be determined. If the head, neck, or extremities were irradiated, then the risk of fetal exposure is minimal, even though scattered radiations reach the fetus.

4. CT

            CT has higher radiation doses than X-rays (Table 3). The use of CT is increasing and that’s why we should pay attention to this.  The effects of CT radiation are the same as what we discussed above. Higher dose examinations are abdominal, pelvic CT-s, and PET CT.

            The most common acute conditions that theoretically require CT examination are appendicitis, pulmonary embolism, urolithiasis, and trauma. Figure 2 illustrates their diagnostic algorithm (Figure 2):

Pulmonary embolism is the most common cause of pregnancy-related mortality. Clinical symptoms are atypical and the D-dimer test cannot be used to exclude pulmonary embolism. It is sufficient if ultrasound is positive for deep vein thrombosis [10]. In negative cases, chest CT is preferred to ventilation-perfusion scintigraphy. However, this is controversial. Some authors believe that the absorbed dose to the fetus is significantly lower in the first trimester than with lung scintigraphy, the doses are comparable in the second trimester, and the dose to the fetus is slightly higher with CT in the third trimester. However, the dose to the mother's breast is significantly higher with CT than with scintigraphy, resulting in a higher effective dose [6].

For appendicitis, ultrasound is a better option than CT. Appendicitis is confirmed when the appendix looks like a non-compressible tubular structure of more than 6 mm in diameter, with a thickened wall. This is highly specific, but its sensitivity continuously decreases from the 1st to the 3rd trimester. In order to make clear the diagnosis, an MRI or CT is required. Definitely, MRI is first preferred over CT.

When a pregnant patient is suspected of urolithiasis, an ultrasound is required, which can be repeated in 24 hours. In negative cases MR is preferred over CT, however, CT is more accurate. Low-dose CT can be an alternative imaging method.

In trauma, the benefit-risk relation is dominated by the benefit of imaging, and when life is in danger radiation exposure should not be a reason for omitting investigations that are justified without pregnancy. Depending on the suspected locations of injury, different imaging procedures will be used.

            Iodinated contrast agents could be used freely, it does not harm the fetus.

5. Nuclear medicine

            In this section, the physical and biochemical properties of the radioisotope determine fetal risk, not ionizing radiation.  However, not all radioisotopes can be used safely during pregnancy. Radioactive iodine (iodine-131) readily crosses the placenta, its half-life is 8 days, and adversely affects the fetal thyroid gland, especially after 10-12 weeks of gestation. Iodine 131 should not be used during pregnancy, indifferently to its purpose. If a diagnostic scan of the thyroid gland is essential, technetium-99m is the isotope of choice.

At 10-12 gestational weeks the fetal thyroid accumulates  I123 (it also crosses the placenta). Therefore,  the fetal thyroid dose will be much higher than the total fetal body dose, resulting in hypothyroidism, dwarfism, and an increased risk of thyroid cancer. If pregnancy is detected within 12 hours of I123 administration, rapid and repeated oral administration of stable KI (60-130 mg) to the mother may reduce the fetal thyroid dose [6].

            If radioisotopes are excreted by the kidneys, the pregnant patient should drink more fluid than usual, approximately 1-2 L extra fluid, to reduce and prevent fetal damage.