Complicating Effects of Radiation

The major factor determining the degree of risk to the fetus in an imaging technique is the amount of ionizing radiation involved in the test. Exposure to ionizing radiation occurs with plain x-ray films, angiography, fluoroscopy, nuclear medicine, and computed tomography (CT). Nonionizing studies include magnetic resonance imaging (MRI) and ultrasound.

The risks of radiation exposure vary with gestational age.

The predominant deterministic effect due to exposure during the first 2 weeks of pregnancy is resorption of the embryo.

The second to eighth week postconception is the period of organogenesis. Significant x-ray exposure during this period may result in teratogenesis (birth defects). Examples include gross malformation and growth retardation, the latter of which can occur both at term and later at adulthood. Neuropathology and small head size may also occur as a result of significant exposures during weeks 2 to 8.

The embryo has developed into a fetus at about the seventh to eighth week, and neurologic development occurs during the next 7 weeks. Significant x-ray exposure at this time (between weeks 8 and 15) may result in mental retardation, small head size, and decreased IQ. Other possible but less likely effects due to significant exposure during this period include growth retardation as an adult and sterility. Even less likely but still possible are cataracts, neuropathology, and growth retardation at term.

Most of the deterministic effects mentioned above are either not observed or observed much less frequently when the fetus receives significant radiation dose beyond 15 weeks postconception. Mental retardation has been observed as a result of significant exposure during the eighth to twenty-fifth weeks but not beyond. Other effects that have been observed because of exposure after week 15 include sterility and growth retardation as an adult. Less likely effects that have been demonstrated are cataracts, neuropathology, and growth retardation at term.

The most recent evidence suggests that 10 rad is a threshold for human teratogenesis, and the fetus appears to be most vulnerable at 8 to 15 weeks' gestation. The American College of Radiology's position states that there is no single diagnostic test that results in radiation doses that threaten the well-being of the developing embryo or fetus. However, cumulative doses from multiple procedures may enter the harmful range. The radiation doses involved in commonly used diagnostic tests are given in Table 102:6.

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