hese studies are performed by "tagging" a radioactive element to a carrier that can be injected, inhaled, or swallowed. For example, the radioisotope technetium-99m may be tagged to red blood cells, sulfur colloid, or pertechnetate. he method used to tag the agent determines fetal radiation exposure. he amount of placental transfer is obviously important, but so is renal clearance because of fetal proximity to the maternal bladder. Measurement of radioactive technetium is based on its decay, and the units used are the curie (Ci) or the becquerel (Bq). Dosimetry is usually expressed in millicuries (mCi). he efective tissue dose is expressed in sievert units (Sv) with conversion as discussed in Table 46-4: 1 Sv = 100 rem = 100 rad.
Depending on the physical and biochemical properties of a radioisotope, an average fetal exposure can be calculated (Wagner, 1997; Zanzonico, 2000). Commonly used radiopharmaceuticals and estimated absorbed fetal doses are given in Table 46-8. The radionuclide dose should be kept as low as possible (Adelstein, 1999; Zanotti-Fregonara, 2017). Exposures vary with gestational age and are greatest earlier in pregnancy for most radiopharmaceuticals. One exception is the later efect ofiodine-131 on the fetal thyroid (Wagner, 1997).
Discussed earlier, some still use the ventilation-perfusion lung scan for suspected pulmonary embolism. It is also used if CT angiography is nondiagnostic. Perfusion is measured with injected 99T c-macroaggregated albumin, and ventilation is measured with inhaled xenon-127 or xenon-133. Fetal exposure with either is negligible (Chan, 2002; 110untford, 1997).
Thyroid scanning with iodine-123 or iodine-131 seldom is indicated in pregnancy. With trace diagnostic doses used, however, fetal risk is minimal. Importantly, therapeutic radioiodine in doses to treat Graves disease or thyroid cancer may cause fetal thyroid ablation and cretinism.
amCi = millicuries. To convertto mrad, mUltiply by 100.
bExposures are generally greater prior to 12 weeks compared with increasing gestational ages.
cSome measurements account for placental transfer.
dThe uptake and exposure of 1 increases with gestational age.
Tc04 = pertechnetate; TI = thallium.
Data from Adelstein, 1999; Schwartz, 2003; Stather, 2002; Wagner, 1997; Zanzonico, 2000.
he sentinel lymphoscintigram, which uses 99mT c-sulfur indispensable in everyday practice. Its wide-ranging clinical colloid to detect the axillary lymph node most likely to have uses are further discussed in Chapter 10 and in most other secmetastases from breast cancer, is a commonly used preoperative tions of this book. study in nonpregnant women (Newman, 2007; Spanheimer, 2009; Wang, 2007). As shown in Table 46-8, the calculated dose is approximately 0.014 mSv or 1.4 mrad, which should MAGNETIC RESONANCE IMAGING not preclude its use during pregnancy.