Imaging modalities are used as adjuncts for diagnosis and therapy during pregnancy. Options include sonography, radiography, and magnetic resonance (MR) imaging. Of these, radiography is the most problematic. Inevitably, some radiographic procedures are performed before recognition of early pregnancy, usually because of trauma or serious illness. Fortunately, most diagnostic radiographic procedures are associated with minimal fetal risks. As with drugs and medications, however, these procedures may lead to litigation if pregnancy outcome is adverse. And, x-ray exposure may lead to a needless therapeutic abortion because of patient or physician anxiety.
Since 2007, the American College of Radiology (ACR) has addressed the growing concern of radiation dose in all ields of medicine (Amis, 2007). Some of its goals were to limit exposure through radiation safety practices and promote lifelong accumulated records of exposures in any given patient. Task Force recommendations included additional considerations for special radiosensitive populations, such as children and pregnant and potentially pregnant women. At our institutions, special recommendations are made for gravidas. Radiation exposure values and duration are recorded and monitored in high-exposure areas such as computed tomography (CT) and luoroscopy.
he term radiation literally refers to energy transmission and thus is often applied not only to x-rays, but also to microwaves, ultrasound, diathermy, and radio waves. Of these, x-rays and gamma rays have short wavelengths with very high energy and are ionizing radiation forms. The other four energy forms have rather long wavelengths and low energy (Brent, 1999b, 2009).
Ionizing radiation refers to waves or particles-photons-of signiicant energy that can change the structure of molecules such as those in DNA or that can create free radical or ions capable of secondarily damaging tissue (Hall, 1991; National Research Council, 1990). Methods of measuring the efects of x-rays are summarized in Table 46-4. The standard terms used are exposure
TABLE 46-4. Some Measures of Ionizing Radiation
Exposure Number of ions produced by x-rays per kg
Dose Amount of energy deposited per kg of tissue
Relative Amount of energy deposited per kg effective of tissue normalized for biological (in air), dose (to tissue), and reative fictive dose (to tissue). In the range of energies for diagnostic x-rays, the dose is now expressed in grays (Gy) , and the relative efective dose is expressed in sieverts (Sv). hese can be used interchangeably. For consistency, all doses discussed subsequently are expressed in contemporane ously used units of gray (1 Gy = 100 rad) or sievert (1 Sv = 100 rem) . To convert, 1 Sv = 100 rem = 100 rad.
As noted, the biological efects of x-rays are caused by an electrochemical reaction that can damage tissue. According to
Brent (1999a, 2009), x-and gamma-radiation at high doses can create two types of biological efects and reproductive risks in the fetus. These are deterministic eicts and stochastic focts, which are both described in the next sections.
One potential harmful efect of radiation exposure is determin istic, which may result in abortion, growth restriction, congeni tal malformations, microcephaly, or mental retardation. hese deterministic efects are threshold efects, and the threshold level is the NOAEL-No Observed Adverse Eict Level (Brent, 2009). lthough controversial, the NOEL concept supports that there is no risk below a threshold dose (0.05 Gy or 5 rad).
It also suggests that the threshold for gross fetal malformations is more likely to be 0.2 Gy (20 rad).