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INTRODUCTION

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Frequently clinical conditions arise during pregnancy that require radiologic evaluation in order to make an accurate diagnosis and formulate a treatment plan. Many of these studies involve radiation; the latter immediately evokes consternation from patients, radiologists, and obstetricians. The most effective way to counteract this sense of trepidation is to first develop an understanding of the biologic effects of radiation. Next, a familiarity with which particular studies are indicated when evaluating for a particular diagnosis will also help minimize the exposure to unnecessary procedures. Finally, physicians should reassure patients that exposure to most imaging procedures are not thought to result in adverse pregnancy outcomes and have only a small effect on risk of childhood malignancies if the fetus is exposed in utero.

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RADIATION BASICS

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Radiation is “energy that comes from a source and travels through some material or through space.”1 Radiation can be divided into ionizing and nonionizing. Nonionizing radiation can cause an atom to vibrate but does not produce enough energy to remove electrons. It includes visible light, ultraviolet rays, and radio frequency. Ionizing radiation does contain enough energy to release electrons resulting in the production of “ions.” These ions produce damage by producing free radicals, breaking chemical bonds, and damaging molecules that play a role in DNA, RNA, and protein synthesis. The cell cycle is divided into four phases: mitosis (M), G1, DNA synthesis (S), and G2. Cells are most susceptible to the effects of radiation when actively dividing. This makes M phase the most sensitive, and S phase the least sensitive. Nonionizing radiation is used for common everyday tasks such as microwaves for heating food and radio waves for broadcasting. In medical imaging, nonionizing radiation is found in ultrasound waves and magnetic resonance imaging. Ionizing radiation can be found in the form of particulate or short-wave electromagnetic energy. Short-wave electromagnetic energy includes x-rays and gamma rays. In medicine, ionizing radiation is used both for imaging (eg, x-ray and computed tomography) and for therapy in the treatment of some malignancies.

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To understand the biology of ionizing radiation, it is important to understand the nomenclature and dosimetry. Radiation energy is discussed most often in terms of the type of ionizing radiation producing the energy as well as the amount absorbed. For example, alpha particles, one type of ionizing radiation, have a greater ability to cause tissue damage than x-rays, another type of ionizing radiation. The absorbed dose is a measure of the energy deposited and provides a way to gauge biologic effects. This measurement is usually made in “gray.” The “outdated” unit of the absorbed dose is the “rad” or “radiation absorbed dose”; however, this is the measurement familiar to most Obstetrician/Gynecologists. The biologic effect is best approximated by what is known as the “equivalent dose” which takes the type of energy into account as well as how much is absorbed. The equivalent dose is expressed in Sieverts and is ...

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