Risk of low-level ionizing radiation from medical imaging procedures.

Medical imaging procedures with x-ray, computed tomography (CT) and nuclear isotopes use ionizing radiation.1–4 On the other hand, ultrasound and magnetic resonance imaging (MRI) do not. When biological tissues are exposed to radiation, the effects are related to the type of radiation and the amount absorbed. Units of absorbed dose are grays (Gy) (joules/kilogram). One rad (radiation absorbed dose) is 0.01 Gy.5 As not all types of radiation affect tissue the same, the equivalent dose (measured in Sieverts) takes this into account. X-ray, CT, and gamma ray radiation have a “weighting factor” of 1.0, therefore with these medical procedures 1 Gy = 1 Sv. Medical imaging doses are usually measured in units of milliSieverts (mSv). In addition, one REM is 0.01 Sv (10 mSv). An average exposure from natural background radiation in the United States for an individual is ∼3–5 mSv/year.5–8 Also, a small amount of radiation exposure occurs with tobacco, the domestic water supply, building materials, and to a lesser extent televisions, and computer screens.9 The usual radiation dose of a chest radiograph is 0.02–0.1 mSv.10–12 Representative radiation doses for other radiological studies are shown in Figure 1.13–17 These medical studies contribute up to 20% of the total annual radiation exposure to the population of the United States.18 The National Research Council’s Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation has published their most recent report, the BEIR VII Report.19 The purpose of this report is to advise the U.S. Government on the relationship between exposure to ionizing radiation and human health. The BEIR VII Report describes a cancer risk model (linear no-threshold model) in which cancer risk diminishes proportionately to diminishing radiation exposure.