In what units is radiation dose expressed?
The modern International System of Units (SI) unit for absorbed radiation dose is the gray (Gy), where 1 Gy = 1 Joule (J)/kilogram (kg) = 100 rad. The sievert (Sv) is the SI unit for equivalent dose, which describes the risk of stochastic biological adverse events, such as cancer and genetic damage, when tissue is irradiated. For ionizing radiation used in diagnostic imaging (x-rays, gamma rays), 1 Sv = 100 rem = 1 Gy. Absorbed dose from a medical imaging test is typically on the order of milligrays (mGy), and the associated biological risk is therefore on the order of millisieverts (mSv). The Sv is also the unit for effective dose, which takes into account the different sensitivities of different types of tissue to ionizing radiation.