Various radiation dose parameters are used in diagnostic radiology, the most commonly being absorbed dose and effective dose. Absorbed dose, expressed in grays (Gy), is a measure of the energy per unit of mass deposited in the tissue and organs of the body. Radiation dose from ionising radiation is frequently quantified in terms of the effective dose. The effective dose, expressed in sieverts (Sv), is calculated from information about absorbed doses to the organ or tissue exposed to X-rays and the relative radiation risk assigned to each of these organs or tissues. Appropriate weighting factors related to radiogenic risk for body organs and tissues have been published by the International Commission on Radiological Protection (ICRP) [19]. The effective dose was introduced to allow estimation of radiogenic risks when various organs receive different levels of dose. This commonly occurs with partial body exposures, which is always the case with DXA. The effective dose is a useful quantity for comparison among different sources of ionising radiation, such as that from DXA and QCT or DXA and natural background radiation. The worldwide average effective dose from natural background radiation is 2.4 mSv/year.