Radiation dose quantities and units
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.
Two dosimetric quantities are utilised in CT, CT dose index (CTDI) and dose–length product (DLP) [20]. The CTDI represents the average absorbed dose, along the z axis, from a series of contiguous exposures. CTDI measurements are performed at the periphery (CTDIP) and at the centre (CTDIC) of cylindrical poly(methyl methacrylate) phantoms representing the human head and body by using a pencil ionisation chamber with a length of 100 mm. From these measurements, a weighted CTDI (CTDIW) representing the average dose to a single slice can be derived as follows:
To take into account the effect of pitch on radiation dose, CTDI volume (CTDIV) has been introduced for imaging performed in the spiral mode, which is defined as CTDI divided by pitch. CTDIV is the dose quantity displayed by the operator’s console of most CT systems. The DLP is defined as the CTDIV multiplied by the imaging length. The SI unit for DLP is mGy cm. Thus, DLP is an indicator of the integrated dose of an entire CT examination. Broad estimates of effective dose E can be derived from DLP values using conversion coefficients: E = DLP × k where k is the normalised effective dose (mSv mGy−1 cm−1) that is a function of body region [21].