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Table I: Common Terminology Used in Radiation Dosimetry

Absorbed dose: mean energy imparted to the matter in a volume by ionizing radiation, divided by the mass of the matter in the volume.

    Gray (Gy): special name for 1 Joule / kg, the SI unit of absorbed dose

    Rad (radiation absorbed dose): 0.01 Gy, traditional unit of absorbed dose

Activity (A): average number of spontaneous nuclear decays in a given period of time

    Bequerel (Bq): 1 decay per second, the SI unit of activity

    Curie (Ci) : 3.7 × 1010 Bq, traditional unit of activity

Air kerma: sum of the kinetic energy of all charged particles liberated per unit mass in air, measured in Gy or rad.

Cumulative dose (CD): total air kerma during a procedure, commonly measured at the interventional reference point.  Measured in Gy, also called cumulative air kerma.

Deterministic effect: an effect that occurs predictably when a dose exceeds a threshold, e.g. skin injury and posterior subcapsular cataracts.

Dose-area product (DAP): air kerma multiplied by x-ray beam cross-sectional area, commonly measured in mGy × cm2.

Dose coefficient: a measure of dose per unit activity, estimated from mathematical biokinetic models.  May reflect equivalent dose per unit activity to an individual tissue (DT /A), or effective dose per unit activity to a person (E/A).

Dose-length product (DLP): integrated radiation dose for an entire CT examination, measured in units of mGy × cm and calculated by multiplying the CTDIvol by the length irradiated.

Effective dose (E): sum of the weighted equivalent doses for all tissues, and also measured in Sv or rem.  Similar to the older term effective dose equivalent.

Entrance skin dose (ESD): absorbed dose to skin at a particular location on the entrance surface of the x-ray beam, measured in Gy.  Measures both incident and backscattered radiation.

Equivalent dose: product of the absorbed dose and the radiation weighting factor.

    Sievert (Sv): special name for 1 Joule/kg, the SI unit of equivalent dose

    Rem (roentgen equivalent man): 0.01 Sv, traditional unit of equivalent dose

Exposure: the total charge of ions of one sign produced in a unit of dry air by a given amount of x-ray or γ-ray radiation; SI unit is Coulombs / kg.

    Roentgen (R): conventional unit of exposure, equal to 2.58 × 10-4 C/kg.

Incident dose (ID): air kerma along the x-ray beam axis at the level of the skin surface, measured in Gy.  In comparison with ESD at the same point, ID does not measure backscattered radiation.

Interventional reference point (IRP): the point on the x-ray beam axis lying 15 cm from the imaging system’s isocenter on the x-ray tube side.

Kerma-area product (KAP): synonym for dose-area product.

Peak skin dose (PSD): the highest absorbed dose received by any location of a patient’s skin, measured in Gy.  Also called maximum skin dose (MSD).

Radiation weighting factor (wR): factor quantifying the biological effect of a given absorbed dose, reflecting the type and quality of radiation emitted by a radionuclide or external radiation field.  Dimensionless, ranging from 1 for photons and electrons to 20 for alpha particles.

Stochastic effect: an effect that occurs with a probability that increases with dose, e.g. malignancy and germ cell mutations.

Tissue weighting factor (wT): factor reflecting the relative probability of stochastic effects, such as cancer incidence, in a given organ or tissue.  Dimensionless, the sum of wT over all tissues is 1.

Weighted equivalent dose: product of the equivalent dose and the tissue weighting factor.  Also measured in Sv or rem.

Volume computed tomography dose index (CTDIvol): average radiation dose over the volume scanned in a helical sequence, commonly measured in mGy.

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