|Quantity||Units of Measurement||What It Is||What It Measures||Why It's Useful||Conversion Between Old and New Units|
|Absorbed dose||gray (Gy) or milligray (mGy) [rad or millirad (mrad)]||The amount of energy locally deposited in tissue per unit mass of tissue||Measures concentration of energy deposition in tissue||Assesses the potential biological risk to that specific tissue||1 rad = 10 mGy|
|Effective dose||sievert (Sv) or millisievert (mSv) [rem or millirem (mrem)]||An attributed whole-body dose that produces the same whole-person stochastic risk as an absorbed dose to a limited portion of the body||Converts any localized absorbed or equivalent dose to a whole-body risk factor||Permits comparison of risks among several exposed individuals, even though the doses might be delivered to different sets of organs in these individuals||1 rem = 10 mSv|
|Air kerma*||gray (Gy) or milligray (mGy) [rad or millirad (mrad)]||The sum of initial kinetic energies of all charged particles liberated by the X-rays per mass of air||Measures amount of radiation at a point in space||Assesses the level of hazard at the specified location†||1 rad = 10 mGy|
|Exposure||millicoulomb·kg−1[roentgen (R) or milliroentgen (mR)]||The total charge of ions of one sign produced by the radiation per unit mass of air||Measures amount of radiation at a point in space||Assesses the level of hazard at the specified location†||1 millicoulomb·kg−14 Roentgen (R)|
|Equivalent dose‡||sievert (Sv) or millisievert (mSv) [rem or millirem (mrem)]||A dose quantity that factors in the relative biological damage caused by different types of radiations||Provides a relative dose that accounts for increased biological damage from some types of radiations||This is the most common unit used to measure radiation risk to specific tissues for radiation protection of personnel‡||1 rem = 10 mSv|
↵* Air kerma can be presented in two separate ways. Incident air kerma is the kerma to air from an incident X-ray beam measured on the central beam axis at the position of the patient and excludes backscattered radiation. Entrance surface air kerma is the kerma to air from an incident X-ray beam measured on the central beam axis at the position of the patient with backscattered radiation included. The two may differ from each other by up to about 40%.
↵† Exposure and air kerma are both used for the same purpose. Exposure used to be the most common measure, but with the switch to international units, air kerma is the preferred unit.
↵‡ For X-rays, gamma rays, and electrons, there is no difference between absorbed dose and equivalent dose, i.e., 1 mGy = 1 mSv. This is not the case for neutrons and alpha particles, but these radiation types are not relevant to X-ray exposure. The important issue is that cardiologists recognize that for their interests there is no practical difference between a measurement of mGy and that of mSv.