Since both equivalent dose and effective dose cannot be measured directly, to determine external exposure, the ICRP relies on the operational quantities, defined by the International Commission on Radiation Units and Measurements, Inc. (ICRU): Ambient dose equivalent, H(10), and directional dose equivalent, H’(0.07, O), are for area monitoring, and personal dose equivalent, Hp(d), is for individual monitoring. Compliance with dose limits can be ascertained with the use of properly worn dosimeters. To link the protection and operational quantities to physical quantities that characterize the radiation field (such as tissue absorbed dose, air kerma free-in-air and particle fluence), the ICRU computed conversion coefficients. To assess internal exposure, the ICRP recommends the use of activity quantities in combination with dose coefficients based on physiological models and 4-D computations. The unit for all the ICRP and ICRU quantities listed above is the sievert (Sv).

Effective dose should be used only for occupationally exposed workers and members of the public, where doses are assumed to be low, well below 100 mSv, and thus, only stochastic effects are considered. At doses above about 0 .5-1 Sv, where tissue reactions (deterministic effects) may occur, the dosimetric quantity to use is the absorbed dose in the irradiated tissue modified by the radiobiological effectiveness of the radiation for the biological endpoint of concern. The unit is the gray (Gy).

Effective dose should not be used for retrospective evaluation of exposed populations or to assess individual risks, as is the case in medical exposures, which are not subject to dose limitations. Exposures in radiotherapy are clearly expressed in absorbed dose to the irradiated tissue. Since both the irradiation conditions and the exposed group of patients are known, exposures to individual patients from medical imaging, even those at low levels, should also be expressed as absorbed doses to the irradiated organs, as the ICRU recommends. The methods of organ dose calculations include placing external dosimeters such as TLD or OSL on the patient’s skin, making measurements in physical phantoms that simulate patients and performing Monte Carlo radiation transport calculations on mathematical phantoms. BEIR VII has calculated stochastic risks for many organs/tissues exposed to low doses of low LETradiations . ICRP has just published new threshold dose values for tissue reactions .With these values, patient risks can be estimated. However, if the goal is not to assess risk, but to reduce patient exposure, diagnostic reference levels can be easily determined, as they are always expressed in machine parameters, such as incident air-kerma for radiography/fluoroscopy and CT air-kerma index and air-kerma length-product for CT.