Dosage is an important factor in the use of ionization radiation as well as in application of contrast agents or radiopharmaceuticals and the dosage should be comply with the ALARA principle (As Low As Reasonably Achievable).
Ionizing radiation comes from natural and artificial sources. Radiation effects depend on the type of radiation, and various units are used for measurement of dosages including gray, sievert, radiation absorbed dose (RAD), roentgen equivalent in man (REM), and roentgen. The amount of radiopharmaceutical given to a patient is measured in becquerels (Bq).
The dosage of contrast media in radiographic or computer-tomographic procedures should be tailored according to the diagnostic indications, the iodine concentration, and the patient's body size and age.

See also Administrative Dose Guidelines.

(rem) The rem is the dose equivalent of the absorbed dose considering the biological efficiency of the radiation, also called quality factor. With a quality factor of 1 is 1 rem equal to 1.07185 roentgens. One millirem is one thousandth of one rem.
In the SI system of units the rem is replaced by the sievert (Sv), which is numerically equal to the absorbed dose in gray modified by a radiation weighting factor. The conversion factor has been readjusted from 1 to 1.07185 so that 100 rem are equal to 1 Sv.