Rad (Radiation Absorbed Dose):
Takes into account the fact that different absorbers that receive an identical exposure will absorb different amounts of energy. A Rad therefore represents the amount of energy from ionizing radiation that is transferred to a mass of material, typically patients undergoing diagnostic or therapeutic studies.

Rem (Roentgen Equivalent Man):
Is a unit that correlates the dose of any radiation to the biological effect of that dose. To relate the absorbed dose of specific types of radiation to their biological effect, a "quality factor" must be multiplied by the dose in rad, which then shows the dose in rems. Thus, rem = rad x Q. For gamma rays, X rays, and beta particles, 1 rad of exposure results in 1 rem of dose. X rays and gamma rays have a Q about 1, so the absorbed dose in rads is the same number in rems. Neutrons have a Q of about 5 and alpha particles have a Q of about 20. An absorbed dose of 1 rad of these particles is equivalent to 5 rem and 20 rem, respectively.

International Commission on Radiation Units and Measurements:

Gray (Gy): defined as 100 Rads

Sievert (Sv): defined as 100 Rem

1 mGy = 100 mRads
1 mSv = 100 mRem

1 μGy = 100 μRads
1 μSv = 100 μRem

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