The unit to measure radiation doses to tissue is the sievert (Sv). Since 1 Sv is a large quantity, radiation doses normally encountered are expressed in the millisievert (mSv) or microsievert (µSv) unit, which are respectively one-thousandth or one millionth of a sievert.
1Sv = 1,000 mSv = 1,000,000 µSv
Gray (Gy) is the unit of absorbed radiation dose of ionising radiation, and is defined as the absorption of one joule of ionising radiation by one kilogram of matter (usually human tissue).
1 Gy = 1 Sv = 1 J/kg
The roentgen equivalent in man (rem) is a unit of radiation dose equivalent. It is the product of the absorbed dose in rads and a weighting factor, WR, which accounts for the effectiveness of the radiation to cause biological damage. A rem is a large dose of radiation, so the millirem (mrem), which is one thousandth of a rem, is often used. Rem and millirem are variant metric units in widest use in USA.
1 rem = 1000 mrem