There are a number of units to measure radiation dose and exposure:

• rad or radiation absorbed dose:
  The amount of radiant energy absorbed in a certain amount of tissue.
• gray (Gy):
  A unit of absorbed radiation equal to the dose of one joule of energy absorbed per kilogram of matter, or 100 rad. The unit is named for the British physician L. Harold Gray (1905-1965), an authority on the use of radiation in the treatment of cancer.
• milligray (mGy):
  A unit of absorbed radiation equal to one thousandth of a gray, or 0.1 rad.
• rem or roentgen-equivalent-man:
  A unit of measurement that takes into account different biological responses to different kinds of radiation. The radiation quantity measured by the rem is called equivalent dose.
• Millirem:
  One thousandth of a rem, the unit for measuring equivalent dose.
• roentgen (R, r) (rent-gen, rent-chen):
  The international unit of exposure dose for x-rays or gamma rays. Roentgens are named after Professor Wilhelm Konrad Roentgen, the man who discovered x-rays in 1895.
• sievert (Sv) (see-vert):
  The unit for measuring ionizing radiation effective dose, which accounts for relative sensitivities of different tissues and organs exposed to radiation. The radiation quantity measured by the sievert is called effective dose.
• millisievert (mSv) (mill-i-see-vert):
  One thousandth of a sievert, the unit for measuring effective dose.