Units of Measurement
Radiation dose is measured in two ways:
Entrance dose and Effective dose.
Some tissues in the human body are more susceptible to damage from radiation than others. For example, reproductive organs are more sensitive to radiation than are the extremities. The Entrance dose describes the total amount of radiation entering the body during a procedure, without accounting for these differing levels of potential damage. This is sometimes called the Absorbed dose.
On the other hand, Effective dose calculates the effect of radiation on the body by accounting for the response by different organs to the dose. It represents the radiation risk of the procedure. Effective dose is calculated by multiplying the Entrance dose by a quality factor, which represents the relative biological effect of a type of radiation. Effective dose is also called Dose Equivalent.
Entrance dose is given in units called Grays (Gy). A Gray is a fairly large unit, so most common radiation exposures are in the range of several milliGrays (mGy) or microGrays (µGy).
Effective dose is given in units called Sieverts (Sv). Like Grays, most procedure are in mSv or µSv. One Sievert of radiation produces the same biological effect regardless of what type of radiation was present.