Radiation Dose Units

Radiation absorbed dose and effective dose in the international system of units (SI system) for radiation measurement uses “gray” (Gy) and “sievert” (Sv), respectively.

In the United States, radiation absorbed dose, effective dose, and exposure are sometimes measured and stated in units called rad, rem, or roentgen (R).

For practical purposes with gamma and x rays, these units of measure for exposure or dose are considered equal.

This exposure can be from an external source irradiating the whole body, an extremity, or other organ or tissue resulting in an external radiation dose. Alternately, internally deposited radioactive material may cause an internal radiation dose to the whole body, an organ, or a tissue.

Smaller fractions of these measured quantities often have a prefix, such as milli (m) that means 1/1,000. For example, 1 sievert = 1,000 mSv. Micro (μ) means 1/1,000,000. So, 1,000,000 μSv = 1 Sv, or 10 μSv = 0.000010 Sv.

Conversions from the SI units to older units are as follows:

• 1 Gy = 100 rad
• 1 mGy = 100 mrad
• 1 Sv = 100 rem
• 1 mSv = 100 mrem

With radiation counting systems, radioactive transformation events can be measured in units of “disintegrations per second” (dps) and, because instruments are not 100 percent efficient, “counts per second” (cps).

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