Units of Radiation Part Two

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How can different types of radiation exposures be compared?

In the diagram at left, a man is exposed to a collimated x-ray beam. In the diagram at right, he is exposed to a non-collimated beam. For this example, we shall assume the exposure in the beam is 50 mR in both cases.

Radiation exposures

Although the radiation output of the x-ray machine is the same in both cases (50 mR), it is evident that the biological risks from these two exposures is NOT EQUAL, since a greater portion of the body is exposed to the x-rays in the second case.

To be meaningful, a measure of radiation dose should be adjusted for these types of factors. This "adjusted" dose is known as the Effective Dose Equivalent.

Effective Dose Equivalent [measured in rem]

This is a risk-based measure of radiation dose. It is adjusted for:

  1. the type of radiation, and
  2. the relative radio-sensitivity of the exposed portion of the body.

Effective Dose Equivalent = rad x Quality Factor x Weighting Factor

Quality Factor is a factor that expresses the relative biological effectiveness for a specific type and energy of radiation. For x-rays and gamma rays the Quality Factor (QF) is 1.

Weighting Factor is a number that adjusts for the relative radio-sensitivity of the exposed organ or part of the body.

For whole body exposures (WBE) the weighting factors for organs/body parts area added together and give a Weighting FactorWBE = 1

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