Ionizing Radiation: Basic Concepts

Measurement of Radiation

Radiation exposure can be measured and monitored. The exposure measurement, using the systeme international (SI) units, is determined by the Coulombs/kilogram (C/kg). However, this is a measurement of the administered dose when all of the x-rays are stopped by air not by human tissues.

In an attempt to correlate the biologic effects of the radiation absorbed in human tissues one can use an assessment of absorbed dose per unit of mass called the gray. One gray = 1 Joule/kilogram of absorbed radiation. This absorbed dose is different for different tissues in the body i.e. fat absorbs less of the administered x-ray dose than bone.

As tissues absorb x-rays differentially and human tissues have different potential for experiencing damage from x-rays i.e. the gonadal tissues are more at risk for radiation damage than are muscle cells, Thus, the concept of dose equivalency has been developed. The most common derived unit of ionizing radiation that can be used to assess the health effects of radiation on the human body is the Sievert (Sv). This unit of measurement has been called an Effective Dose Equivalent as it attempts to correlate the dose of the x-rays administered with the radiosensitivity of different tissues i.e. gonads have a higher sensitivity to radiation vs. muscle.

The measurement of radiation exposure using the Sievert, the effective dose, follows the Stochastic school of thought in regard to the potential adverse effects of radiation. Using the linear, no-threshold, model for radiation injury it has been estimated that 1 Sievert of radiation carries a 5.5% risk of developing a future cancer. (1)

Radiation Exposure

The comparison of the dose of radiation that one receives from an imaging examination to the equivalent number of days of exposure to background radiation is an excellent way to conceptualize the quantity of radiation administered to a patient for diagnostic purposes. (1)

(Table 2.1) Exposure equivalent to background ionizing radiation for common imaging examinations