Radiation Units and Measurements

Radiation  Measurements

Radiation Measurements can be made in two different but related, system: the traditional (Britist) system, still commonly used in the United states, and the Systeme Internationale (SI) units established by the International Commission on Radiation Units in 1981. These units and their relationships are summarized in below.

The traditional unit of radiation exposure is the roentgen (R), a measurement of radiation intensity in air. The roentgen is equal to the quantity of radiation that will produce 2.08 x 10⁹ (more than 2 billion) ion pairs in 1 cm³ of dry air. The SI unit for measuring radiation intensity is coulombs per kilogram (C/kg), specifying the quantity of electrical charge in coulombs produced by the exposure of 1 kg of dry air. Once roentgen equals 2.58 x 10^-4 (.000258) C/kg.

The roentgen is useful for measuring the quantity of radiation present but is not a useful dose measument. Dose varies with the depth of measurement and the quantity of radiation energy absorbed in the exposed tissue. The measure therapeutic radiation doses as well as specific tissue doses received in diagnostic applications, the traditional unit is the rad. Rad stands for radiation absorbed dose and equal to 100 ergs (an energy unit) per gram of tissue. One roentgen of exposure will result in approximately 1 rad of absorbed dose in muscle tissue. The SI unit for dose measurement is the Gray (Gy). One Grey equals 100 rad, and conversely, 1 rad equals to 1 centigray.

Biologic Effect of Radiation

The biologic effect of radiation exposure varies according to the type of radiation involved and its energy. Equal doses of various types of radiation, as measured in rad or Gy, will not necessarily result in equal biologic effects. Some radiation workers, such as engineers in nuclear power plants, nuclear submarine construction workers, or technologist in nuclear medicine laboratories may be exposed to several types of radiation with unequal level of biologic effect. Neither the roentgen not the rad is a useful unit for measuring the occupational dose of combined radiations with different levels of effects.

To simplify the process of measuring occupational dose, a weighting factor (WF) number is assigned to each type of radiation based upon its absorbed energy in a mass of tissue and its relative biologic effect as compared to xrays. Formerly, weighting factors were called quality factors. The weighting factors for different types of ionizing radiation are listed below.

For example, note that alpha particles have a WF of 20. This is because 1 rad of alpha particles causes biologic effects that are approximately equal to those produced by 20 rad of xray energy. The absorbed dose is multiplied by the WF to obtain the dose equivalent. The resulting unit is called the rem, which stands for roentgen equivalent in man. Thus, the worker exposed to 1 rad of alpha particles would receive 20 rem of occupational exposure.

Because the radiation quantities involved in diagnostic radiology are so small, radiographers commonly use units that are 1 / 1000 of the common unit (example, milliroentgens [mR], millirad [mrad], and millirem [mrem]). For example, a chest radiograph may result in a skin entrance dose of 15 mrad, or 0.015 rad.

Students often find it confusing to determine which radiation units should be used in a given situation. This is made more difficult by the tendency of many radiographers to use the traditional roentgen, rad, and rem units interchangeable. This does not cause serious inaccuracy when speaking only of diagnostic xray, because exposure to 1 roentgen of xray energy will result in approximately 1 rad of absorbed dose on muscle. Because the WF of diagnostic xray equals one, 1 rad is also equal to 1 rem.

In general, the reason for the measument determines which unit is most appropriate. The R and C/kg units are used to measure the presence of x-radiation without any reference to its absorption or attenuation (that is, the quantity of radiation present in air). The rad and the Gy are used to measure radiation dose. These units are used to prescribe radiation therapy. The amount absorbed by a specific tissue is what is being measured, so a statement indicating the part of the body involved usually modifies the rad dosage. For example, a radiation oncologist may prescribe a treatment involving 150 centigray to the liver, or a research report might state that the average patient who has a routine chest radiograph receives a thyroid dose of 4 mrad. The laboratory that processes personal radiation monitor badges will report occupational dose in dose equivalent units: rem, mrem, or mSv.