RADIATION MEASUREMENTS
Two systems are used to quantify radiation: the Système International d’Unités , referred to as SI or international units, and traditional or standard units. International units are based on a modernized version of the metric system and are the preferred method for expressing radiation measurement. The term dosimetry refers to the scientific determination of the amount or dose of ionizing radiation. In measuring radiation, Exposure Exposure refers to the measurement of radiation quantity or the ability of x-rays to ionize air. In terms of dental radiography, exposure represents the amount of radiation emitted from the tubehead that reaches the patient. The measurement is taken at the surface of the skin before the radiation has penetrated the tissues. The SI unit of measurement for exposure is coulombs per Absorbed dose The absorbed dose measures the radiation energy absorbed per unit mass of matter, regardless of the type of ionizing radiation and the type of matter. This is the actual amount of radiation absorbed by the tissues when a person is exposed to radiation. Equivalent dose The equivalent dose compares the biologic effects of different types of ionizing radiation on living tissues and organs. Some types of ionizing radiation produce more biologic damage than others. The degree of damage varies according to the linear energy transfer rate (LET) for each type of radiation. As the LET increases, the frequency of ionization and the ability to produce biologic damage also increase. This effect is quantified by a factor referred to as the relative biologic effectiveness or RBE (Tarrence, 2020). The RBE is the weighting factor used to reflect the differences in biologic damage as a consequence of the variation in the LET. For example, the RBE (or weighting factor) for fast neutrons (produced by nuclear weapons and some nuclear reactors) is 10, while the RBE (or weighting factor) for x-rays is 1. This means that fast neutrons produce 10 times more Effective dose The effective dose takes into account the type of radiation and the radiosensitivity of the tissue being irradiated. The effective dose is used to estimate radiation risk or to evaluate the biologic consequences of radiation in humans. The effective dose was developed for use in radiation protection, but it also facilitates comparison of the risk of radiation exposure from one area of the body to another (Fisher & Fahey, 2017). The effective dose (E) is the sum of the products of the equivalent dose to each tissue or organ (HT) and the tissue-weighting factor (WT), a numerical value given to different tissues such as the brain, gonads, bone Radioactivity Radioactivity refers to the rate of decay or disintegration of a sample of radioactive material. The SI unit used to measure radioactivity is the becquerel (Bq), named for the French physicist who discovered radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus
the terminology varies depending on whether the radiation is being measured at the source, the radiation is absorbed by a patient, or the risk of detrimental health effects is being estimated. The common units used to measure radiation include exposure, absorbed dose, equivalent dose, effective dose, and radioactivity.
kilogram (C/kg). The traditional unit of exposure is the roentgen , named after the scientist who discovered x-rays. The roentgen measures the amount of energy that reaches the skin surface, rather than the amount of radiation that is absorbed by the patient.
The biologic effects of radiation are most commonly related to the absorbed dose. The SI unit used to measure the absorbed dose is the gray (Gy). The traditional unit is the radiation absorbed dose (rad). One Gy equals 100 rads. biologic damage than x-rays. The purpose of the equivalent dose is to compare the biologic effects of these differing types of radiation and to bring all forms of radiation to the same dose level (Jones and Hacking, 2020). The equivalent dose (HT) is the product of absorbed dose averaged over a tissue or organ (DT) and the weighting factor (WR) for the type and energy of the radiation: HT = WR × DT (Jones and Hacking, 2020). The SI unit for the equivalent dose is the sievert (Sv). The traditional unit is the roentgen equivalent man (rem). For the purposes of conversion, 1 Sv equals 100 rem. To summarize using the previous example, the equivalent dose would imply that 1 mSv of fast neutron radiation would produce the same amount of biologic damage as 1 mSv of x-radiation despite their difference in LET. marrow, thyroid gland, and skin: E = Σ WT × HT. The tissue- weighting factors are defined by the International Commission on Radiological Protection (ICRP). In 2007, the ICRP revised its estimates of tissue radiosensitivity and its corresponding tissue- weighting factors. Several oral and maxillofacial structures – such as oral mucosa, salivary glands, and airways of the respiratory tract – were included for the first time (ICRP, 2007). The SI unit of measurement for effective dose is the Sv. There is no traditional unit of measurement for effective dose. decays per second. The traditional unit for this measurement is the curie (Ci). Table 2 provides a summary of the units of radiation measurement. These units of measurement may be expressed in smaller quantities by using the prefixes milli (m, e.g., mrem) or micro (µ, e.g., µSv).
SOURCES OF RADIATION EXPOSURE
Radiation is emitted from either naturally occurring or human- made sources. Although many types of radiation are innocuous, ionizing radiation can cause injury in humans. The sources of radiation exposure that the U.S. population is exposed to includes (a) background radiation; (b) medical procedures; (c) consumer products; (d) industrial, security, medical, educational, and research; and (e) occupational (United States Nuclear Regulatory Commission [USNRC], 2020). For the purposes of this course, the broad categories of natural background radiation and artificial radiation sources will be discussed.
Patients may ask about the potential sources and amounts of radiation to which they may be exposed during dental treatment. It might surprise them to learn that, for people living in the United States, 50% of radiation exposure comes from natural background radiation (USNRC, 2020). However, medical sources of radiation exposure have steadily increased over time and now account for levels of exposure equivalent to natural radiation exposure United States Environmental Protection Agency, 2021b; U.S. Nuclear Regulatory Commission, 2020).
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