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
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).
Table 2: Radiation Quantities and Units of Measurement Quantity Measure
International Unit
Traditional Unit
Conversion
Exposure
Ionization of air
C/kg
R
1 C/kg = 3,876 R
Absorbed dose
Energy absorbed
Gy
rad
1 Gy = 100 rads
Equivalent dose
Biologic effects
Sv
rem
1 Sv = 100 rems
Effective dose
Estimated risk
Sv
None
None
Radioactivity
Decay rate
Bq
Ci
1 Bq = 2.7 × 10-11 Ci
C/kg = coulomb/kilogram
R = roentgen
Gy = gray
rad = radiation absorbed dose
Sv= sievert
rem = roentgen equivalent man
Bq = becquerel
Ci = curie.
Note . From Radiation Emergency Medical Management. (2016). Radiation units and conversions . Retrieved from https://www.remm.nlm.gov/ radmeasurement.htm
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