Radiologic Units

Intensity Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time. The intensity of an X-ray or gamma-ray source can easily be measured with the right detector.

Exposure One way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air. The amount of ionization in air produced by the radiation is called the exposure. Exposure is expressed in terms of a scientific unit called a roentgen (R). The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0°C and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge.

In 1981, the International Commission on Radiation Units and Measurements (ICRU) issued standard units based on SI that have since been adopted by all countries except the United States. Most U.S. scientific journals and societies have adopted Le Système International d'Unités (The International System, SI), but regulatory agencies and the American Registry of Radiologic Technologists (ARRT) have not.

Roentgen (R) (Gy a ) The roentgen is equal to the radiation intensity that will create 2.08×10 8 ion pairs in a cubic centimeter of air; that is, 1 R=2.08×10 8 ip/cm 3. The official definition, however, is expressed in terms of electric charge per unit mass of air (1 R=2.58×10 −4 C/kg). The charge refers to the electrons liberated by ionization. The roentgen was first defined as a unit of radiation quantity in Since then, the definition has been revised many times. Radiation monitors usually are calibrated in roentgens. The output of x-ray imaging systems is usually specified in milliroentgens (mR). The roentgen applies only to x- rays and gamma rays and their interactions with air. The SI unit of air kerma (mGy a ) is used.

This device determines radiation exposure by measuring the amount of ionization an x-ray beam produces within its air collection volume. The instrument consists of a box containing a known quantity of air, two oppositely charged metal plates, and an electrometer, an instrument that measures the total amount of charge collected on the positively charged metal plate. The chamber measures the total amount of electrical charge of all the electrons produced during the ionization of a specific volume of air at standard atmospheric pressure and temperature. The electrical charge is measured in units called coulombs (C) (charge of an electron = −1.6 × 10 −19 C). A collected electrical charge of 2.58 × 10 −4 C/kg of irradiated air constitutes an exposure of 1 roentgen (R).

Rad (Gy t ) Biologic effects usually are related to the radiation absorbed dose; therefore, the rad is the unit most often used when one is describing the quantity of radiation received by a patient. The rad is used for any type of ionizing radiation and any exposed matter, not just air. One rad is equal to 100 erg/g (10 −2 Gy t ), where the erg (joule) is a unit of energy and the gram (kilogram) is a unit of mass. The units Gy a and Gy t refer to radiation dose in air and tissue, respectively.

Rem (Sv) Occupational radiation monitoring devices are analyzed in terms of rem (radiation equivalent man). The rem is used to express the quantity of radiation received by radiation workers and populations. Some types of radiation produce more damage than x-rays. The rem accounts for these differences in biologic effectiveness. This is particularly important for persons working near nuclear reactors or particle accelerators.

Diagnostic radiology is concerned primarily with x-rays. We may consider 1 R is equal to 1 rad is equal to 1 rem (1 mGy a =1 mGy t =1 mSv). With other types of ionizing radiation, this generalization is not true. Type of Ionizing Radiation Quality Factor X-ray photons1 Beta particles1 Gamma photons1 Thermal neutrons5 Fast neutrons20 High-energy external protons1 Low-energy internal protons2 Alpha particles20 Multiple charged particles of unknown energy 20

Curie (Ci) (Bq) The curie is the unit of quantity of radioactive material, not the radiation emitted by that material. One curie is that quantity of radioactivity in which 3.7×10 10 nuclei disintegrate every second (3.7×10 10 becquerels [Bq]). The millicurie (mCi) and the microcurie (μCi) are common quantities of radioactive material. Radioactivity and the curie have nothing to do with x- rays.

QuantityCustomarySymbolSI Symbol ExposureroentgenRair kermaGy a Absorbed dose rad grayGy t Equivalent dose rem seivertSv RadioactivitycurieCibecquerelBq

SI unitCustomary unit 1 air kerma = 100 R 1 gray = 100 rad 1 sievert = 100 rem Conversions