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) (Gya)
The roentgen is equal to the radiation intensity that will create 2.08×108 ion
pairs in a cubic centimeter of air; that is, 1 R=2.08×108 ip/cm3. 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 1928. 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 xrays and gamma rays and their interactions with air. The SI unit of air
kerma (mGya) 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 (Gyt)
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 Gyt), where the erg (joule) is a unit of energy and the gram
(kilogram) is a unit of mass. The units Gya and Gyt 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 mGya =1 mGyt =1 mSv). With other types of
ionizing radiation, this generalization is not true.
Type of Ionizing Radiation
Quality
Factor
X-ray photons
1
Beta particles
1
Gamma photons
1
Thermal neutrons
5
Fast neutrons
20
High-energy external protons
1
Low-energy internal protons
2
Alpha particles
20
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
3.7×1010 nuclei disintegrate every second (3.7×1010 becquerels [Bq]). The
millicurie (mCi) and the microcurie (μCi) are common quantities of
radioactive material. Radioactivity and the curie have nothing to do with xrays.
Quantity
Customary
Symbol
SI
Symbol
Exposure
roentgen
R
air kerma
Gya
Absorbed
dose
rad
rad
gray
Gyt
Equivalent
dose
rem
rem
seivert
Sv
Radioactivity
curie
Ci
becquerel
Bq
Conversions
SI unit
Customary unit
1 air kerma
= 100 R
1 gray
= 100 rad
1 sievert
= 100 rem