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CHAPTER 4 Radiation Quantities and Units
57
A
Fig. 4.4 (A) Clarence Madison Dally (1865–1904), the first American radiation fatality. (B) Dally, assistant to
Thomas A. Edison, is seen holding his hand over a box containing an x-ray tube while Edison examines the
hand through a fluoroscope that he invented. (A, From Brown P: American martyrs to science through
the Roentgen rays, Springfield, IL, 1936, Charles C Thomas. B, From Eisenberg RL: Radiology: an illustrated
history, St. Louis, 1995, Mosby.)
anemia, which results from bone marrow failure, and
leukemia, an abnormal overproduction of white blood
cells, were also more common among early radiologists
than among nonradiologists.
Investigation of Methods for Reducing Radiation Exposure
Alarmed by the increasing number of radiation injuries
reported, the medical community decided to investigate
methods for reducing radiation exposure from all
sources of radiation. In 1921 the British X-Ray and
Radium Protection Committee was created to perform
this task. The committee planned to formulate guidelines for the manufacture and use of radium and x-ray
equipment and devices to reduce the chance of occupational injury. Even though the committee members
recognized the danger of excessive radiation exposure,
they were handicapped because they did not have
accurate measurement techniques or adequate background knowledge of radiobiology. Ultimately, because
they could not agree on a workable unit of radiation
exposure, the members of the committee were unable
to fulfill their responsibility.
B
Skin Erythema Dose
From 1900 to 1930, the unit in use for measuring radiation exposure was called the skin erythema dose, defined
as the received quantity of radiation that causes diffuse
redness over an area of skin after irradiation. This
amount of absorbed radiation corresponds roughly
to a skin dose that would be specified as several gray
today. The radiation unit, gray (Gy), is discussed later
in this chapter. Because the amount of radiation
required to produce an erythema reaction varied from
one person to another, the skin erythema dose was
often a crude and inaccurate way to quantify radiation
exposure. Scientists felt compelled to continue searching for a more reliable unit. The new unit selected was
to be based on some exactly assessable effect produced
by radiation, such as ionization of atoms or energy
absorbed in the irradiated object.
Early Definition of Quantities and Units
The First International Congress of Radiology was held
in London, England, in 1925. This meeting allowed
radiologists from all over the world to collaborate. Unfortunately, no definite decisions for quantifying the
58
CHAPTER 4 Radiation Quantities and Units
Fig. 4.5 Lesions of the fingers induced by ionizing radiation. (From Gusev IA, Guskova AK, Mettler FA, Jr.,
editors. Medical management of radiation accidents, ed 2, New York, 2001, CRC Press, Inc.)
effects of ionizing radiation were made based on the
recommendations presented. The International Commission on Radiation Units and Measurements (ICRU)
was also formed in 1925. In 1928 a Second International
Congress of Radiology was held in Stockholm, Sweden.
Although at this time a unit of measure, the “roentgen,”
was in place as a unit associated with a certain degree of
exposure, it was not as yet by any means scientifically
well defined. The 1928 congress charged the ICRU with
precisely defining this unit of exposure. The congress
also established the International X-Ray and Radium
Protection Commission, predecessor of the International Commission on Radiological Protection (ICRP).
Since the early days of radiology, biologic effects in
humans caused by exposure to ionizing radiation were
only too apparent. These early tissue reactions (Box 4.1),
which appeared within minutes, hours, days, or weeks
of the time of radiation exposure, were believed to be
preventable if doses to radiation workers were limited.
A tolerance dose is a radiation dose to which occupationally exposed persons could be subjected without
any apparent harmful acute effects, such as erythema of
the skin. The general belief was that no adverse effects
from radiation exposure would be demonstrated at
doses lower than this level. Alternatively, this tolerance
exposure level could be regarded as a threshold dose,
that is, a dose of radiation lower than which an individual has a negligible chance of sustaining specific
biologic damage. At the time of the 1928 International
Congress, the tolerance dose was specified in roentgen
units, which were then an imprecise measure of the
quantity called exposure. Even with this uncertainty, the
roentgen remained the principal guideline for occupational radiation exposure tolerance levels during the
CHAPTER 4 Radiation Quantities and Units
59
BOX 4.1 Effects of Ionizing Radiation
Early Tissue Reactions
• Nausea
• Fatigue
• Diffuse redness of the skin
• Loss of hair
• Intestinal disorders
• Fever
• Blood disorders
• Shedding of the outer layer of skin
Late Tissue Reactions
• Cataract formation
• Fibrosis
• Organ atrophy
• Loss of parenchymal cells
• Reduced fertility
• Sterility
Stochastic Effects
• Cancer
• Genetic (hereditary) effects
1930s. Neither tolerance dose nor threshold dose is
presently used for the purposes of radiation safety.
In 1934 the International X-Ray and Radium Protection Commission recommended a tolerance dose daily
limit of 0.2 roentgen. In the United States, the Advisory
Committee on X-Ray and Radium Protection, which
was formed in 1931 to formulate recommendations for
radiation control, also recommended a tolerance dose
equal to 0.2 roentgen per day.
In 1936 the committee reduced this dose to 0.1
roentgen per day. As scientists began to recognize the
late tissue reactions and stochastic effects of ionizing
radiation that appeared months or years after exposure
and the possibility of genetic, or hereditary, effects, they
began to focus on finding ways to minimize the risk of
sustaining such damage (see Box 4.1). The search was on
for a more reliable unit to replace the tolerance dose.
In 1937 the ICRU finished its assignment from
the Second International Congress of Radiology, and,
although still not accurately defined but now much
better quantified, the roentgen became internationally
adopted as the unit of measurement for exposure to
x-radiation and gamma radiation, (short-wavelength,
higher-energy electromagnetic waves emitted by the
nuclei of radioactive substances). In 1962 the roentgen
was conceptually revisited and more rigorously defined
in scientific terms.
In 1946 the US Advisory Committee on X-Ray and
Radium Protection became known as the National
Committee on Radiation Protection. The name of this
radiation standards organization underwent another
change in 1956 and again in 1964, when it became
the National Council on Radiation Protection and
Measurements (NCRP).
The General Conference of Weights and Measures,
which was responsible for the development and international unification of the metric system, assigned its
International Committee for Weights and Measures
the responsibility of developing guidelines for the units
of measurement in 1948. To fulfill this responsibility,
the committee developed the International System
of Units (SI), from the French “Système International
d’Unités.” This system makes possible the interchange
of units among all branches of science throughout the
world.
The Modern Era of Radiation Protection
By the early 1950s, maximum permissible dose (MPD)
replaced the tolerance dose for radiation protection
purposes. MPD essentially indicated the largest dose
of ionizing radiation that an occupationally exposed
person was allowed within a certain period that was
not anticipated to result in major adverse biologic
effects according to the best available data. However,
the concept of an MPD did not mean that some small
risk of damage would not exist with radiation doses
at the MPD level. MPD was initially expressed in a unit
called rem† (an acronym for radiation equivalent man,
also historically known as Roentgen equivalent man), the
traditional British unit used for radiation protection
purposes at that time. The rem has since been replaced
by the SI unit, sievert. Please refer to Appendix A for all
relationships between original, or traditional, units and
the current standard SI units.
Removing the notion of “tolerance dose” and adopting the statistical MPD concept in its place ultimately
†One rem is defined as the dose that is equivalent to any type
of ionizing radiation that produces the same biologic effect
as 1 rad (radiation absorbed dose) of x-radiation. One rad
corresponds to an energy transfer of 100 ergs per gram to an
irradiated object. The rad is identical to the subunit centigray
or cGY.
60
CHAPTER 4 Radiation Quantities and Units
meant that no amount of radiation was considered completely safe. The probability of long-term harm, such
as the development of cancer, was expected to decrease
as the dose decreased, but it was not expected to become
zero at any dose level! This raised a dilemma: If no
amount of radiation exposure was safe, and if it was
impossible to design a work environment where the dose
was zero, and still be able to perform procedures that
unavoidably included some degree of exposure, then
what would determine the maximum allowed occupational exposure? The solution was to compare rates of
death and accident among various occupations. Insurance companies had been using this actuarial method
of comparison for many years to determine insurance
rates. Some occupations are very hazardous. Examples
of such occupations are:
• Deep sea diving
• Professional mountaineering
Some nonhazardous occupations are:
• Trade
• Government desk work
However, even in nonhazardous occupations, there
is still a small risk of fatality or serious injury, approximately one chance in 10,000 each year.1 With this in
mind, the decision was made to base recommendations
for dose limits on the concept that the probability of
harm associated with typical dosimeter readings should
be no more than the amount of harm in industries that
are generally considered reasonably safe.
By the 1970s dosimetry and risk analysis had become
quite sophisticated. Radiation units were developed that
contained factors that accounted for the varied bioeffects of different types of radiation, namely:
• Alpha
• Beta
• Gamma
• X-radiation
• Neutrons
There was also growing recognition that the conse-
quences from radiation exposure to the health of a
human as a whole depended on which organs and organ
systems had been irradiated. For example, irradiation
of the bone marrow was found to be much more significant to the whole-body health of an individual than
irradiation of the skin. Equal doses of radiation to
bone marrow and to skin had very different penalties!
In the late 1970s, using these concepts of different
organ radiosensitivity expressed numerically in terms
of tissue-specific weighting factors, dose limits were
calculated and established to ensure that the overall risk
from radiation exposure acquired on the job did not
exceed risks encountered in “safe” occupations, such as
clerical work, in which the risk is approximately 1024
(one chance in 10,000) per year.
1
In 1991 the ICRP revised the values of the tissue
radiosensitivity weighting factors. The revision was
based on data from more recent epidemiologic studies
of the atomic bomb survivors. The ICRP also adopted
the term effective dose (EfD). Based on the energy
deposited in biologic tissue by ionizing radiation,
it takes into account both of the following:
1. The type of radiation (e.g., x-radiation, gamma,
neutron)
2. The variable sensitivity of the tissues exposed to the
radiation
This quantity, EfD, is therefore the best measure of
the overall risk arising from the simultaneous irradiation of various biologic tissues and organs within an
individual. EfD is expressed in the SI unit, sievert (Sv),
or in subunits of the sievert. The Sv is discussed further
in the next section.
Quantities and Units in Use Today
In 1980 the ICRU adopted SI units, a unified system
of metric units, for use with ionizing radiation and
urged full implementation of the units as soon as possible. Many developed countries, particularly in Europe,
have already made a complete transition to SI units.
In the United States, SI units, such as the Gy and the
centigray (cGy), are now used routinely in therapeutic
radiology to specify absorbed dose. Even though the
NCRP adopted the internationally accepted SI units for
use in 1985, traditional units, older units associated
with radiation protection and dosimetry, such as the
roentgen (R)* and its subunit the milliroentgen (mR),
are being utilized. In addition, the rem, the traditional
unit for the radiation quantity equivalent dose (EqD)
currently remains a used quantity. This is especially
so in radiation dosimetry reports for occupationally
exposed personnel. In the SI system of units, the Sv has
*One roentgen is the photon exposure that under standard
conditions of pressure and temperature produces a total
positive or negative ion charge of 2.58 3 1024 coulombs
per kilogram of dry air.
CHAPTER 4 Radiation Quantities and Units
61
replaced the rem for radiation protection purposes. Like
the rem, it provides a common scale whereby varying
degrees of biologic damage caused by equal absorbed
doses of different types of ionizing radiation can be
compared with the degree of biologic damage caused by
the same amount of x-radiation or gamma radiation.
One Sv is the same as 100 rem.
Fluoroscopic patient entrance radiation levels are
now specified in milligray per minute (mGy-/min), but
in many facilities they are still measured as exposure
rates in roentgens per minute (R/min), and essentially
all radiation survey instruments, even the newer SIoriented devices, continue to also provide readings in
traditional units. Furthermore, many regulatory criteria
are given in terms of traditional units. In this text, in
an effort to advance the full conversion to SI units, all
dosimetry information will be presented as much as
possible in terms of SI units. Appendix A contains a
complete discussion of both SI and traditional units and
the relationship between them. Examples of conversion
between various units are also included.
Box 4.2 presents an overview of the important dates
in the historical evolution of radiation quantities and
units and an overview of terminology used in a given
time period to describe radiation dose limitation.
The SI unit of absorbed dose, the gray (Gy), was
named after the English radiobiologist, Louis Harold
Gray (1901–1965), who was instrumental in developing
what is arguably the most important theory in all of
radiation dosimetry. The Bragg–Gray theory (1936) re-
lates the ionization produced in a small cavity within an
irradiated medium or object to the energy absorbed in that
medium as a result of its radiation exposure. With the use
of appropriate correction factors, the theory essentially
links the determination of the absorbed radiation dose
in a medium to a relatively simple measurement of
ionization charge.
RADIATION QUANTITIES AND THEIR SI UNITS OF MEASURE
Diagnostic imaging professionals must have a clear understanding of the following basic radiation quantities:
• Exposure (X)
• Air kerma
• Absorbed dose (D)
• Equivalent dose (EqD)
• Effective dose (EfD)
In everyday usage, it is commonly said that an
“exposure” has occurred when ionizing radiation
strikes an object such as a human body. However, the
quantity, “exposure”, has a rigorous scientific meaning related to ionization produced in air. Absorbed
dose is the deposition of energy per unit mass in any
material from exposure to ionizing radiation. EqD is
a quantity that builds upon D but then takes into
account the type of radiation striking an object. Different types of radiation affect molecules and cells of
the body in different ways. EfD builds upon EqD by
adding an attempt to take into account the different
harmful degrees of radiation effects on the parts
of the body that are being irradiated to arrive at an
index of overall harm to a human (Box 4.3). Each
radiation quantity has its own special unit of
measure. These quantities and units are discussed in
detail in the following sections.
Exposure
When a volume of air is irradiated with x-rays or with
gamma rays, the interaction that occurs between the
radiation and neutral atoms in the air causes some
electrons to be liberated from those air atoms as they
are ionized. Consequently, the ionized air can function as a conductor and carry electricity because of
the negatively charged free electrons and positively
charged ions that have been created. As the intensity
of x-ray exposure of the air volume increases, the
number of electron–ion pairs produced also increases.
Thus, the amount of radiation responsible for the
ionization of a well-defined volume of air may be determined by measuring the number of electron–ion
pairs, or charged particles of either sign, in that volume of air. This radiation ionization in air is termed
exposure.
Exposure (X) is defined as the total electrical charge
of one sign, either all plus or all minus, per unit mass
that x-ray and gamma ray photons with energies up
to 3 million electron volts (MeV) generate in dry (i.e.,
nonhumid) air at standard temperature and pressure
(760 mm Hg or 1 atmosphere at sea level and 22°C). It
is a radiation quantity “that expresses the intensity of
radiation delivered to a specific area, such as the surface
of the human body.”
As it is defined, the exposure quantity, X, is based on
a response produced when radiation interacts with air.
For a precise measurement of X, the total amount of
2
62
CHAPTER 4 Radiation Quantities and Units
BOX 4.2 Historical Evolution of Radiation Quantities and Units
Year Event
1895 X-rays are discovered, and the discovery is announced.
1896 Initial cases of somatic damage caused by exposure to ionizing radiation are reported in Europe.
1900 Skin erythema dose becomes the unit for measuring radiation exposure.
1904 Clarence Madison Dally becomes the first American radiation fatality.
1910 First cancer deaths among physicians that are attributed to x-ray exposure are reported.
1921 The British X-Ray and Radium Protection Committee is formed to investigate methods for reducing
radiation exposure.
1925 The First International Congress of Radiology is held in London, England; radiologists from all over
the world collaborate, but no definite system for measuring ionizing radiation exposure is identified.
The International Commission on Radiation Units and Measurements (ICRU) is formed.
1928 The ICRU is charged by the Second International Congress of Radiology (Stockholm, Sweden) with
defining a unit of exposure. The International X-Ray and Radium Protection Commission (predeces-
sor of the ICRP) is established by the Second International Congress of Radiology.
1930 Tolerance dose is used for radiation protection purposes.
1931 The US Advisory Committee on X-Ray and Radium Protection is formed to formulate recommenda-
tions for radiation control.
1934 A tolerance dose of 0.2 R per day is recommended.
1936 The tolerance dose is reduced to 0.1 R per day.
The Bragg–Gray theory is introduced.
1937 The roentgen (R) becomes internationally accepted as the unit of measurement for exposure to
x-radiation and gamma radiation.
1946 The US Advisory Committee on X-Ray and Radium Protection becomes known as the National
Committee on Radiation Protection and Measurements (NCRP).
1948 The International System of Units (SI) is developed.
Early 1950s Maximum permissible dose (MPD) replaces the tolerance dose for radiation protection purposes.
1962 The roentgen (R) is redefined to increase accuracy and acceptability.
1963 The National Committee on Radiation Protection and Measurements becomes the National Council
on Radiation Protection (NCRP).
1977 The International Commission on Radiological Protection (ICRP) recommends that the dose equiva-
lent limit or effective dose equivalent replace the MPD.
1980 The ICRU adopts SI units for use with ionizing radiation.
1985 The National Council on Radiation Protection (NCRP) adopts SI units for use.
1991 The ICRP replaces effective equivalent dose with the term effective dose (EfD).
History of Terminology Used to Determine Radiation Dose Limitation
1900–1930 Skin erythema dose (SED)
1930–1950 Tolerance dose (TD)
1950–1977 Maximum permissible dose (MPD)
1977–1991 Effective dose equivalent
1991–present Effective dose (EfD)
ionization (charge) an x-ray beam produces in a known
mass of air must be obtained. This type of direct measurement is normally accomplished in an accredited
dosimetry calibration laboratory (ADCL) by using
a standard, or free-air, ionization chamber (Fig. 4.6).
The chamber contains a known quantity of air with
precisely measured temperature, pressure, and low
humidity. If in that specified volume of dry air the total
charge of all the ions of one sign (either all plus or all
minus) produced is collected and measured, the total
amount of radiation exposure may be accurately determined. Lastly, the free-air chamber response is modified
CHAPTER 4 Radiation Quantities and Units
63
BOX 4.3 Difference Between Equivalent
Dose and Effective Dose
The quantity equivalent dose uses radiation weighting
factors (WR) to adjust the value of the absorbed dose to
reflect the different capacity for producing biologic harm
by various types and energies of ionizing radiation.
The quantity effective dose uses tissue weighting fac-
tors (WT) to adjust the quantity equivalent dose to reflect
the difference in harm to the person as a whole depending on the tissues and organs that have been irradiated.
Therefore, effective dose takes into account both the
type of radiation and the part of the body irradiated.
X-ray
source
X-ray
beam
to correspond to standard temperature and pressure of
dry air in accordance with the definition of exposure.
Such an instrument, however, is not a practical
device at locations other than a standardization laboratory. As a result, much smaller and less complicated
instruments have been developed for use away from
the laboratory. Although very convenient, these instruments must be periodically recalibrated in an ADCL
against a free-air chamber.
The coulomb (C) is the basic unit of electrical
charge. It is equal to the “amount” of electrical charge
“Q” moving past a point in a conductor in 1 second
Lead
diaphragm
Negative plate
Guard plate
Electrometer
D
E
Collecting
Air-filled
lead-lined
box
Fig. 4.6 This device determines radiation exposure by measuring the amount of ionization (charge) 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 5 1.6 3 10
an exposure of 1 roentgen (R).
–19
C). A collected electrical charge of 2.58 3 10–4 C/kg of irradiated air constitutes
plate
(positive)
Measured
volume
A
BC
1500 volts
electrical
supply
+−
64
CHAPTER 4 Radiation Quantities and Units
when an electrical current “I” amounting to 1 ampere is
used. The ampere* is the SI unit of electrical current.
Current consists of moving electrical charges, the most
common of which is the electron (the charge carried by
an electron is equal to: 21.6 3 10
219
C). Essentially, the
ampere quantifies the flow rate of electric charge and
mathematically is simply given by: I 5 Q/t where Q is
the total net electrical charge in coulombs moving past
a location in a conductor in a time t (given in seconds).
In the International System, the exposure unit is
coulombs per kilogram (C/kg). No special name for
this SI quantity has been assigned. This exposure unit is
simply equal to an electrical charge of 1 C produced in
a kilogram of dry air by ionizing radiation. Appendix A
provides examples of numeric conversions between C
per kilogram and the roentgen traditional unit. Both of
these remain very useful for x-ray equipment calibration because x-ray output intensity is measured directly
with an ionization chamber.
Air Kerma
Air kerma is another SI quantity that is used to express
how energy is transferred from a beam of radiation to
air. It is mostly replacing the traditional quantity, expo-
sure. Because of this, “x-ray tube output and inputs
to image receptors are now often given in air kerma.”3
A standard, or free-air, ionization chamber is the instrument that can be calibrated to read air kerma.2 “A conversion factor can also be used to change between air
kerma and exposure values.”
“Kinetic energy released in air,” “kinetic energy
released in material,” and “kinetic energy released per
unit mass” all use the word kerma as an acronym. In
simple terms, air kerma is the total kinetic energy released in a unit mass (kilogram) of air and is expressed
in metric units of joules per kilogram (J/kg).2 In a similar way tissue kerma can be defined as the total kinetic
energy released in a unit mass of tissue. Tissue kerma
is also given in units of joules per kilogram. This unit
*The ampere is precisely defined as follows: Between two very
long and very thin parallel wires carrying moving electric
charges (i.e., a current), there is observed to be either a force
of attraction or repulsion depending on the relative directions
of motion of the charges in the wires. One ampere is defined
to be that amount of constant wire current that leads to a force
of 231027 newtons per meter of wire length when the wires
are situated in a vacuum and separated by 1 meter.
2
for kerma is in fact the same radiation unit, the Gy,
which was previously defined as the SI unit for the
radiation quantity, absorbed dose. With respect to
radiographic and fluoroscopic units, however, “air”
kerma, not “tissue” kerma, is the primary concept
because in these situations the main concern is with
exposure and the patient’s resulting entrance dose.
Modern radiographic and fluoroscopic units have
incorporated an ability to determine the entire amount
of energy delivered to the patient by the x-ray beam.
This quantity is often referred to as the dose area
product (DAP). It is the sum total of air kerma over the
exposed area of the patient’s surface or, in other words,
a measure of the amount of radiant energy that has
been thrust into a portion of the patient’s body surface.
DAP is usually specified in units of mGy-cm2. As an
illustration of this concept, consider a patient whose
irradiated surface receives an air kerma dose of 20 mGy.
If the area of the irradiated surface is 100 cm2, then
the DAP will be 20 mGy 3 100 cm2 5 2000 mGy-cm2.
Absorbed Dose
As ionizing radiation passes through an object such as
a human body, some of the energy of that radiation is
absorbed by the body and stays within it. The quantity
absorbed dose (D) is defined as the amount of energy per
unit mass absorbed by an irradiated object. Therefore, D
indicates the energy that the patient actually receives from
an exposure to ionizing radiation. This absorbed energy is
what causes damage in biologic tissues of the patient.
Because many x-ray examinations require relatively
small radiation doses, smaller units, which are only a
fraction of a specific unit, may frequently be used to indicate D values. Examples of some of these subunits are
provided in Box 4.4. Box 4.5 demonstrates conversion
BOX 4.4 Subunits of the Gray
Smaller fractions of measured quantities such as the
gray (Gy) will have a prefix. Examples follow.
Prefix Subunit Symbol Fraction Factor
centi- centigray
(cGy)
milli- milligray
(mGy)
micro- microgray
(µGy)
c
m
µ
1
⁄100 10
1
⁄1000 10
1
⁄1,000,000 10
22
23
26
CHAPTER 4 Radiation Quantities and Units
65
BOX 4.5 How to Convert Gray to
Milligray and to Centigray
Rule: Number of gray 3 1000 5 Number of milligray
Example: 0.010 Gy 3 1000 5 10 mGy
Rule: Number of gray 3 100 5 Number of centigray
Example: 0.100 Gy 3 100 5 10 cGy
between decimal values of gray and the numerically
smaller units: centigray (cGy) and milligray (mGy). It
is also easy to convert milligray or centigray to gray.
This is done just by dividing the number of milligray
by 1000 or the number of centigray by 100. SI subunits
can also facilitate conversion from traditional units of D
to SI units of absorbed dose, especially in therapeutic
radiology (see Appendix A for an example).
Equivalence of Radiation-Produced Damage From Different Sources of Ionizing Radiation
Equal absorbed doses of different types of radiation
produce different amounts of biologic damage in body
tissue. For example, in laboratory experiments it has
been shown that a 1-Gy D of fast neutrons causes much
more biologic damage than a 1-Gy D of x-rays. A 1-Gy
dose of neutrons would kill a laboratory rat, but a 1-Gy
dose of x-rays would not. The concept of dose equivalence takes this varied biologic impact into consideration by using a specific modifying factor termed a
quality factor. The quality factor (Q) is an adjustment
multiplier that is employed in the calculation of dose
equivalence to detail the specific ability of a dose of any
kind of ionizing radiation to cause biologic damage.
X-rays, beta particles (high-speed electrons), and
gamma rays produce virtually the same biologic effect
in body tissue for equal absorbed doses. In terms
of quality factor, these radiations have been given a
numeric adjustment value of 1 (i.e., Q 5 1) and are
the basis, or standard, against which to compare the
effectiveness of other types of ionizing radiation in
producing biologic damage. The quality factors of different kinds of ionizing radiations are listed in Table 4.1.
The concept of linear energy transfer (LET) helps
explain the need for a quality, or modifying, factor.
LET is the amount of energy transferred on average
by incident radiation to an object per unit length of
track, or passage, through the object and is expressed in
units of kiloelectron volts per micrometer (keV/µm)
TABLE 4.1 Quality Factors for Different
Types of Ionizing Radiation
Type of Ionizing Radiation Quality Factor
X-ray photons
Beta particles 1
Gamma photons 1
Thermal neutrons 5
Fast neutrons 20
High-energy external protons
Low-energy internal protons* 20
Alpha particles 20
Multiple charged particles
of unknown energy
*Protons produced as a result of neutrons interacting with
the nuclei of tissue molecules.
Data from National Council on Radiation Protection and
Measurements (NCRP): Limitation of exposure to ionizing
radiation, Report No. 116, Bethesda, MD, 1993, NCRP.
1
1
20
(see Appendix C). Radiation with a high LET transfers
a large amount of energy into a small area and can
therefore potentially do much more biologic damage
than radiation with a low LET. As a result, a high-LET
radiation has a quality factor that is noticeably greater
than the quality factor for a low-LET radiation.
Equivalent Dose
EqD is the product of the average D in a tissue or organ
in the human body and its associated radiation weight-
ing factor (WR) chosen for the type and energy of the
radiation in question. X-radiation and gamma radiation have a WR of 1, whereby 1 Gy equals 1 Sv. Other
types of radiation have different radiation weighting
factors.
The WR takes into consideration the fact that some
types of radiation are more efficient at causing biologic
damage than other types of radiation for a given dose.
Values for the radiation weighting factors are selected
by national and international scientific advisory bodies
(NCRP, ICRP) and are based on quality factors and
LET. The NCRP, in Report No. 116, described the WR
as “a dimensionless factor” (a multiplier) that was chosen for radiation protection purposes to account for
differences in biologic impact among various types of
ionizing radiations.1 This factor places risks associated
with biologic effects on a common scale. Each type and
energy of radiation has a specific radiation weighting
66
CHAPTER 4 Radiation Quantities and Units
TABLE 4.2 Radiation Weighting Factors
for Different Types and Energies of Ionizing
Radiation
Radiation Type and Energy
Range
X-ray and gamma ray photons
and electrons (every energy)
Neutrons, energy ,10 keV 5
10 keV–100 keV 10
.100 keV–2 MeV 20
.2 MeV–20 MeV 10
.20 MeV
Protons 2
Alpha particles 20
Data adapted from International Commission on Radiological
Protection (ICRP): Recommendations, ICRP Publication No.
60, New York, 1991, Pergamon Press.
Radiation Weighting
Factor (W
1
5
)
R
BOX 4.6 Subunits of the Sievert
Smaller fractions of measured quantities such as the
sievert (Sv) will have a prefix. Examples follow.
Prefix Subunit Symbol Fraction Factor
centi- centisievert
(cSv)
milli- millisievert
(mSv)
micro- microsievert
(µSv)
c
m
µ
1
⁄100 10
1
⁄1000 10
1
⁄1,000,000 10
22
23
26
factor, the numeric value of which may be found in
Table 4.2. The WR actually has the same numeric value
as the quality factor that was previously used for determining dose equivalence.
EqD is used for radiation protection purposes when
a person receives exposure from various types of ionizing radiation. EqD is expressed in sieverts or in a subunit of the Sv (Box 4.6). EqD is obtained by multiplying
the D by the WR as follows:
EqD 5 D 3 W
R
which in terms of units corresponds to:
Sv 5 Gy 3 W
R
An example of determining and expressing EqD using gray and Sv is provided in Box 4.7. Because radiation
BOX 4.7 Determining and Expressing
Equivalent Dose Using Gray and Sievert
Example: An individual received the following absorbed
doses: 0.1 Gy
and 0.2 Gy
lent dose (EqD)?
(The radiation weighting factor for each radiation in
question may be obtained from 4.2.)
Answer:
Radiation Type D 3 WR5 EqD
X-radiation 0.1 Gy
Fast neutrons 0.05 Gy
Alpha particles 0.2 Gy
of x-radiation, 0.05 Gyt of fast neutrons,
t
of alpha particles. What is the total equiva-
t
EqD (D W ) (D W ) (D W )
R 1 R 2 R 3
3 1 5 0.1 Sv
t
3 20 5 1.0 Sv
t
3 20 5 4.0 Sv
t
Total EqD 5 5.1 Sv
doses for radiation workers employed in diagnostic radiology are normally relatively small, they may be specified
in terms of millisievert. To change Sv to millisievert,
multiply the number of Sv by 1000, whereas millisievert
can be converted to Sv by dividing the number of millisievert by 1000.
Effective Dose
EfD is the best measure of the overall risk of exposure
to humans from ionizing radiation. The NCRP, in
Report No. 116, defines it as “the sum of the weighted
equivalent doses for all irradiated tissues or organs.”1
EfD incorporates both the effect of the type of radiation
used (e.g., x-radiation, gamma, neutron, alpha) and the
variability in radiosensitivity of the specific organ or
body part irradiated through the use of appropriate
weighting factors. These factors quantify the overall
potential harm to those biologic components and the
risk of developing a radiation-induced cancer or, for
the reproductive organs, the risk of genetic damage. The
term that specifically takes into account the relative
detriment to each specific particular organ and tissue is
called the tissue weighting factor (WT). More precisely,
each WT value (Table 4.3) denotes the percentage ratio
of the summed stochastic (cancer plus genetic) risk
stemming from irradiation of a specific tissue or organ
to the all-inclusive risk when the entire body is irradiated in a uniform fashion. As a result, EfD accounts for