Radiobiology Radiation Dosimetry Radiation Protection 0 The

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Radiobiology Radiation Dosimetry Radiation
Protection
0
The following slides describe the biological
effects of radiation, radiation dosimetry and
protective measures to reduce exposure to
patients and operators.
In navigating through the slides, you should
click on the left mouse button when you see the
mouse holding an x-ray tubehead or you are done
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Radiobiology
When x-rays enter the body, they interact at the
atomic level to cause ionization. Radiobiology is
the response of living systems to ionizing
radiation.
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Ionization
0
Ionization is the process of removing an electron
from an electrically neutral atom to produce an
ion pair. An ion is an atom or subatomic particle
with a positive or negative charge.
Ionization
negative ion (electron)
X-ray enters atom and strikes electron, knocking
it out of its orbit and creating two ions (ion
pair). The ejected electron is the negative ion
and the atom with a net positive charge is the
positive ion.
positive ion atom with 3 protons, 2 electrons
4
Ionizing Radiation
0
The two types of ionizing radiation are
electromagnetic and particulate. Electromagnetic
Electromagnetic radiation has both electrical
and magnetic properties. It includes x-rays and
gamma rays. X-rays are produced by a machine and
gamma rays are produced when radioactive
materials decay. Neither one has any mass.
Particulate Particulate radiation consists of
particles that have mass and travel at high
speeds. Included are alpha particles (helium
nuclei), electrons (beta particles also called
beta rays), protons and neutrons.
5
Attenuation
0
Attenuation is the reduction of the x-ray beam
intensity by interaction with the tissue that the
x-rays pass through. The three types of
interactions are 1. Coherent scattering 2.
Compton scattering 3. Photoelectric
absorption Approximately 9 of the x-rays pass
through the tissues without any interactions.
6
Coherent Scattering
0
A low-energy x-ray interacts with an outer-shell
electron and causes it to vibrate briefly. A
scattered x-ray of the same energy as the primary
x-ray is then emitted, going in a different
direction than the primary x-ray. An electron is
not ejected from the atom. (No ionization). This
interaction occurs about 8 of the time.
7
Coherent Scattering
0
8
0
Compton Scattering
In Compton Scattering, an outer shell electron is
ejected when struck by an x-ray, creating an ion
pair (ionization). The primary x-ray loses some
of its energy and continues in a different
direction as a scattered x-ray. Compton
Scattering accounts for about 62 of the
interactions occurring within the tissues.
Approximately 30 of the scattered x-rays exit
the head.
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Compton Scattering
0
ejected electron (negative ion)
The primary x-ray strikes an outer-shell
electron, knocking it out of its orbit
(ionization). The primary x-ray loses some of its
energy and continues in a different direction as
a scattered x-ray.
10
Photoelectric Absorption
0
In Photoelectric Absorption, an inner-shell
electron is ejected when struck by an x-ray,
creating an ion pair (ionization). The primary
x-ray loses all of its energy in knocking the
electron out of its orbit the x-ray ceases to
exist (no scatter radiation). Photoelectric
Absorption accounts for about 30 of the
interactions occurring within the tissues.
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Photoelectric Absorption
0
ejected electron (negative ion)
The primary x-ray strikes an inner-shell
electron, knocking it out of its orbit
(ionization). The x-ray loses all of its energy
and disappears. There is no scatter.
12
Dose-Response Curves
0
Dose-Response curves represent the relationship
between the dose of radiation a person receives
and the cellular response to that exposure. These
responses may be linear or non-linear and may, or
may not, have a threshold dose the responses
(effects) may be stochastic or deterministic.
(See next two slides for definitions of these
terms).
linear
Response
non-linear
non-threshold
Dose
threshold
13
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Linear the response is directly related to the
dose. As the dose increases, the response
increases proportionately. Non-linear the
response is not proportionate to the dose. An
increase in dose may result in a larger or
smaller increase in the response depending on
the location on the dose-response
curve. Threshold this represents the dose at
which effects are produced below this dose,
there are no obvious effects. Non-threshold
any dose, no matter how small, will produce a
response.
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Stochastic effect occurs by chance, usually
without a threshold level of dose. The
probability of a stochastic effect is increased
with increasing doses, but the severity of the
response is not proportional to the dose (e.g.,
two people may get the same dose of radiation,
but the response will not be the same in both
people). Genetic mutations and cancer are the two
main stochastic effects. Deterministic effect
health effects that increase in severity with
increasing dose above a threshold level. Usually
associated with a relatively high dose delivered
over a short period of time. Skin erythema
(reddening) and cataract formation from radiation
are two examples of deterministic effects.
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DNA
Radiation effects at the cellular level result
from changes in a critical or target molecule.
This target molecule is DNA (deoxyribonucleic
acid), which regulates cellular activity and
contains genetic information needed for cell
replication. The DNA molecule is called a
chromosome. Permanent changes in this molecule
will alter cell function and may result in cell
death.
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Direct vs. Indirect Effect
If an x-ray or some type of particulate radiation
interacts with the DNA molecule, this is
considered a direct effect. Particulate
radiation, because of its mass, is more apt to
cause damage to the DNA by this direct effect.
Other molecules that contribute to cell function,
such as RNA, proteins, and enzymes, may also be
affected by the direct effect.
DNA
Direct effect

x-ray or particulate radiation
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0
Direct vs. Indirect Effect
Most of the damage to DNA molecules from x-rays
is accomplished through the indirect effect. When
x-rays enter a cell, they are much more likely to
hit a water molecule because there are a large
number of water molecules in each cell. When the
x-ray ionizes the water molecule, ions and free
radicals are produced which in turn bond with a
DNA molecule, changing its structure. Since the
x-ray interacted with the water molecule before
the DNA was involved, this is considered an
indirect effect.
ions and
Indirect effect
H2O

DNA
free radicals
x-ray or particulate radiation
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0
Free Radical
A free radical is an atom or molecule that has an
unpaired electron in the valence shell, making it
highly reactive. These free radicals aggressively
join with the DNA molecule to produce damage. In
the presence of oxygen, the hydroperoxyl free
radical is formed this is one of the most
damaging free radicals that can be produced. Free
radicals are the primary mediator of the indirect
effects on DNA.
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Cellular Effects
Cells undamaged ionization alters the structure
of the cells but has no overall negative
effect. Sublethal injury cells are damaged by
ionization but the damage is repaired. Mutation
cell injury may be incorrectly repaired, and cell
function is altered or the cell may reproduce at
an uncontrolled rate (cancer). Cell death the
cell damage is so extensive that the cell is no
longer able to reproduce.
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Sublethal Injury Cellular Repair
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• Ionization causes damage to DNA
• (single-strand break of DNA).
• 2. Cellular enzymes recognize the damage and
  coordinate the removal of the damaged section.
• 3. Additional cell enzymes organize replacement
  of the damaged section with new material.

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Mutation
When the DNA is damaged, cell function may be
altered or reproductive capacity may be
accelerated. Cancer is the most harmful result of
cellular mutation.
Normal
Mutation
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Cell Death
If there is extensive damage to the cell
following irradiation or if cell division
(mitosis) is disrupted, the cell may die. This
will depend on how sensitive the cells are to
radiation. The loss of a few cells or small group
of cells is usually of no consequence, since
there are so many cells present in the body. In
most cases, the dead cells will soon be replaced
through normal reparative processes.
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Cell Cycle
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More damage results when the cell is irradiated
during the G1/early S portion of the cell cycle
(before DNA synthesis) the damaged DNA
(chromosome) will be duplicated during DNA
synthesis and will result in a break in both arms
of the chromosome at the next mitosis.
Cell most sensitive to radiation
G1 gap phase 1 in which nuclear components are
replicated
S synthesis phase DNA is synthesized during
the last 2/3 of this phase
G2 gap phase 2, a preparatory stage to cell
division
M mitosis, during which cells divide
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Radiosensitive Cells
Cells that are more easily damaged by radiation
are radiosensitive. The characteristics of
radiosensitive cells are
1. High reproductive rate (many mitoses) 2.
Undifferentiated (immature) 3. High metabolic rate
Lymphocytes, germ cells, basal cells of skin and
mucosa, and erythroblasts are examples of
radiosensitive cells.
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Radioresistant Cells
0
Cells that are not as susceptible to damage from
radiation are radioresistant. The characteristics
of radioresistant cells are
1. Low reproductive rate (few mitoses) 2. Well
differentiated (mature) 3. Low metabolic rate
Nerve and muscle cells are examples of
radioresistant cells.
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Radiation Effect Modifiers
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• The biological response to radiation is dependent
  on several different factors. These include
• Total Dose the higher the radiation dose, the
  
• greater the potential cellular damage.
• Dose Rate A high dose given over a short
• period of time (or all at once) will produce
  more
• damage than the same dose received over a
• longer period of time.
• Total Area Covered the more cells that are
• exposed to radiation, the greater the effects
  will
• be.

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Radiation Effect Modifiers (continued)
0

• Type of tissue As discussed earlier,
• radiosensitive cells are more likely to be
• damaged by radiation than are radioresistant
• cells.
• Age Because the cells are dividing more
• frequently in a growing child, young people
  are
• affected more by radiation than are older
  people.
• Linear Energy Transfer This measures the rate
  
• of the loss of energy as radiation moves
• through tissue. Particulate radiation (alpha
• particles, electrons, etc.) has a higher LET
• because it has mass and interacts with
• tissues much more readily than do x-rays.

28
Radiation Effect Modifiers (continued)
0

• Oxygen Effect Radiation effects are more
  pronounced
• in the presence of oxygen. Oxygen is required
  for the
• formation of the hydroperoxyl free radical,
  which is
• the most damaging free radical formed
  following
• ionization.

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Latent Period
The amount of exposure a patient receives from
dental diagnostic radiography (effective dose) is
relatively small. Most of the radiation damage
will be repaired. The effects of the radiation
damage that is not repaired may not show up for
many years. The time between the exposure and the
appearance of the effects of that exposure is
called the latent period. In general, the higher
the dose, the shorter the latent period.
30
Since repair of radiation injury is not 100,
radiation effects are accumulative. However,
these effects will usually not be noticeable,
since they are masked by the normal aging
processes. The effects from extreme levels of
radiation exposure are potentially life
threatening. Since these high levels will never
be seen with diagnostic radiography, the effects
will not be discussed. Check radiology texts or
online sources for more information.
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0
Somatic Cells vs. Germ Cells
There are two general types of cells in the body
these are somatic and genetic. Somatic cells are
all the cells except for the germ (reproductive)
cells. If somatic cells are irradiated, only the
person exposed will be affected. Germ cells are
the sperm and ova. If the germ cells are
irradiated, the offspring of the individual may
be affected.
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Hormesis
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Hormesis is a dose response phenomenon in which
small doses of a toxin have the opposite effect
of large doses. For example, exposing mice to
small doses of radiation shortly before exposing
them to very high levels of radiation actually
decreases the likelihood of cancer. The initial
low dose of radiation may activate certain repair
mechanisms in the body and these mechanisms are
efficient enough to not only neutralize the
radiation effects but may even repair other
defects not caused by the radiation. There is a
lot of debate about hormesis, but the general
opinion is that this is not something that can be
relied on when discussing the effects of
radiation exposure.
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Dosimetry
Measuring the dose of radiation emitted by a
radioactive source.
As mentioned previously, radiation effects are
dependent on the total area covered. If the
entire body is exposed, it would be considered
whole-body radiation. If only a localized area is
exposed, as in dental radiography, it would be
called specific-area radiation. The effects from
a given dose of radiation would be expected to be
more severe if the whole body is exposed to that
dose rather than a specific area.
34
0
Units of Radiation Measurement
Traditional Units SI Units
Roentgen (R) Coulombs per
kilogram rad
Gray rem
Sievert
SI International System of Units used
worldwide
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0
Roentgen
The Roentgen (R) is the traditional unit of
measuring radiation exposure. This measures the
ionization of air. (The exact definition of
Roentgen is complicated and not worth
remembering). The Roentgen measures radiation
quantity before the radiation enters the body.
There is no exact SI unit comparable to the
Roentgen, but in keeping with the metric system
it is measured in coulombs per kilogram.
36
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rad/Gray
The rad (radiation absorbed dose) is the
traditional unit used to measure the energy
absorbed by the body. The SI unit is the Gray
(Gy). 1 Gray 100 rads 1 cGy (centiGray) .01
Gray 1 rad.
37
0
rem/Sievert
The rem (roentgen equivalent man) is the
traditional unit used for comparing the effects
of different types of ionizing radiation
(electromagnetic and particulate). The dose (in
rads) is multiplied by a quality (weighting)
factor. The quality factor for x-rays is 1.
Therefore the dose in rems (dose equivalent) is
the same as the dose in rads. For alpha particles
the quality factor is 20. Therefore the dose in
rems (dose equivalent) would be 20 times the dose
in rads for alpha particles. The higher the LET,
the higher the qualifying factor. The SI unit is
the Sievert (Sv). 1 Sievert 100 rems 1cSv (.01
Sievert) 1 rem.
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Conversions
1 R 1 rad 1 rem 1 Gray 1 Sievert 100
rads 100 rems c (centi) .01 m (milli)
.001 µ (micro) .000001 1000 mrem 1 rem
1 cSv 100 mrem 1 mSv 1000 µSv 1 Sv 100
cSv 1,000 mSv 1,000,000 µSv For x-rays,
not particulate radiation
39
Annual Radiation Exposure
Each year, people are exposed to various types of
ionizing radiation (listed below) and receive an
average dose of 3.6 mSv (360 mrem ) per year. The
actual dose depends on the degree of exposure to
the ionizing radiation sources.
40
Natural (Background) Radiation
Environmental radiation that we are exposed to
daily is called natural or background radiation.
It is composed of both external and internal
sources. Background radiation averages 3.0 mSv
(300 mrem) per year. External Sources
Cosmic (8) Ionizing radiation from space.
Increased exposure at higher altitudes and
during airline travel. Terrestrial
(8) Results from radioactive materials
in soil and rocks. May be incorporated
into some building materials. (See next slide).
Percent of average annual radiation exposure
41
Certain black sand beaches in Brazil produce
radiation levels as high as 5 mrem/hour. This
would be equivalent to getting a full series of
x-ray films every hour (photo left).
Some plants in another area of Brazil have
absorbed so much radium that they will produce an
autoradiograph when placed on photographic paper
(photo right).
42
Natural (Background) Radiation
(continued)

Internal Sources Radon (55) Radon and
its decay products enter our homes via the
atmosphere and water. Inhalation of these
products contributes more than half of our
average annual radiation exposure.
Food/Water (11) Some of the food and water
we ingest contains radioactive materials.
Percent of average annual radiation exposure,
both natural and artificial
43
Artificial (Man-made) Radiation
Artificial radiation results in an annual
exposure of about 0.6 mSv (60 mrem). Included
are Medical X-rays (11) Diagnostic medical
x-rays are the major component of artificial
radiation. Therapeutic x-rays contribute a small
portion. Dental x-rays account for only 0.1 of
the total annual exposure. Nuclear medicine
(4) Diagnostic and therapeutic Consumer
Products (3) Dental porcelain, smoke alarms,
televisions, airport inspections, etc.. Other
Sources (lt1) Primarily nuclear fallout
Percent of average annual radiation exposure,
both natural and artificial
44
Effective Dose Equivalent
Exposure and dose are not related to the amount
or type of tissue covered by the x-ray beam. A
dose (or exposure) of 1 Sv could cover just the
teeth or the entire body. Obviously, the overall
effects would be different, even though the dose
is the same. The effective dose equivalent takes
into account the dose, the volume of tissue
covered and the radiosensitivity of the cells.
Using the effective dose equivalent, different
types of x-ray examinations can be more
realistically compared regarding the risk factor
of each. The following slide lists the effective
dose equivalents for some typical radiographic
exams.
45
Effective Dose Equivalent
AFM (round, F) 60 µSv AFM (rect., F)
27 µSv Panoramic
7 µSv Ceph 220 µSv Chest
80 µSv Upper GI
2400 µSv Natural Radiation 3000 µSv
AFM adult full mouth series of x-ray films
Notice that the effective dose equivalent for
natural (background) radiation (to which everyone
is exposed) is 50 times as much as that for an
AFM with round collimation and using F-speed
film.
46
The preceding slide shows that taking a complete
series of radiographs increases the average
yearly exposure to the patient by a very small
amount. The risk ( of cancer formation) is
increased slightly. The following table compares
this risk with the risk involved in common
activities or habits.
Comparable risk from AFM (cancer) Smoking 1
cigarette (cancer) Drinking 30 cans of diet soda
(cancer) Riding a bicycle 10 miles
(accident) Driving a car 300 miles
(accident) Flying 1000 miles (accident)
47
Pregnancy
The accepted cumulative dose of ionizing
radiation during pregnancy is 5 rad (.05 Sv).
According to the American Academy of Family
Physicians, you would need 50,000 dental x-ray
examinations to reach the 5-rad cumulative dose
to the fetus. An airline flight of 5 hours
results in an exposure of 25 µSv. The exposure to
the pelvic region from a full-mouth series of
radiographs (done properly) is 1 µSv. (average
natural (background) radiation is 8 µSv per
day). The decision to order films during
pregnancy is a personal one. Because of the
relatively low dose, it is not expected that
there will be any harm to the fetus. However, my
recommendation is to limit the films to those
needed to treat the patient during the pregnancy
(symptomatic teeth or very active caries).
48
0
Radiation Protection for Patient
and Operator
49
0
Maximum Permissible Dose (MPD)
The maximum permissible dose is the amount of
radiation (dose limit) that a person can receive
from artificial radiation (effective dose
equivalent). These dose limits are recommended by
the NCRP and required by the state in which a
dentist practices. The dose limits may vary
between the NCRP and the state. There are no
dose limits for patients being radiographed. The
dentist should only order films that are needed
for a diagnosis, and thus keep patient exposure
to a minimum (See ALARA).
National Council on Radiation Protection and
Measurements
50
0
Maximum Permissible Dose (MPD)
Dose limits (MPDs) are set for occupationally
exposed personnel (dentist, dental hygienist, and
dental assistant) and for non-occupationally
exposed individuals (front-office staff, people
in waiting room, etc.). The dose limits are as
follows
Occupationally exposed Adult 50
mSv (NCRP Ohio) Minor 5 mSv
(Ohio) Pregnant 5 mSv
(Ohio) Non-Occupationally exposed NCRP
5 mSv Ohio 1 mSv
51
Patient Protection
It is important to do everything we can to reduce
the amount of exposure when a patient has dental
radiographs taken. The following slides identify
the ways in which we can do this.
52
ALARA
ALARA stands for As Low As Reasonably
Achievable. If we assume that there is no
threshold for stochastic effects (mutations and
cancer) to occur, then it is important to keep
the exposure to the minimum needed to provide an
accurate diagnosis. In other words, take only
those films needed to properly identify patient
problems.
53
Professional Judgment/Selection Criteria
Deciding which films are needed for a particular
patient is dependent on two things Professional
Judgment and Selection Criteria. Professional
Judgment Through education and experience, each
dentist develops an expertise in deciding which
films will be needed to obtain an accurate
diagnosis. Selection Criteria In 2005, the ADA,
in conjunction with the Food and Drug
Administration, released updated guidelines for
prescribing dental radiographs. These guidelines
identify which films should be taken, based on
clinical findings. (For more detailed discussion,
see Self-study Patient Management/Film
Ordering)
54
Equipment Reliability
X-ray equipment must be functioning properly to
insure that the patient does not receive
unnecessary radiation exposure. The settings for
the exposure factors (exposure time, mA, kVp)
must accurately reflect the output. Each state
has requirements for the inspection of x-ray
equipment to make sure that everything is working
properly. The Ohio Department of Health requires
that equipment be checked every five years (fee
charged) and that the operator renew the
registration of the equipment every two years
(fee charged). If new x-ray equipment is
purchased or if an x-ray unit is received from
another dentist, the state must be notified.
Disposal of old units must also be documented.
55
0
Constant Potential X-ray Machine
Many machines now convert the alternating current
into a direct current (constant potential).
Instead of cycles going from zero to the maximum,
both positive and negative, the voltage stays at
the maximum positive value, creating more
effective x-ray production. This allows for
shorter exposure times and a 20 reduction in
patient exposure.
56
Filtration
0
Low-energy x-rays do not contribute to the
formation of an x-ray image all they do is
expose the body to radiation. Therefore, we need
to get rid of them. The process of removing these
low-energy x-rays from the x-ray beam is known as
filtration. Filtration increases the average
energy (quality) of the x-ray beam. The x-ray
beam becomes more penetrating, providing good
image formation on the film with reduced patient
exposure. (See Self-study X-ray Production for
more information on filtration).
Low-energy x-rays
high-energy x-ray
57
Collimation
0
Collimation is used to restrict the size of the
x-ray beam, covering the entire film with the
x-ray beam but not exposing unnecessary tissue.
By reducing the amount of tissue exposed, the
production of scatter radiation is also reduced.
The shape of the opening (round or rectangular)
in the collimator determines the shape of the
x-ray beam. The size of the opening determines
the size of the beam at the end of the PID. If
you switch from a 7 cm diameter round PID to a 6
cm diameter round PID, the patient receives 25
less radiation. Rectangular collimation results
in the patient receiving 55 less radiation when
compared to what they would receive with a 7 cm
round PID. (See Self-study X-ray Production
for more on collimation).
58
Focus Film Distance
Extending the distance between the target of the
x-ray tube (focal spot) and the teeth makes the
beam less divergent as it passes through the
head, exposing a smaller area of the patient.
(The diameter of the beam at the skin surface is
the same for both distances. The beam from the 8
target-teeth distance spreads out much more as it
passes through the head).
16
8
Target-teeth 16
Target-teeth 8
59
Intraoral Film Speed
Using a faster film requires less radiation.
Using F-speed film (Insight) instead of D-speed
film reduces patient exposure by 60. F-speed
film has larger silver halide crystals, which
more readily intercept the x-rays. (See
Self-study Film and Screens for more on
films).
60
Extraoral Screen Speed
Extraoral films are exposed by light from
intensifying screens this light is produced when
x-rays contact phosphor crystals on the surface
of the screens. The light is either blue or
green, depending on the type of screen.
Intensifying screens have different speeds,
depending on the type of phosphor crystal (rare
earth recommended) and the thickness of the
phosphor layer. The faster the screen is, the
less the patient exposure will be. However, image
detail decreases as the speed of the screens
increases. It is important to make sure that the
film is compatible with the color of light coming
from the screen. (See Self-study Film and
Screens for more on screens).
61
Lead Apron/Thyroid Collar
The American Dental Association recommends that a
lead apron and thyroid collar be used on all
patients. The actual exposure from scatter
radiation to other parts of the body is minimal,
but considering the ease of placing the lead
apron and thyroid collar, there is no reason not
to use them. Patients will appreciate your
efforts in keeping their exposure to a minimum.
(The thyroid collar is not used for panoramic
films).
62
Technique
Good technique in taking films is essential in
producing diagnostic radiographs. Proper film
placement and selection of the correct exposure
factors will maximize the value of the films and
will reduce or eliminate the need for retakes,
which would increase the patients overall
exposure.
63
Processing
Processing films for the correct amount of time
and at the proper temperature produces films of
good diagnostic quality, assuming the films were
exposed properly. It is necessary to have
appropriate safelighting in a light-tight
darkroom. Inadequate processing will result in
retaking films which will add to the patients
overall radiation exposure. (See Self-study
Processing).
64
X-ray Protection for the Operator
The operator should never hold films in the
patients mouth during an exposure. Some
patients, due to physical or mental impairments,
may need help in stabilizing the films, but this
assistance should be provided by a friend or
relative of the patient. This person should wear
a lead apron and leaded gloves when holding films
in the patients mouth.
The photo at right shows a squamous cell
carcinoma on the finger of a dentist who
routinely held films for the patient.
65
X-ray Protection for the Operator
The operator should stand behind a protective
barrier if available. It has been determined that
drywall is adequate protection for this purpose.
The operator must be able to observe the patient
during the exposure to make sure the patient
doesnt move prior to or during the exposure. If
a direct line of sight is not possible, mirrors
can be mounted on a wall opposite the doorway to
allow visualization of the patient.
66
0
Inverse Square Law
If barriers are not available, the operator
should follow the position and distance rule.
The x-ray beam spreads out as it moves away from
the target (focal spot, source, focus), covering
a larger area and diluting the effects of the
x-ray beam in a given area. The farther you get
from the target , the weaker the x-ray beam will
be. The inverse square law is a formula used to
identify the strength (intensity) of the x-ray
beam at a given distance from the target. (See
next slide).
67
Inverse Square Law
The formula above shows that the intensity of
radiation varies inversely as the square of the
target-film distance. D represents distance, KI
is the known intensity (x-ray beam strength) and
UI is the unknown intensity. If you know the
intensity at one distance, you can compute the
intensity at any other distance by using the
formula. (See example on following slide).
68
Inverse Square Law
0
2
D
KI
X KI
UI
D
UI
1
X 100 1
100
The intensity of the beam at 1 ft. is 100 (KI).
What is the intensity of the beam at 10 ft. (UI)?
The distance of known intensity (DKI) is 1. The
distance with unknown intensity (DUI) is 10.
Using the formula, as seen at right, the unknown
intensity is found to be 1.

69
Inverse Square Law
0
The inverse square law also works by looking at
the magnitude of change in the distance. In the
diagram below, the distance D2 is 2 times the
distance of D1. The x-ray beam covers 4 squares
at D1 and 16 squares at D2, or four times as
many the intensity is ¼ as much because the beam
is spread out over four times as many squares.
The change in distance is two times the square
of 2 is 4 and the inverse of 4 is ¼. If the
change in distance is 3
times as much,
calculate as
follows the square of 3 is 9,

and the inverse is 1/9. The
intensity at three
times the
distance would be 1/9 what
it was
at the initial distance.
70
Position and Distance Rule
To take advantage of the reduction in beam
intensity due to the Inverse Square Law, the
operator should stand at least six feet away from
the patient at an angle between 90 and 135
degrees. As the tubehead is moved, this safe
position will change relative to the patients
head (see below).
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State Requirements
All states have some regulations concerning the
operation of x-ray equipment. In Ohio, each
dental office must designate a Radiation Safety
Officer (any office employee dentist, dental
assistant, hygienist, etc.) who is responsible
for maintaining records relating to the x-ray
practices. Each office must post a Notice to
Employees, which briefly lists the
responsibilities of the dentist and all employees
who take radiographs as detailed in the states
radiation protection rules. Each office must have
a Safe Operating Procedures manual and each
operator must sign an Instruction of
Individuals form to indicate that they have read
the manual.
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Film Badges
Personnel-monitoring devices (film badges) can be
used to determine the exposure an operator
receives during a given period (often quarterly).
Film badges are required in some states if you
expect to exceed 25 of the MPD during any
calendar quarter (12.5 mSv). Although you should
not expect to exceed this dose following normal
safe operating procedures, it is beneficial to
have a dosimetry service. The cost is minimal and
the reports, which hopefully identify the lack of
exposure to the operators, reduces any
apprehension the office staff may have about
radiation exposure.
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NCRP Report 145
0
The NCRP Report 145 was released in December,
2003. It listed the following recommendations. Th
e lead apron is not required. The thyroid collar
is required for children and should be used for
adults. (The ADA, in response to the National
Academy of Science report on low-level radiation
effects, recommends lead apron/thyroid collar for
all patients). Rectangular collimation is
required for periapicals and should be used, when
feasible, for bitewings.
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NCRP Report 145
D-speed film is not to be used. E-speed film or
faster is required. (Kodak no longer makes
E-speed film F-speed film, which is faster, is
available along with D-speed). Rare earth screens
are to be used for pans. Sight development (dip
tanks) is not acceptable. Shielding design for
new or remodeled dental offices is to be done by
a qualified expert. Film badges are required for
pregnant personnel.
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0
This concludes the section on Radiobiology,
Dosimetry, and Radiation Protection. Additional
self-study modules are available at
http//dent.osu.edu/radiology/resources.php If
you have any questions, you may e-mail me at
jaynes.1_at_osu.edu Robert M. Jaynes, DDS,
MS Director, Radiology Group College of
Dentistry Ohio State University