|
<== Previous
page
Next page ==>
There are three ways x-rays interact with organic
matter: Classical Scattering, the Compton Effect, and the Photoelectric Effect.
A
photon may come into contact with an atom and
interact with an electron. If the photon does not have enough energy to actually
displace the electron from the atom it gives its energy to the electron. The
electron then produces another photon with the same energy as the other and
sends it off in a different direction. This is called Classical Scattering.
If the photon has enough energy it will displace
the electron from its orbit around the atom. The electron, called a recoil
electron, is lost from the atom. The atom absorbs the energy from the photon,
but is now missing one electron. This atom will now have a net positive charge
and is called an ion. This is called Photoelectric Effect.
(image below)
If the photon collides with an atom or electron and has
enough energy to displace it, but does not transfer all of its own energy to the
atom or the electron, it will continue on weaker as scattered radiation. This is
called the Compton Effect.
These three reactions in themselves are not life
threatening. However, the molecular interactions of these altered atoms can result in
breaking molecules into smaller pieces, disrupting molecular bonds, and forming
new bonds within or between molecules. Radiation can also interact with the
water or oxygen in cells to disturb their delicate balance and damage DNA
molecules.
High doses of radiation to the entire body can
cause acute effects. Long term or chronic effects come from repeated exposure to
radiation. The body attempts to repair the damage but cannot keep up if the
exposures are regular enough or strong enough.
Operators of x-ray devices should monitor the
amount or radiation they are exposed to by using a film badge. These badges are
worn while at work and then sent in to a company at regular times to be
evaluated for radiation exposure. Operators should step behind a lead barrier
when exposing films. If no barrier is available, stand at least 6 feet away and
between 90 and 135 degrees to the primary beam. Operators should never hold the
film for a patient during exposure.
Radiographs should not be taken unless the
benefit for the patient outweighs the risk of the radiation exposure. Lead
aprons must be used on all patients, and a thyroid collar used while taking intraoral
films. A patient would have to have 25 complete mouth series in a short time to
significantly increase his or her risk of skin cancer. The benefit of detecting
disease that may not be otherwise detected far outweighs the risk of radiation
in the small doses used for dental radiography.[i]
Radiation exposure varies according to the
technique, the amount of collimation, the film speed, and the kilovoltage. The
paralleling technique using a "long cone" provides the least amount of radiation
and the best quality radiograph. Rectangular collimation reduces the area of
tissue exposed to the x-ray beam by 60 to 70%.[ii]
|
The practical aspects of radiation
safety
Some people do not want you to take
diagnostic x-rays
because they have heard that the radiation is dangerous. In fact,
dental x-rays pose very little danger. It is important for the
dental professional to know the various terms used when speaking about
the effects of diagnostic x-rays on the human body.
There are five units used in measuring radiation: The
Roentgen, The Gray, the Rad, the Sievert, and the REM. Their
definitions and relationships re explained below.
-
Exposure:
The measure of radiation quantity, the capacity of radiation to
ionize air (Roentgen [R]).
-
Absorbed
Dose: The measure of energy imparted by any type of ionizing
radiation per unit mass of any type of matter. Its SI (Systeme
Internationale) unit is Gray [Gy], where one Gy= 1
joule/kg. Its traditional unit is rad (radiation
absorbed dose). 1 Gy= 100 rads. The absorbed dose is generally
applied to any type of mass, biological or otherwise. It has
no real value when comparing the "dangerousness" of different forms
of radiation exposure to biological organisms.
-
Equivalent dose is based on an average
absorbed dose in a tissue or an organ and weighted by the radiation
weighting factor (i.e. the type of radiation). This term has
largely been replaced by effective dose. The units used to define
equivalent dose are the REM and the Sievert, both defined below.
-
Effective dose: The effective dose is the sum of the weighted
equivalent doses for all irradiated tissues or organs. The tissue
weighting factor takes into account the relative detriment to each
organ and tissue including the different mortality and morbidity
risks from cancer, the risk of severe hereditary effects for all
generations, and the length of life lost due to these effects.
Effective dosing units are used to compare radiation doses on
different body parts on an equivalent basis because radiation does
not affect different parts in the same way.
Effective
dose is
a useful
term that allows
comparisons
to be made between
sources
of radiation
exposure
which
expose
only portions of the body,
such as radiographic techniques,
and whole-body exposures, including those resulting
from natural
or background radiation.
The units
of effective dose are the same ones used for equivalent dose.
They are the
sievert
(Sv)
and the REM.
For diagnostic x-ray purposes, 1 Sv = 1
Gy. The unit traditionally used in the United states is the rem (radiation equivalent
man). 1Sv = 100 REM. A millisievert
(mSv)
is one thousandth
of a sievert,
and one miliREM (mREM) is one thousandth of a REM.
-
The REM and the SIEVERT are large units, so exposure
to medical radiation is generally measured in milliREMs (mREM)
and milliSIEVERTs (mSV).
1 mSV = 100 mREM.
The average dental x-ray taken with
film delivers
between about .75 and .95 mREM effective dose per exposure depending on angulation.
This estimate is based on an average exposure because different
speed films (D, E, & F) require different exposure settings on the
machine, and because dental x-rays taken at one angle may expose
different parts of the body to radiation than another taken at a
different angle. For simplicity, we will say that the average
dental intraoral radiograph exposes the patient to about 1 mREM. A
dental x-ray taken digitally delivers between a third and half of this
value (.3 to .5 mREM).
Based on this estimate, a
full mouth series
of dental x rays (18 intraoral films) taken with x-ray film delivers about 18 mREM. A
panorex film delivers
about 2 mREM.
By comparison, the average person in the
US is exposed to about 300 mREM per year just
from naturally occurring background sources. By this measure, it would take approximately
17 full series of dental radiographs or 150 panoramic x-rays to equal the background radiation
that the average citizen is exposed to on a yearly basis. Note that
most dentists take a new full series and/or a new panoramic every three to five years on
average.
Background radiation comes from outer space, the
earth, natural materials (including natural foods), and even other
people. For example, flying cross country exposes a person to about 5
mREM over and above the normal radiation he receives from outer space
while simply walking outdoors for the same length of time. Cooking
with natural gas exposes us to about an additional 10 mREM per year
because of the naturally occurring radon gas the cooking gas contains.
Living in a brick building adds an additional 10 mREM per year over and
above the radiation you would receive from living in a wooden
structure. Simply sleeping next to another person exposes each bed
partner to an extra 2 mREM per year.
The American Nuclear Society has an
excellent web page that allows you to
calculate your own yearly exposure to ionizing radiation.
The Washington State Department of Health has set the maximum
safe occupational whole body radiation exposure to 5000 mREM per
year. This is the dose considered safe for people who work with
ionizing radiation in their professional lives, including x-ray and
nuclear technicians. By this reckoning, it would take over 278 full mouth series of
dental x-rays to equal one years maximum safe radiation level for an
person employed in a radiation intensive occupation. It would take
2500 panorex films to get to this limit, or over 5000
individual x-ray films.
The use of digital radiography further reduces the
exposure to about one third of the values in the chart below. This
would mean that it would take about 50 full series of x-rays (taken with
a digital sensor) to equal the amount of radiation the average citizen
picks up from naturally occurring background sources each year---that
means 360 intraoral films:
The table below is adapted and updated from the
website of the American Dental Association
(which was in turn adapted from Adapted from Frederiksen NL.
X-Rays: What is the Risk? Texas Dental Journal. 1995;112(2):68-72).
A table of equivalent doses is quite helpful in comparing the
amount of radiation received from dental x-rays to other medical and
natural sources.
|
Dental radiographs exposure:
Single
intraoral (film averages)
Single
intraoral (digital sensor average)
Bitewings (4 films-D-Speed)
Bitewings (4 digital radiographs)
Full-mouth series (about 19 films)
Full-mouth series (19 taken digitally)
Panorex (panoramic jaw film)
|
(mSV)
.0075
to
.0095
.0025
to
.0032
0.038
0.013
0.150
0.050
0.019 |
(mREM) .75
.95
.25
.32
3.8
1.3
15
5
1.9 |
|
Medical radiographs exposure:
Lower GI series
Upper GI series
Chest
|
4.060
2.440
0.080 |
406
244
80 |
|
Average radiation from outer
space In Denver, CO (per year) |
0.510 |
51 |
|
Average radiation in the U.S.
from Natural sources (per year) |
3.00 |
300 |
Adapted from Frederiksen NL. X-Rays: What is the Risk? Texas
Dental Journal. 1995;112(2):68-72
|
For more accurate and detailed charts and
explanations, I have provided a documentation page: Please
click here |
What about danger to the x-ray technician?
The x-radiation figures mentioned above pertain to
the patient who is in the direct line of fire from the x-ray tube.
The radiation received by the person taking the x-ray comes
exclusively from scatter, which is most easily understood by
thinking about a flashlight aimed at a wall in a completely darkened
room. The spot on the wall where the flashlight is aimed is the
brightest because it is in the direct line of fire, however, the
rest of the room is also dimly illuminated by the light that
scatters off the wall. This scatter is what concerns us since
nothing but the patient's face and jaws is directly in the line of
fire of the beam. The flashlight analogy is inexact since x-ray
beams are better collimated (they form a tighter beam), and much
less x-radiation is scattered from the target than light from the
wall because of the nature of the x-radiation itself. But the
analogy still helps you to understand the concept of scatter versus
direct illumination. Furthermore, the strength of the radiation (or
light) hitting any unit area falls off geometrically depending on
the distance from the source of scatter. Think of the flashlight
analogy again. In a very large, dark room the area of the wall two
feet from the bright spot is much brighter than an area 20 feet
away. The "brightness" of the scatter illumination falls off as the
square of the distance. A person standing 6 feet away from
the target receives one ninth (1/9) as much scatter radiation as a
person standing two feet away from the target (6 feet is 3 times
further away than 2 feet, and 3 squared is 9). A person standing 10
feet away (5 times further away) from the target receives one
twenty-fifth (1/25).
|
|
Dentists and allied dental professionals often
seek CE courses from ADA CERP recognized providers to fulfill their
CE requirements for re-licensure. Most state and
provincial licensing boards will accept CE credits issued by ADA
CERP recognized providers. In the spring of 2003, the FDI World
Dental Federation became the first internationally based CE provider
to be granted ADA CERP recognition.
Please contact your state board directly for their specific rules
and regulations. Most states approve supervised self-study courses
that are ADA CERP accredited.
Those dentists, hygienists, dental assistants
and radiographers interested in receiving 3 continuing
education credits for this course may take a 10 question test at a
cost of $35 and receive their certificate immediately by clicking
here.
Those dentists, hygienists, dental assistants
and radiographers interested in receiving 8 continuing
education credits for this course may take a 25 question test at a
cost of $66 and receive their certificate immediately by clicking
here.
Note: There are no questions on tables or
Glossary. |
<== Previous
page
Next page ==>
[i]
Mile, van Dis, Jensen. “Radiographic Imaging for Dental Auxiliaries”
1993.
[ii]
Olson, S. “Dental Radiography Laboratory Manual.” W. B. Saunders Company
1995
|
