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Are dental x-rays
dangerous?
Some people do not want diagnostic x-rays because they have
heard that the radiation is dangerous. In fact, they pose very little
danger. There are currently two units used to measure the exposure of
biological organisms to
radiation.
These units are measures of equivalent dose. Equivalent dose
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. Equivalent dosing
units make possible the comparison of radiographs of different types and sizes
in different parts of the body. They also allow comparison with exposure from
natural background radiation. They allow for a more meaningful comparison
between radiation sources that expose the entire body (such as natural
background radiation) and those that only expose a portion of the body (such as
dental vs. medical radiographs).
The first, oldest and still probably the most frequently used unit of
equivalent dose in the US is called a rem. A second unit,
used outside of the US is the sievert. 1 sievert = 100 rems. A rem is a large unit,
(And a sievert is an even larger unit), so exposure
to medical radiation is generally measured in millirems (mREM)
and millisieverts (mSV).
Other units you may hear about are measures of radiation called rads
and grays. These are units of absorbed dose, and are
generally applied to non biological bodies. They do not take into account
the differing effects of radiation on different tissues in the body. This
type of measurement does not concern us in the study of dental x-rays.
The average dental x-ray delivers about 1 mREM per exposure. Thus a
full mouth series
of dental x rays (18 intraoral films) delivers about 18 mREM.
(Note: These figures are based on the use of
E-speed film.
Kodak InSight, an F-Speed dental
film, lets you reduce radiation exposure by up to 60 percent as compared to
Kodak Ultra-Speed dental film, a D-speed product.) A
panorex film delivers
about 2 mREM. By comparison,
the average person in the US is exposed to about 360 mREM per year just
from naturally occurring background sources. By this measure, it would take approximately
20 full series of dental radiographs 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 every three to five years on
average.
The average person in the US is exposed to about 360 mREM per
year just from background sources, but the actual amount of background radiation
received by any given person varies quite a bit depending upon that individual's
lifestyle choices. 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 3-5 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 Washington State Department of Health has set the maximum
safe occupational whole body radiation exposure to 5000 mREM (5 rem)per year.
The same limit holds true for other states as well (ex.
New york -- see section 16.6). Finally,
5000 mREM is the federal total effective, whole body, yearly
occupational dose limit. By this reckoning, it would take about 278
full mouth series of dental x-rays (18 films per survey) over the course of a
year to equal one years maximum safe occupational radiation level. It
would take 2500 panorex films or about 5000 individual intraoral
x-rays to get to this limit. The 5000 mREM yearly limit applies to persons
who are routinely exposed to ionizing radiation in the course of their jobs. This
is not to suggest that a member of the general public should routinely expect to
be exposed to 5000 mREM per year of diagnostic x-rays, but it is an
indication that the benefits of routine yearly diagnostic x-rays far, far
outweigh the dangers posed by the radiation.
Dental x-rays are aimed in a tight beam at a small spot on the face.
The only structures that receive the full dose of x-radiation are the tissues in
the direct line of fire. The rest of the body receives only the radiation
that is scattered off of the structures in the line of fire. (Much less
radiation scatters from an object in an x-ray beam than from an object in a beam
of ordinary light due to the difference in the nature of the respective
radiation sources. Click
here for a better understanding of scatter
radiation.) Furthermore, the tissues at which dental x-rays are aimed are
much less prone to injury from x-radiation than are tissues in other parts of
the body, such as the intestinal lining or reproductive organs and other
constantly reproducing tissues. The newest unit of measurement, the
milisievert was designed to take this factor into account.
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
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 950 intraoral films:
The table below is adapted and updated from the
website
of the American Dental Association which in turn took its information from
Frederiksen NL. X-Rays: What is the Risk? Texas Dental Journal.
1995;112(2):68-72, It is quite helpful in comparing the amount
of radiation received from dental x-rays to other medical and natural
sources. As you can see, by this more realistic measure, it would take 20
full series of x rays to equal the amount of radiation the average citizen picks
up from background sources each year:
Note also that radiation to the gastrointestinal (GI) tract is MUCH more
damaging than radiation to the chest. This is due to the increased
vulnerability of the lining of the intestine because the cells there are
constantly reproducing and being replaced while the cells in the lungs are less
frequently replaced.
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Dental radiographs exposure:
Single
intraoral (d-speed film)
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)
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(mSV)
.0095
0.038
0.013
0.180
0.060
0.019 |
(mREM) .95
3.8
1.3
18
6
1.9 |
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Medical radiographs exposure:
Lower GI series
Upper GI series
Chest
|
4.060
2.440
0.080 |
406
244
80 |
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Average radiation from outer
space In Denver, CO (per year) |
0.510 |
51 |
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Average radiation in the U.S.
from Natural sources (per year) |
3.000 |
360 |
Adapted from Frederiksen NL. X-Rays: What is the Risk? Texas
Dental Journal. 1995;112(2):68-72
A good reference for persons looking for the relative dosage from other
sources of medical diagnostic and treatment procedures should consult the
website of the
Health Physics Society
The American Nuclear Society also offers an
excellent web page that allows you to calculate your own exposure
to ionizing radiation.
|
This site offers a course in dental radiology
This course is designed for radiology technicians,
dentists, hygienists and dental assistants, and covers all aspects
of taking intraoral x-rays. The most intriguing part of the
course for most trained professionals is an emphasis on shadow
casting and how to use this knowledge to advantage while taking
those difficult intraoral films. |
What about danger to the x-ray technician? (scatter
radiation)
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). Dental radiography
using film A majority of dental offices still use
intra oral film to take their x-rays. There are three standard film
speeds. D speed film is the slowest, E speed is midrange and F speed
is the fastest. Each jump in speed has two consequences. First,
Each succeeding speed film requires less radiation to expose than the one
before. Thus, switching from D to E speed produces a 30-40% reduction
in exposure. Switching from E to F speed produces a 20-25% reduction in
exposure, and switching from D to F-speed film produces a 60% reduction in
exposure. Second, the faster the film, the larger the grain size (the
size of the silver nitrate particles on the surface of the film), and thus
the lower the film resolution. While lowering the patient exposure to
x-rays is obviously a good thing, the lower resolution (the amount of
clarity) the less diagnostic information is available to make the diagnosis.
Therefore, about 70% of offices using film still use D speed film, 21% use E
speed and only 9% use F speed. Digital X-rays
In
digital radiography, a sensor replaces the film normally used for
traditional radiographs. The sensor plugs into the USB port on an ordinary
computer. The most common type of Intraoral sensors are solid-state
electronic devices called “charged-coupled devices” (CCD). A CCD is
composed of millions of light sensitive silicon cells arranged in a
rectangular array on the face of the sensor. Each cell on the
face of the sensor will eventually result in one pixel (picture element) in
the final image.
The x-ray photons falling upon each cell create an analog (continuous) electrical
voltage. The level of the voltage produced depends on the number of
photons reaching the cell, and this in turn depends on the density of the
structures (teeth and bone) between the x-ray source and the CCD. The
voltage level for each pixel is converted to digital data (numbers between 0
and 65,536) by a relatively simple device called an "analog to digital
converter".
Each value is interpreted by the computer as a shade of gray. Zero
corresponds to pure white, and 65,536 corresponds to pure black with
intermediate values corresponding to varying shades of gray. In this way, the image is converted to millions of tiny digital picture
elements (pixels) which are reassembled by the computer into a coherent
image.
CCD's used in dental imaging are essentially the same as the CCD's
used in digital cameras. In your home camera, the CCD contains color filter
arrays for each pixel so the image can be reassembled in color. Since
dental radiographs are monochrome (shades of gray), the dental CCD does not
contain these filters While digital radiography is a newer
technology than the film it replaces, it must be stressed that the image
obtained on a digital x-ray is not necessarily any better than one taken
using standard x-ray film. Digital technology does, however, require substantially
less radiation than film. Digital radiography requires only about a quarter as much
exposure time as D speed film, and a little more than half the time as the
newest F-speed films, while delivering about the same resolution as D speed
film. The largest benefit of digital x-rays is the ability to
computer-enhance the images, making them larger, clearer, or higher contrast
at will. This can be helpful, particularly for dentists with less
experience in reading traditional film, but it is rarely essential in making
a correct diagnosis. Larger, sharper images are helpful in patient
education and in helping patients to accept a treatment plan. There is
no darkroom developing of the images, and the sensor can be moved about in
the mouth more quickly than films, which must be exchanged for new ones for
each shot. Thus digital radiography cuts down on the time it takes to
expose and process a series of intraoral films. For these reasons,
digital radiography is gaining increasing acceptance in dental offices
throughout the US and Europe.
The major dental series
There are three major types of dental x-ray surveys: the
initial full mouth series, the yearly bite wing series, and the Panoramic x-ray
film.
The Full Mouth Series (FMX)

This is an example of the full mouth series we
take in our office. It consists of 4
bite wing films which are taken at an
angle specifically to look for decay, and 14
periapical films
which are
taken from other angles to show the tips of the roots and the supporting
bone. Not all full series look exactly like this one, but they all use
some combination of bite wing and periapical x-rays to show a complete survey of
the teeth and bones. We take a full mouth series on everyone over the age
of 25 at the initial oral examination, and retake it again every 3 to 5
years.
Notice that each tooth is seen in multiple
films. This redundancy is important because it gives us lots of
information we would not otherwise have. Each x-ray is shot from at least
a slightly different angle and the difference in angulation can reveal many
different aspects of the tooth in question. X-rays are not ordinary 2
dimensional pictures. They are actually 2 dimensional shadows
of 3 dimensional objects.
As you know, shadows may be longer or shorter than the
object which casts them depending on the angle of the light source and the
screen upon which they are projected. They may also be distorted in other
ways as well. The shadow of your hand may show all 5 fingers spread out if
you hold it palm forward facing the light source with the screen directly
behind the back of the hand. On the other hand, the fingers will not be
visible at all if the hand is turned so that the thumb is facing the light
source and the little finger is facing the screen. This happens with
x-rays also, except that the objects which cast the shadow appear translucent on
the film, and it is actually possible to see several objects superimposed over
each other. This is what gives x-rays their 3 dimensional quality, and
this is why it is very helpful to have several views, taken from different
angles, of any given tooth.
The bitewing series

A bitewing series consists of either 2 or 4 films
taken of the back teeth (although some offices take them on front teeth as
well), with the patient biting down so the films contain images of both the top
and bottom teeth. A bitewing series is the minimum set of x-rays that most
offices take to document the internal structure of the teeth and gums. In
our office, we take 2 on children under the age of 12, and 4 on everyone older, supplemented by the other periapical films associated with a full series of x
rays if the patient is over the age of 25.
The difference
between bitewing and periapical films
| In a bitewing film, all three
elements, the teeth, the film, and the x-ray beam are optimized to give
the most undistorted shadows possible. (The film and teeth are
parallel, and the beam is aimed directly at both; at a 90 degree angle.)
Thus bitewing films afford the most accurate representation of the true
shape of the teeth and associated structures such as decay, fillings,
shape of nerves and bone levels. (To see how the big cavity in the lower tooth was filled, click
here.) |

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A periapical
film like the one on the right is shot from an angle in which the three
elements are not necessarily aligned parallel. Some distortion is
introduced on purpose to be sure that the shadow of the entire tooth or
teeth in question falls on the film. This is done because in many
instances, the space available in the mouth, or the curvature of the roof
of the mouth will not permit parallel placement of the film. This patient
had an abscess and was in pain when the film was shot. (To see how this
situation is treated, click
here.) |
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The Panoramic Film (Panorex)

As you can see from the image above, the Panorex is a large,
single x-ray film that shows the entire bony structure of the teeth and
face. It takes a much wider area than any intra oral film showing
structures outside of their range including the
sinuses, and the
Temperomandibular Joints. It shows many pathological structures such as
bony tumors and cysts, as well as the position of the
wisdom teeth. They are
quick and easy to take, and cost a little more than a full series of intraoral
films. In addition to medical and dental uses, panoramic films are
especially good for forensic (legal) purposes in the identification of otherwise
unrecognizable bodies after plane crashes or other mishaps.
Panoramic films differ from the others in
that they are entirely extraoral, which means that the film remains
outside of the mouth while the machine shoots the beam through other
structures from the outside. It fits into a broad category of
medical x-rays called tomographs. A tomograph is a computer
assisted method of focusing x-rays on a particular slice of tissue
and showing that slice on the film as if there were no other structures
outside of that slice. It has a number of real advantages over
the intraoral variety of film discussed above. Since it is entirely
extraoral, it works quite well for gaggers who could not otherwise
tolerate the placement of films inside their mouths. The patient
stands in front of the machine (pictured on the right), and the x-ray tube
swivels around behind his head. Another advantage of the panoramic
film is that
it takes very little
radiation to expose it. The amount of radiation needed to expose
a panoramic x-ray film is about the same as the radiation needed to expose
two intraoral films (periapical or bitewing). The reason for this is
that the film cassette contains an intensifying screen which fluoresces
upon exposure to x-rays and exposes the film with visible light as well as
x-rays.
For much more on how a panoramic unit actually creates its image, click on
the image of the machine above, or click
here.
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The film on the right is a panoramic view of
a child under the age of 12. You can see the adult teeth that are forming
underneath the baby teeth. You can also see the adult second molars
which are the 4 half formed teeth toward the outside of the film.
The fact that the second molars are not yet erupted is the reason a
dentist or anthropologist can tell that this child is under the age of
12. For a better understanding of this film click
here.
These films have one major disadvantage. The
panoramic film is a lower resolution picture than the intraoral
films. This means that the individual structures which appear on them
(such as the teeth and bone) are somewhat fuzzy, and structures like
caries (tooth decay) and bony trabeculation (the spongelike bone inside the
marrow spaces) are imaged without the fine detail seen on intraoral films. They are
not considered sufficient for the diagnosis of decay, and must be accompanied by
a set of bitewing x-rays if they are to be used as an aid for full diagnostic
purposes. The combination of a set of bitewings and a panoramic film is particularly useful for those patients who are
to be referred for orthodontic consult, and for extraction of wisdom teeth. We use the bitewing/panorex
combination frequently instead of a full series of intraoral films on patients
between the ages of 13 and 30.
The CAT scan
A tomograph is a two-dimensional image of a slice or section
through a three-dimensional object. An example of a primitive tomograph is
the panoramic x-ray defined above.
While the panoramic x-ray utilizes a photographic film or an electronic array of
charged-coupled devices (CCD) to make an image directly from a fan shaped beam
of x-rays which sweeps around the jaw, Computerized Tomography (CT) uses fan
shaped, or cone shaped beams of x-rays that scan each point in an object from
multiple angles to create an array of data points.
The detector array is the same width as the fan shaped beam. The detector
and the x-ray source are mounted on opposite sides of a gantry which moves
around the subject. This arrangement allows objects within plane of the
beam to
be scanned from numerous angles as the gantry rotates around them. The
detector takes numerous "snapshots" called "views." About 1,000
views are
taken in one rotation. Each profile is analyzed by computer software, and the
full set of profiles from each rotation is compiled into a two dimensional image
representing a "slice" through the subject in the same plane as the beam.
For much more on the CT scan and the theory behind it, see
this page
on my course
on dental radiography, or click on the icon above.
Cone Beam CT scanning
Cone
beam computerized tomography has been available in the United states since 2001. The cone beam CT scanner
(CBCT) does not image slices. Instead
its cone shaped beam scans a complete volume at once. By rotating the beam
around the subject and creating a very large array of data points, the area of interest
is observed from a large number of different angles. The cone beam scans both
the maxilla and mandible at one time, and requires about 2-8 times the amount of
radiation used in a panoramic radiograph. This is still quite low when
compared with the dose supplied by the CT scanner. The data is captured by a two
dimensional array and creates between 150 and 600 high resolution images (also
called "views"). These two dimensional views are then combined to form a
coherent three dimensional image of the bony structures in the field of view.
(Click the icon to read much more on the cone beam and the theory behind it)
Unlike the CT scanner, the cone beam is generally tuned to make images of hard
tissues (bone and teeth), which is the reason that the radiation exposure to the
patient is so low. CT scans expose the patient to much more ionizing
radiation because they are generally tuned to get images of soft tissue.
This means that while a cone beam uses a low intensity, high energy x-ray beam,
the CT scanner uses a high intensity low energy beam which is more efficiently
absorbed and scattered by biological tissues than the higher energy beams used
in cone beam technology.
Cone beam machines are quite expensive, and the clinician who
takes or orders one has a heavy legal liability, so cone beam scans are likely
to be fairly expensive
|
This site offers a course in dental radiology
This course is designed for radiology technicians,
dentists, hygienists and dental assistants, and covers all aspects
of taking intraoral x-rays. The most intriguing part of the
course for most trained professionals is an emphasis on shadow
casting and how to use this knowledge to advantage while taking
those difficult intraoral films. |
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