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One of the most important concepts in dental radiography is the concept of
shadow casting. Once the operator realizes the correlation between the
position and angulation of the various elements in radiography and the way
ordinary shadows are cast (say, the way your own shadow is cast on a wall or the ground
on a sunny day) the entire process of film and source placement becomes easier
to understand.
In this discussion, we will be dealing with four
terms: Source, receptor, object and angulation. We will also be drawing
analogies between the dental radiographic technique and an everyday example of
casting shadows.
The radiographic source of light (x-rays) is the focal point in the x-ray tube. The
receptor in radiographic technique is the film or the CCD of a digital
radiographic sensor. The everyday source in our analogy is the sun, a lamp, or a slide
projector. The everyday receptor is the floor or the ground, or a wall upon
which a shadow is cast. The objects in dental radiography are oral
structures such as bone and teeth, while in the everyday example, we will be
using our own bodies as the object who's shadow is cast on the receptor.
X-rays should be emitted from the
smallest source of radiation possible. Large sources cause fuzzy images. As electrons strike the focal spot
in the x-ray tube,
X-rays are emitted. The smaller the focal spot the greater the detail.
Manufacturers govern the size of the focal spot, and it cannot be changed by the
operator. However, the focal spot can become enlarged over time due to
continuous use. When focal spot enlargement does occur, the image becomes less
sharp. The focal spot must be monitored through a quality assurance program.
Resolution test devices will determine any change in the focal spot size.
Large sources of
light emit rays from their entire surface. In the
illustration above, a round disk is placed in the path of a
large light source and casts a shadow on the blue wall.
The disk is seen edge on, while the shadow is shown as if you
were looking at the wall directly. A penumbra is
the shadow behind an object lit by an area
light source (in contrast to
a point light source). The penumbra doesn't have sharp
boundaries. This is caused by the fact that each point in the
boundary area is only partially shadowed. The area in full
shadow is the umbra. The
penumbra is drawn inaccurately in the illustration since the a
real penumbra has a gradient starting from the dark umbra in the
center where all the light is blocked and becoming brighter
toward the outer edges. This gives the shadow as a whole a
dark center and fuzzy edges. This is the reason that an
incandescent bulb casts a fairly sharp shadow, and a long tube
fluorescent casts diffuse shadows.
Principle Two:
The x-ray source-to-object-distance should be as
long as possible. The use of a long-position indicating device (a long cone
which is also lined with lead) will enable the
x-ray photons to emerge in a less divergent beam, therefore producing a more
accurate shadow. The term "collimation" describes how divergent a
beam is. Long cones produce better collimated (less divergent) beams.
Longer source-to-object distances reduce magnification and increase image
sharpness. The resulting image will be a more accurate and sharper presentation
of the sizes of the various radiographic structures.
In the illustration above, the beam is more divergent from the
short cone in the top figure than it is in the long cone in the
lower figure. The beam from the short cone casts a larger
illuminated circle on the wall than the beam from the long cone.
Therefore, the rays are more divergent with a short
source-to-object distance than they are with a long
source-to-object distance. This introduces a size
distortion to the radiographic image, causing images to be
enlarged. This characteristic is actually quite useful
when taking a
panoramic radiograph.
In our everyday example, substitute a very bright
light source, say a slide projector for the sun, and lets say that you are standing several
feet from a wall. If the projector is located a long distance from you,
your shadow on the wall will be a fairly accurate representation of your height
and width. On the other hand, if someone moves the projector closer to
you, your shadow is magnified in all directions and and is no longer
representative of your height and width. (Note: In some
machines, especially the newer ones, the external cone may appear short, but the
point source is located in the back of the housing which extends the cone length
internally.)
Principle Three:
The object-to-receptor-distance should be as
short as possible. Placing the object close to the receptor reduces
magnification and increases image sharpness. (This translates to placing the
film or digital CCD as close as possible to the tooth.) The less sharp
edges come from an exaggerated penumbra effect, even for fairly small point
sources.
In an everyday example quite familiar to
most modern people, consider that we are flying in an airplane at 10,000 feet on a
sunny day. The shadow of the airplane on the ground may look quite sharp
to us as we gaze down on it from on high, but to an observer on the ground, the
shadow lacks sharp edges and is actually a great deal larger than the actual size
of the airplane itself.
On the other hand, once the
airplane lands, the shadow cast from the sun when it is directly overhead and
unobstructed is almost the same exact size as the airplane itself, and the
plane’s shadow has edges that are sharp.
Principle Four:
The receptor and long axis of the tooth should be
parallel. When the receptor and the long axis of the tooth are parallel, as in
the paralleling technique, the distortion of the recorded image is decreased.
In our everyday example, a projector casting our
shadow on a perpendicular wall shows a reasonable representation of our shape in
the shadow. On the other hand, if we stand upright on the earth and as the sun sets, our shadow on the ground
gets longer and longer. In addition,
the elongation in the shadow is greater at the feet than at the head.
Finally, if the sun is nearly directly overhead, our shadow will be
extremely foreshortened. This sort of distortion is very important when
taking periapical films, since there often is not enough room in the mouth to
place the film exactly parallel to the teeth.
In the images below, the one on the left shows an extracted tooth lying flat on
the film with the x-ray beam aimed at 90 degrees to both. It shows the
truest representation of the tooth size and shape. In the x-ray on the
right, the film and the beam are in ideal alignment, with the beam at 90 degrees
to the film. However, the crown of the tooth was tilted up and lies at
about 30 degrees to the film and beam. You can see that the tooth in this image
is foreshortened. This image shows what happens in the all too
familiar scenario in which a Rinn apparatus is used to keep the film and beam
properly aligned while the apparatus itself is placed in the mouth at an angle
to the teeth because
there is not enough space in the palate or the floor of the mouth to align it properly. The best
(and easiest) method of compensating for this condition is to use a technique
which splits the angle between the film and the tooth.
In the image below, the tooth
was at the same angle as the image on the right above. The difference here was that the beam was repositioned
so that it split the difference in angle between the film and the tooth
itself. Notice that the filling is slightly foreshortened, and the pulp chamber is
visible in this image. The roots, on the other hand, are elongated compared to the
roots on the image on the right above. These effects are due to the
non parallel nature of the beam. (see the photo of the shadow of the
man with the very long legs above.) This is a consequence of adjusting the angle of
the beam so
we are shooting from a higher angle. These distortions are the
price you pay to more accurately gauge the length of the whole tooth if you
are taking the radiograph for an endodontic trial distance. The actual technique is
discussed below.
Principle Five:
The x-ray beam should be perpendicular to the
receptor. When this principle is not followed, the resultant image
will shift and cause overlapping of the adjacent structures on the film. If the beam is at a lateral
angle to the film while trying to take bitewing x-rays, the crowns of the teeth may
appear to be overlapped thus obscuring the contacts. The Rinn filmholder
has the virtue of keeping the beam perpendicular to the film, but unfortunately,
the film is often not perfectly parallel with the teeth.
This is especially important when taking bitewing
x-rays in which the contacts between the teeth must be clearly visible.
Misangulation of the x-ray beam causes the shadows of the adjacent teeth to appear on the
film to overlap obscuring incipient caries and other anatomical structures.
This principle even applies to a single tooth when multiple structures, such as
the nerve space and a filling may overlap in various ways depending on the
relative angulations of the the source and the tooth.
The radiograph on the left was taken with all three
elements, the film, the teeth, and the beam in optimum allignment. The
film is parallel to the teeth, and the beam is perpendicular to both.
Notice that the contact areas between the teeth are clear and there is no
overlap of the teeth. The radiograph on the right was taken with film
and teeth parallel, but the beam is angled about 20 degrees from the mesial. Notice
the overlap of the contacts between the teeth. This overlap tends to
obscure any caries that may be present. Notice the root caries on
#14 which is apparent in the radiograph on the left, but not in the one on
the right. Finally, notice that the teeth have shifted to the mesial
on the film in the image on the right which was shot from a mesial angle.
This is most easily understood using an everyday example.
Picture a sharp shadow
of your hand with the fingers spread apart. As long as the palm of the
hand is perpendicular to the sun or the slide projector, the shadow on the wall
gives an accurate representation of the hand with fingers spread. Now
imagine slowly twisting your hand so that the palm begins to become parallel to
the light coming from the source. Even though you are keeping your fingers
spread, the shadow shows the spaces between the fingers progressively getting
smaller until the fingers overlap entirely you can no longer discern separate
fingers at all.
Note: Even if the beam and the film are exactly
perpendicular to each other, if the film is not fairly parallel with the
mesial-distal plane of the teeth, the adjacent teeth may still overlap at
the contacts.
In the images above, the hand on the left is in the same
configuration as the hand on the right. The position of the light
source and the wall against which the images were shot has not changed.
They are approximately perpendicular. The only thing that has changed
is the angle of the hand itself. In the image on the right, the hand
is no longer parallel to the wall (receptor). Note how the spaces
between the fingers are disappearing as the fingers seem to overlap on the
shadow. The same thing happens with bitewings when the film is not
parallel with the teeth.
The perfect radiographic technique incorporates
all five principles of shadow casting. Unfortunately, researchers have not found
an ideal technique which meets all the requirements for perfectly accurate shadow casting.
The next page in this course helps you to make use of the distortions
to your own advantage.
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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. |
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