There are, however, two things that you CAN do
to produce the best radiograph possible for your purposes:
This page is devoted to helping you to
accomplish the second objective above.
Bisecting the angle:
(When the film can't be placed parallel to the long axis of the teeth)
There is a fairly easily learned
technique in which the operator can overcome the length distortion (forshortening
and elongation) caused by
the near impossibility of keeping all three elements (teeth, film and beam) in an
ideal relationship when taking periapical films. It's called "bisecting the angle", and
once mastered, it can be used to produce the least distorted images on all the
periapical radiographs in a full mouth series.
There is a common misconception that
the bisecting angle technique requires a short cone. Short cones had an
advantage for this technique because their more divergent beams made cone
cutting less of a problem when exposing a film from a distance. Aiming is
a bit more difficult with a long cone, but becomes less of a problem with
practice.
This technique works especially well in
cases in which a low palate, or the floor of the mouth necessitates tilting
a periapical film or sensor medially. While apical parts of the teeth
are slightly foreshortened, the coronal portions are equally elongated
producing an overall image that is quite satisfactory. Once mastered,
it can shorten the time needed to expose the full mouth series.
The technique works especially well when taking periapical films for
endodontic purposes because the overall radiographic length of the tooth
approximates very closely with the actual occlusal-apical length of the
tooth itself.
The Rinn type apparatus may be
used in this procedure, however the x-ray tube is not placed parallel with
the ring.
The ring and alignment arm may be helpful in visualizing the alignment of
the film in the mouth, but in fact, they are not necessary for this
technique, and the full series goes more quickly without them. Instead, the operator uses
the film or sensor holder without the aiming ring aparatus. If you are
using film instead of a digital sensor, is speeds thing up quite a lot to
use a disposable Styrofoam Stabe-type bite block which has the
added advantage of being easily compressible and more likely to apply the
pressures needed to keep the film aligned with the plane of the teeth.
In our everyday analogy, you
are standing upright on a flat, horizontal concrete tarmac. As the sun descends in the sky
from directly overhead, it eventually will reach an angle at which your shadow
on the ground will be exactly as tall as you are. This shadow is not
entirely free from distortion, but this is the least distorted shadow that can be achieved when the
receptor (the ground) and the object (you) are not parallel to one another.
It turns out that this type of image can be produced on an x-ray film by
splitting the difference between the angle of the tooth and the angle of the film. A
simple trick to accomplish this is to use a dual-aiming method. Place the film in the mouth using a Stabe
bite block or the film holder from
a Rinn apparatus without the ring or the metal rod. Position the film as
close to parallel to the long axis of the tooth as is possible.
-
Now, position the
x-ray tube so that it is perpendicular to the film and note the angle of the tube.
Call this position 1.
-
Then reposition the tube so that it is perpendicular
to the tooth itself. call this position 2.
-
Finally, reposition the
tube so that it is at an angle that is exactly between position 1 and position 2. This is the angle which will produce the least distorted shadow of the tooth in
question.
Note that this "triple aiming
technique"
becomes unnecessary as the operator becomes more familiar with bisecting the
angle. Once mastered, this technique is
actually faster and more accurate than using the Rinn since you do not need
to change the apparatus between shots. It always gives the least
distorted shadow possible when the two elements you can't control, the angle
of the film and the angle of the tooth, can be compensated for by the
angle of the beam which you can control.
One place where this technique
becomes essential is with an occlusal film on a child. Occlusal films image the erupted and unerupted
incisors. In this technique, the child is told to bite on the film
like he would bite on a piece of cardboard, flat between his upper and
lower teeth. The film is thus placed in the child's mouth so that it
is almost perpendicular to the long axes of both the upper and lower
incisors.
Aiming the beam perpendicularly
to the film surface would seriously foreshorten the teeth since the teeth are now
nearly
parallel to the beam. Aiming the beam perpendicularly to the teeth
would very seriously elongate them, to the point where the apexes are moved
off the end of the film/sensor. But aiming at an angle that is
midway between perpendicular to the film and perpendicular to the teeth produces a
length corrected shadow of the teeth on the film.
Rinn's XCP-system film holders will
keep the film perpendicular to the x-ray beam which eliminates one source of
distortion, but it cannot eliminate the distortion produced when the film cannot
be placed parallel to the teeth. With practice, developing a technique
utilizing angle splitting produces less distorted intraoral images, and saves
quite a lot of time.
Moving an
image forward, backward, up or down on the film/sensor without repositioning
it
Often, it is nearly impossible
to position a film or sensor far enough posteriorly to get a clear shot of a
maxillary second or third molar due to a patient's gag reflex. It is
also often difficult or nearly impossible to get a periapical of the entire
first premolar due to the curvature of the mandible or the shape of the
palate.
Moving an object up, down, right
or left on a radiograph is a fairly easy trick in radography. It takes
advantage of the fact that the film or sensor is generally at least three or
four millimeters palatal or lingual to the teeth that you want to
move. In fact, the further to the lingual you can move, or tilt the
film or sensor, the further you can move the image of the teeth.
Point the index fingers of both
hands up, close your left eye, and hold the index fingers so that they are
at about 4 inches apart like so:
Now, looking only through one
eye line up the fingers so that one completely covers the other one.
Now shift the hands as a unit to the right so that you are looking at the
fingers from the left side. Notice that the as you look from the left,
the finger closest to your eye seems to shift in the opposite direction to
the right. When you shift the fingers to the left so you are looking
from the right side, the finger closest to your eye seems to shift in the
opposite direction, to the left. The same thing happens when you shift
the hands up or down. When you gaze at the fingers from above, the
finger closest to you seems to move down, while when you gaze at them from
below, the finger closest to you moves up.
This, of course is the effect of
parallax, and we use it to advantage to get that difficult to shoot third
molar, or to move the image on the film so that the root tip is not cut off,
or the crown is entirely on the radiograph. You never have to move the
sensor if you are digital. Just shift the tube head so that the image
shifts in the opposite direction. If you want a third molar to move
mesially, shoot from the distal. If you need to drop the root tip
of a maxillary molar back onto the film if it is cut off the top of the
image, just shoot from a higher angle. Remember that you must re-angle
the tube head toward the film so that the beam is aimed toward it.
The Clark
Shift --Using parallax to determine the buccal-lingual position of an
object in bone.
The Clark Shift is an old trick
used by radiologists to determine whether an impacted tooth, tumor, or other
object is located to the buccal or to the lingual of the roots of the
adjacent teeth, (or to any other other object visible on a radiograph but
not otherwise visible in the mouth). A radiograph is just a shadow,
and a shadow is a two dimensional projection of a three dimensional object
on a screen. When you look at a single x-ray, you see two objects
superimposed over each other. It is impossible to tell from that
single film which of the objects lies to the buccal and which lies to the
lingual or palatal.
On the other hand, if you take
two shots of the same field from two different angles, parallax causes the
buccal object to move distally and the lingual object to move mesially.
This is, in fact, the way that computerized tomographs make three
dimensional reconstructions of large anatomic structures. They
take multiple shots from different angles and using the rules of parallax,
the three dimensional structure of the object is mathematically callculated.
The MBD rule: If you shoot
from the Mesial, a Buccal
object moves Distally
Thus, if you are shooting two
films of an impacted canine, if the canine tooth has shifted distal with
respect to the roots of the lateral and the first premolar on the shot taken
from a mesial angle, then it is located to the buccal of those roots.
The SLOB rule:
Same Lingual,
Opposite Biccal
This is a different way of saying
the same thing as the MBD rule. If an obhect on the film moves in the same
direction as the cone, then it is located on the lingual: (Same, lingual).
If the object moves in the opposite direction, it us located toward the buccal:
(Opposite, Buccal).
This rule is a simple matter of
parallax. When riding in a speeding vehicle and looking out a side window, the
silo on the horizon seems to be almost moving with you in the
same direction as you are going, while the
telephone poles on the side of the road appear to be speeding past you in the
opposite direction.
The two images above were taken from two
different angles. The one on the left was taken straight
on, while the one on the right was taken from a mesial angle.
Notice that the two buccal roots have each moved distally with
respect to the palatal root of the same tooth. When we
shoot from the mesial the buccal objects move distally.
