Radiology Course page 16 – Shadow Casting

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Having read the page on shadow casting principles you can see why many of your radiographs may not come out the way you expected.  The distortions you see in the finished product are the result of incorrectly alignment of the three components of radiographic technique: The beam (source), the object (the tooth or teeth), and the film/sensor (the receptor).  Almost no intraoral radiograph is free from some degree of distortion.

There are, however, two things that you CAN do to produce the best radiograph possible for your purposes:

  • Align the three factors to reduce the distortion as much as possible
  • Use, and even exaggerate the distortions to your advantage

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 (foreshortening 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.

StabeThe 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 apparatus.  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.

Splitting_the_angleNote 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 radiography.  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:

fingers6Now, 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.

perspectiveThis, 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 calculated.

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 Buccal

This is a different way of saying the same thing as the MBD rule.  If an object 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.

clarkshiftThe 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.

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