Radiology Course page 15 – Shadow Casting

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Shadow7Shadow casting Principles

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.

Principle One

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.

penumbraLarge 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.  Apenumbra is the shadow behind an object lit by an arealight 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.

collimationIn 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.

elongated_manIn 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.

forshorteningIn 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 film holder 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.

overlap3The radiograph on the left was taken with all three elements, the film, the teeth, and the beam in optimum alignment.  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.


overlap2In 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 bite wings 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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Matteson S, Whaley C, Secrist V: Dental Radiology, 4th Ed. Chapel Hill, The University of North Carolina Press, pp. 78-81, 1988.

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