As already stated, the radiograph is a two-dimensio- nal representation of a three-dimensional structure. Therefore, to get the greatest amount of information, one must examine one or more two-dimensional ima- ges in order to reconstruct mentally a three-dimen- sional image of the anatomic structures of interest. Obviously, that can be done only if the radiographs have been taken correctly.
One must understand certain principles of projec- tion geometry, which regulate the formation of ima- ges to obtain well-defined radiographs, which are nei- ther enlarged or distorted. It will be seen that the si- ze and position of the focal spot, relative to the object and film, are important in determining the image clari- ty (sharpness and contrast), image magnification, and image distortion.
Image sharpness
“Sharpness” refers to the degree to which the smallest details of an object are reproduced on the radiograph. More specifically, it refers to the degree to which the boundary line between two zones of different densi- ty is well defined.26
Strictly related to this is the resolution, which refers to the extent to which it is possible to distinguish two small nearby objects as separate.
Sharpness and resolution are distinct but interdepen- dent, since both qualities are influenced by the same parameters.
To explain these concepts, a light source, an object, and its shadow will be considered.
Consider the shadow of a human figure that is projected onto a wall when the sun is setting. In this case, the light source is so far as to be considered punctiform; thus the shadow that the figure projects on the wall is well-defined, without blurring. Since the light source is far away from the figure, which in con- trast is extremely close to the wall, the projected sha- dow is the same size as the human figure and thus is neither enlarged nor distorted.
If, on the other hand, one considers the shadow that the same figure projects onto the same wall at the sa- me distance when it is illuminated by a neon light pla- ced only a few meters away, there is a great differen- ce.
First of all, it lacks defined limits and is very blurred, since there is a gradual passage from the black areas
Fig. 5.11. The intensity of an X-ray beam is inversely proportional to the square of the distance (adapted from P.W. Goaz, S.C. White, Oral Radiology, The C.V. Mosby Company, St Louis, 1982).
however small, still represented by a surface, each of its points will emit rays. Thus, the radiographed object will be struck by beams originating from different an- gles. The result will be a “shadow” without defined borders, but surrounded by blurred borders (the so- called “penumbra”) (Fig. 5.12). The shadow zone is that area which does not receive “light” from any of the points of the radiographic source. The zone of the penumbra is that which does not receive “light” from
some parts, but receives it from others. This causes a
loss of image clarity, reducing the sharpness and re- solution.
To reduce these drawbacks as much as possible, three factors must be modified:
1. The size of the focal spot. Just as the sun, which
the penumbra is reduced, with an increase of shar- pness and resolution (Fig. 5.14).
Further, moving the radiographic source away also leads to another advantage, that is to a lesser enlar- gement of the shadow with respect to the object’s actual size, this being due to the fact that the rays that strike the object from far away are more paral- lel to each other.
In practice, the increase of the distance between the focal spot and the object is achieved with the use of a long cone.
3. Distance between object and film. Bringing the film close to the object is equivalent to increasing the distance between the focal spot and the object. The results are exactly the same: an increase of sharp-
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Fig. 5.12. The shadow zone receives no radiation from the points of the radiographic source. The penumbra zone recei- ves radiation from some points, but not from others (adapted from P.W. Goaz, S.C. White, Oral Radiology, The C.V. Mosby Company, St Louis, 1982).
76 Endodontics 5 - Endodontic Radiography 77
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Fig. 5.13. A, B. Reducing the size of the radiographic source, the penumbra zone is also reduced, consequently increa- sing the sharpness and resolution (adapted from P.W. Goaz, S.C. White, Oral Radiology, The C.V. Mosby Company, St Louis, 1982). B ���� �������� �������� ����� A
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Fig. 5.14. A, B. Increasing the distance between the radiographic source and the object, the penumbra zone is reduced, with a consequent increase of sharpness and resolution, and the enlargement of the shadow with respect to the true size of the object is reduced (adapted from P.W. Goaz, S.C. White, Oral Radiology, The C.V. Mosby Company, St Louis, 1982).
B
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78 Endodontics 5 - Endodontic Radiography 79
ness and resolution by diminution of the “penum- bra” zone and lesser enlargement of the shadow with respect to the real size of the object (Fig. 5.15).
Several practical suggestions may be deduced. Obviously, one cannot reduce the size of the target
within the tube to allow one to take advantage of a smaller radiographic source; on the other hand, ma- nufacturers are oriented toward the use of small tar- gets, compatible with their capacity to dissipate heat. Furthermore, the use of X-ray machines with high ki- lovoltage and low milliamperage implies the use of a
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Fig. 5.15. A, B. Reducing the distance between the object and the film, the penumbra zone is again reduced, with a conse- quent increase of sharpness and resolution, and the enlargement of the shadow with respect to the true size of the object is further reduced (adapted from P.W. Goaz, S.C. White, Oral Radiology, The C.V. Mosby Company, St Louis, 1982).
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Image magnification
This reflects uniform enlargement of different parts of the same object, in other words an increase of the si- ze of the image on the film as compared to the actual size of the object. This magnification is due to the di- vergent path of the photons that constitute the X-ray beam and that pass through a cone.
As already mentioned, to reduce this defect, it is ne- cessary to increase the distance between the radio- graphic source and the object and to reduce the di- stance between the object and the film.
Once again, therefore, the use of a long cone is re- commended (which implies the use of more parallel rays placed at the center of the conical X-rays beam), as is positioning of the film as close as possible to the tooth to be radiographed.
a) In the first case, the X-rays are perpendicular to the film, but not to the inclined object (Fig. 5.16). A shortening of the radiographic image will result. This situation can be compared to the shadow that the human body projects onto the ground at noon on a clear Summer day: the shadow is almost entirely beneath the body and thus is significantly shortened. In this case, the rays are parallel to each other and perpendicular to the ground, but the body is inclined with respect to them.
b) In the second case, the X-rays are perpendicular to the object, but not to the film, which is inclined. A lengthening of the radiographic image will result. This situation can be compared to the shadow the body projects onto the ground when it is illumina- ted by the setting sun: the shadow is significantly
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Fig. 5.16. If the X-rays are perpendicular to the object, but not the film, the radiographic image will be elondated. If, instead, the X-rays are perpendicular to the film, but not the object, the radiographic image will be shortened (adapted from P.W. Goaz, S.C. White, Oral Radiology, The C.V. Mosby Company, St Louis, 1982).
80 Endodontics 5 - Endodontic Radiography 81
lengthened, as the ancient Etruscans had already noted (Fig. 5.17).
To eliminate, or in any case minimize, such distor- tions, it is necessary to have parallelism as much as possible between the film and the long axis of the tooth to be radiographed, so that the X-rays are per- pendicular to both. This, obviously, is always possible in the lower arch,
but is quite difficult to achieve in the up- per arch because of the presence of the palate.2