A radiography is a shadow picture of an object that has been placed in the path of an x-ray or gamma-ray beam, between the tube anode and the film or between the source of gamma radiation and the film. It naturally follows, therefore, that the appearance of an image thus recorded is materially influenced by the relative positions of the object and the film and by the direction of the beam. For these reasons, familiarity with the elementary principles of shadow formation is important to those making and interpreting radiographs.
General Principles
Since x-rays and gamma rays obey the common laws of light, their shadow formation may be explained in a simple manner in terms of light. It should be borne in mind that the analogy between light and these radiations is not perfect since all objects are, to a greater or lesser degree, transparent to x-rays and gamma rays and since scattering presents greater problems in radiography than in optics. However, the same geometric laws of shadow formation hold for both light and penetrating radiation.
Suppose, as in Figure 11A below, that there is light from a point L falling on a white card C, and that an opaque object O is interposed between the light source and the card. A shadow of the object will be formed on the surface of the card.
This shadow cast by the object will naturally show some enlargement because the object is not in contact with the card; the degree of enlargement will vary according to the relative distances of the object from the card and from the light source. The law governing the size of the shadow may be stated:
The diameter of the object is to the diameter of the shadow as the distance of the light from the object is to the distance of the light from the card.
Mathematically, the degree of enlargement may be calculated by use of the following equations:
where SO is the size of the object; Si is the size of the shadow (or the radiographic image); D the distance from source of radiation to object; and D the distance from the source of radiation to the recording surface (or radiographic film).
The degree of sharpness of any shadow depends on the size of the source of light and on the position of the object between the light and the card--whether nearer to or farther from one or the other. When the source of light is not a point but a small area, the shadows cast are not perfectly sharp (in Figure 11, B to D) because each point in the source of light casts its own shadow of the object, and each of these overlapping shadows is slightly displaced from the others, producing an ill-defined image.
The form of the shadow may also differ according to the angle that the object makes with the incident light rays. Deviations from the true shape of the object as exhibited in its shadow image are referred to as distortion.
Figure 11, A to F shows the effect of changing the size of the source and of changing the relative positions of source, object, and card. From an examination of these drawings, it will be seen that the following conditions must be fulfilled to produce the sharpest, truest shadow of the object:
1. The source of light should be small, that is, as nearly a point as can be obtained.
Compare Figure 11, A and C.
2. The source of light should be as far from the object as practical. Compare Figure 11, B and C.
3. The recording surface should be as close to the object as possible. Compare Figure 11, B and D.
4. The light rays should be directed perpendicularly to the recording surface. See Figure 11, A and E.
5. The plane of the object and the plane of the recording surface should be parallel.
Compare Figure 11, A and F.
Figure 11: Illustrating the general geometric principles of shadow formation as explained in these sections.
Radiographic Shadows
The basic principles of shadow formation must be given primary consideration in order to assure satisfactory sharpness in the radiographic image and essential freedom from distortion. A certain degree of distortion naturally will exist in every radiograph because some parts will always be farther from the film than others, the greatest magnification being evident in the images of those parts at the greatest distance from the recording surface (See Figure 11).
Note, also, that there is no distortion of shape in Figure 11E above--a circular object having been rendered as a circular shadow. However, under circumstances similar to those shown, it is possible that spatial relations can be distorted. In Figure 12 the two circular objects can be rendered either as two circles (See Figure 12A) or as a figure-eight-shaped shadow (See Figure 12B). It should be observed that both lobes of the figure eight have circular outlines.
Figure 12: Two circular objects can be rendered as two separate circles (A) or as two overlapping circles (B), depending on the direction of the radiation.
Distortion cannot be eliminated entirely, but by the use of an appropriate source-film distance, it can be lessened to a point where it will not be objectionable in the radiographic image.