The radiographic shadows

The amount the X-ray beam is stopped (attenuated) by an object determines the radiodensity of the shadows:

The final shadow density of any object is thus affected by:

The effect of different materials, different thicknesses/densities, different shapes and different X-ray beam intensities on the radiographic image shadows are shown in Figs 1.2–1.5.

The three-dimensional anatomical tissues

The shape, density and thickness of the patient’s tissues, principally the hard tissues, must also affect the radiographic image. Therefore, when viewing two-dimensional radiographic images, the three-dimensional anatomy responsible for the image must be considered (see Fig. 1.6). A sound anatomical knowledge is obviously a prerequisite for radiological interpretation (see Ch. 19).

The limitations imposed by a two-dimensional image and superimposition

The main limitations of viewing the two-dimensional image of a three-dimensional object are:

Appreciating the overall shape

To visualize all aspects of any three-dimensional object, it must be viewed from several different positions. This can be illustrated by considering an object such as a house, and the minimum information required if an architect is to draw all aspects of the three-dimensional building in two dimensions (see Fig. 1.7). Unfortunately, it is only too easy for the observer to forget that teeth and patients are three-dimensional. To expect one radiograph to provide all the required information about the shape of a tooth or a patient is like asking the architect to describe the whole house from the front view alone.

Superimposition and assessing the location and shape of structures within an object

The shadows cast by different parts of an object (or patient) are superimposed upon one another on the final radiograph. The image therefore provides limited or even misleading information as to where a particular internal structure lies, or to its shape, as shown in Fig. 1.8.

In addition, a dense radiopaque shadow on one side of the head may overlie an area of radiolucency on the other, so obscuring it from view, or a radiolucent shadow may make a superimposed radiopaque shadow appear less opaque.

One clinical solution to these problems is to take two views, at right angles to one another (see Figs 1.9 and 1.10). Unfortunately, even two views may still not be able to provide all the desired information for a diagnosis to be made (see Fig. 1.11).

These limitations of the conventional radiographic image have very important clinical implications and may be the underlying reason for a negative radiographic report. The fact that a particular feature or condition is not visible on one radiograph does not mean that the feature or condition does not exist, merely that it cannot be seen. The recently developed advanced imaging modalities such as cone beam computed tomography (CBCT) and medical computed tomography (CT) have been designed to try to overcome some of these limitations (see Chs 16 and 18).

Positioning of the image receptor, object and X-ray beam

The position of the X-ray beam, object and image receptor needs to satisfy certain basic geometrical requirements. These include:

These ideal requirements are shown diagrammatically in Fig. 1.12. The effects on the final image of varying the position of the object, image receptor or X-ray beam are shown in Fig. 1.13.

X-ray beam characteristics

The ideal X-ray beam used for imaging should be:

• Sufficiently penetrating, to pass through the patient and react with the film emulsion or digital sensor and produce good contrast between the different shadows (Fig. 1.14)

• Parallel, i.e. non-diverging, to prevent magnification of the image

• Produced from a point source, to reduce blurring of the edges of the image, a phenomenon known as the penumbra effect.

These ideal characteristics are discussed further in Chapter 3.

Effect of partial images

As mentioned already, the radiographic image only provides the clinician with a partial image with limited information in the form of different density shadows. To complete the picture, the clinician fills in the gaps, but we do not all necessarily do this in the same way and may arrive at different conclusions. Three non-clinical examples are shown in Fig. 1.15. Clinically, our differing perceptions may lead to different diagnoses.

Effect of contrast

The apparent density of a particular radiographic shadow can be affected considerably by the density of the surrounding shadows. In other words, the contrast between adjacent structures can alter the perceived density of one or both of them (see Fig. 1.16). This is of particular importance in dentistry, where metallic restorations produce densely white radiopaque shadows that can affect the apparent density of the adjacent tooth tissue. This is discussed again in Chapter 20 in relation to caries diagnosis.

Effect of context

The environment or context in which we see an image can affect how we interpret that image. A non-clinical example is shown in Fig. 1.17. In dentistry, the environment that can affect our perception of radiographs is that created by the patient’s description of the complaint. We can imagine that we see certain radiographic changes, because the patient has conditioned our perceptual apparatus.

These various perceptual problems are included simply as a warning that radiographic interpretation is not as straightforward as it may at first appear.