Optimum viewing conditions

For film-captured images these include:

• An even, uniform, bright light viewing screen (preferably of variable intensity to allow viewing of films of different densities) (see Fig. 19.1)

• A quiet, darkened viewing room

• The area around the radiograph should be masked by a dark surround so that light passes only through the film

• Use of a magnifying glass to allow fine detail to be seen more clearly on intraoral films

• The radiographs should be dry.

These ideal viewing conditions give the observer the best chance of perceiving all the detail contained within the radiographic image. With many simultaneous external stimuli, such as extraneous light and inadequate viewing conditions, the amount of information obtained from the radiograph is reduced. Film-captured radiographs should be viewed once they have dried as films still wet from processing may show some distortion of the image.

Digital images should be viewed on bright, high-resolution monitors in subdued lighting (see Fig. 19.2). Table 19.1 outlines the minimum and ideal specifications of the monitor (image display device) as recommended in the UK by the Health Protection Agency and by the Royal College of Radiologists.

The nature and limitations of different radiographic images

The importance of understanding the nature of different types of radiographic images – film-captured or digital (depending on the type of image receptor used) and their specific limitations was explained in Chapter 1. How the visual images are created by processing – chemical or computer – was explained in Chapter 5. Revision of both of these chapters is recommended. To reiterate, the final image whether captured on film or digitally is ‘a two-dimensional picture of three-dimensional structures superimposed on one another and represented as a variety of black, white and grey shadows’ – a shadowgraph.

Critical assessment of image quality

To be able to assess and interpret any radiographic image correctly, clinicians have to know what that image should look like, how it was captured, and which structures should be shown. It is for this reason that the chapters on radiography included:

With this practical knowledge of radiography, clinicians are in a position to make an overall critical assessment of individual film-captured and digital images.

Film-captured images

The practical factors that can influence film quality were discussed in Chapter 17, and included:

• The X-ray equipment

• The image receptor-film or film/screen combination

• Processing

• The patient

• The operator and radiographic technique.

A critical assessment of radiographs can be made by combining these factors and by asking a series of questions about the final image. These questions relate to:

• Radiographic technique

• Exposure factors and film density

• Processing.

Here are some typical examples.

Radiographic Technique (see Fig. 19.3)

• Which technique has been used?

• How were the patient, film and X-ray tubehead positioned?

• Is this a good example of this particular radiographic projection?

• How much distortion is present?

• Is the image foreshortened or elongated?

• Is there any rotation or asymmetry?

• How good are the image resolution and sharpness?

• Has the film been fogged?

• Which artefactual shadows are present?

• How do these technique variables alter the final radiographic image?

Exposure factors and film density (see Fig. 19.4)

• Is the radiograph correctly exposed for the specific reason it was requested?

• Is it too dark and so possibly overexposed?

• Is it too light/pale and so possibly underexposed?

• How good is the contrast?

• What effect will exposure factor variation have on the zone under investigation?

Processing

• Is the radiograph correctly processed?

• Is it too dark and so possibly overdeveloped?

• Is it too pale and so possibly underdeveloped?

• Is it dirty with emulsion still present and so underfixed?

• Is the film wet or dry?

Detailed knowledge of normal anatomy

A detailed knowledge of the radiographic appearances of normal anatomical structures is necessary if clinicians are to be able to recognize the abnormal appearances of the many diseases that affect the jaws.

Not only is a comprehensive knowledge of hard and soft tissue anatomy required but also a knowledge of:

Only with all this information can clinicians appreciate how the various normal anatomical structures, through which the X-ray beam has passed, will appear on any particular radiograph.

Detailed knowledge of pathological conditions

Radiological interpretation depends on recognition of the typical patterns and appearances of different diseases. The more important appearances are described in Chapters 20–32.

Systematic approach

A systematic approach to viewing radiographs is necessary to ensure that no relevant information is missed. This systematic approach should apply to:

The entire radiograph

Any systematic approach will suffice as long as it is logical, ordered and thorough. Several suggested sequences are described in later chapters. By way of an example, a suggested systematic approach to the overall interpretation of panoramic radiographs (see Ch. 15) is shown in Fig. 19.5.

This type of ordered sequential viewing of radiographs requires discipline on the part of the observer. It is easy to be sidetracked by noticing something unusual or abnormal, thus forgetting the remainder of the radiograph.

Comparison with previous images

The availability of previous images for comparative purposes is an invaluable aid to radiographic interpretation. The presence, extent and features of lesions can be compared to ascertain the speed of development and growth, or the degree of healing.

Note: Care must be taken that views used for comparison have been taken with a comparable technique and are of comparable density.