The radiographic image
Introduction
• Basic physics and equipment – the production of X-rays, their properties and interactions which result in the formation of the radiographic image
• Radiation protection – the protection of patients and dental staff from the harmful effects of X-rays
• Radiography – the techniques involved in producing the various radiographic images
• Radiology – the interpretation of these radiographic images.
Understanding the radiographic image is central to the entire subject. This chapter provides an introduction to the nature of this image and to some of the factors that affect its quality and perception.
Nature of the radiographic image
However the final image is captured, it can be described as a two-dimensional picture made up of a variety of black, white and grey superimposed shadows and is thus sometimes referred to as a shadowgraph (see Fig. 1.1).
• The three-dimensional anatomical tissues
• The limitations imposed by a two-dimensional picture and superimposition.
The radiographic shadows
• The white or radiopaque shadows on a film represent the various dense structures within the object which have totally stopped the X-ray beam.
• The black or radiolucent shadows represent areas where the X-ray beam has passed through the object and has not been stopped at all.
• The grey shadows represent areas where the X-ray beam has been stopped to a varying degree.
The final shadow density of any object is thus affected by:
• The specific type of material of which the object is made
• The thickness or density of the material
• The intensity of the X-ray beam used
• The position of the object in relation to the X-ray beam and image receptor
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).