The radiographic image

Published on 12/06/2015 by admin

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Last modified 12/06/2015

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Chapter 1

The radiographic image

Introduction

The use of X-rays is an integral part of clinical dentistry, with some form of radiographic examination necessary on the majority of patients. As a result, radiographs are often referred to as the clinician’s main diagnostic aid.

The range of knowledge of dental radiography and radiology thus required can be divided conveniently into four main sections:

• 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

Traditionally the image was produced by the X-rays passing through an object (the patient) and interacting with the photographic emulsion on a film, which resulted in blackening of the film. Film is gradually being replaced by a variety of digital sensors with the image being created in a computer. Those parts of the digital sensor that have been hit by X-rays appear black in the computer-generated image. The extent to which the emulsion or the computer-generated image is blackened depends on the number of X-rays reaching the film or the sensor (either device can be referred to as an image receptor), which in turn depends on the density of the object.

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).

Fig. 1.1 A typical dental radiograph. The image shows the various black, grey and white radiographic shadows. The metallic amalgam fillings have totally stopped the X-ray beam so they appear white or *radiopaque*.

Understanding the nature of the shadowgraph and interpreting the information contained within it requires a knowledge of:

• The radiographic shadows

• The three-dimensional anatomical tissues

• The limitations imposed by a two-dimensional picture and superimposition.

The radiographic shadows

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

• The intensity of the X-ray beam used

• The position of the object in relation to the X-ray beam and image receptor

• The sensitivity and type of 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.