The radiographic image
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.
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.
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.
Fig. 1.2 (i) Front view and (ii) plan view of various cylinders of similar shape but made of different materials: A plaster of Paris, B hollow plastic, C metal, D wood. (iii) Radiographs of the cylinders show how objects of the same shape, but of different materials, produce different radiographic images.
Fig. 1.3 (i) Front view of four apparently similar cylinders made from plaster of Paris. (ii) Plan view shows the cylinders have varying internal designs and thicknesses. (iii) Radiographs of the apparently similar cylinders show how objects of similar shape and material, but of different densities, produce different radiographic images.
Fig. 1.4 (i) Front view of five apparently similar cylinders made from plaster of Paris. (ii) Plan view shows the objects are in fact different shapes. (iii) Radiographs show how objects of different shape, but made of the same material, produce different radiographic images.
Fig. 1.5 (i) Front view and (ii) plan view of four cylinders made from plaster of Paris but of different diameters. (iii) Four radiographs using different intensity X-ray beams show how increasing the intensity of the X-ray beam causes greater penetration of the object with less attenuation, hence the less radiopaque (white) shadows of the object that are produced, particularly of the smallest cylinder.
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. 16).