The quality of radiographic images and quality assurance
The effects of poor radiographic technique are the same whatever type of image receptor is used. These technique errors have already been covered in detail in relation to the three main projections used in dentistry, namely: periapicals (Ch. 7), bitewings (Ch. 8) and panoramic radiographs (Ch. 12).
The creation of the visual digital image was described in Chapter 3, together with how computer software can be used to alter and manipulate the image with regards to contrast, brightness (degree of blackening), magnification, inversion, enhancement and pseudocolourization.
Creation of the black/white/grey image on film using chemical processing was also described in Chapter 3. These various images can, however, be affected by many other factors. This chapter therefore is designed for revision, bringing together and summarizing from earlier chapters all these various factors. It also includes a quick reference section as an aid to fault-finding of film-captured images. Various image faults are illustrated together with their possible causes. This is followed by a section on quality assurance and suggested quality control measures.
This is an inherent property of the film itself (see Ch. 3). It determines how the film will respond to the different exposures it receives after the X-ray beam has passed through the patient. Film contrast depends upon four factors:
As mentioned and illustrated in Chapter 1, the geometric accuracy of an image depends upon the position of the X-ray beam, object and image receptor (film or digital) satisfying certain basic geometrical requirements:
• Poor resolution. Resolution, or resolving power of the film, is a measure of the film’s ability to differentiate between different structures and record separate images of small objects placed very close together, and is determined mainly by characteristics of the film including:
As a result of all these variables, film faults and alterations in image quality are inevitable. However, since the diagnostic yield from radiography is related directly to the quality of the image, regular checks and monitoring of these variables are essential to achieve and maintain good quality radiographs. It is these checks which form the basis of quality assurance (QA) programmes (see later).
Fig. 14.5 Examples of marked films.
These errors can happen whatever image receptor is being used and were described in detail and illustrated in Chapters 7, 8 and 12. They are summarized below and can be divided into intraoral and panoramic technique errors.
Fig. 14.6 Two examples of positioning (radiographic technique) errors. A Intraoral – coning off or cone cutting – X-ray tubehead incorrectly positioned, anterior part of image receptor not exposed. B Panoramic – patient too far away from the image receptor – incisor teeth magnified (anteroposterior error) and patient rotated to the LEFT, left molars narrowed, right molars widened (horizontal error).