14: The quality of radiographic images and quality assurance

The quality of radiographic images and quality assurance

Introduction

The factors that can affect the quality of radiographic images depends on:

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.

Film-based image quality

As mentioned in Chapter 1, image quality and the amount of detail shown on a radiograph depend on several factors including:

Contrast

Radiographic contrast, i.e. the final visual difference between the various black, white and grey shadows, depends on:

Film contrast

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:

Image sharpness and resolution

Sharpness is defined as the ability of the X-ray film to define an edge. The main causes of loss of edge definition include:

• Geometric unsharpness including the penumbra effect (see above)

• Motion unsharpness, caused by the patient moving during the exposure

• Absorption unsharpness – caused by variation in object shape, e.g. cervical burn-out at the neck of a tooth (see Chs 8 and 17)

• Screen unsharpness, caused by the diffusion and spread of the light emitted from intensifying screens (see Ch. 3)

• 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:

Practical factors influencing film-based image quality

In practical terms, the various factors that can influence overall film-captured image quality can be divided into factors related to:

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).

Dental care professionals need to be able to recognize the cause of the various film faults so that appropriate corrective action can be taken.

Typical film faults

Examples of typical film faults are shown below and summarized later in Table 14.1.

Film too dark (Figs 14.1 and 14.2)

Film marked (Fig. 14.5)

Possible causes

Patient preparation and positioning (radiographic technique) errors (Fig. 14.6)

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.

Intraoral technique errors

These can include:

• Failure to position the image receptor correctly to capture the area of interest

• Failure to position the image receptor correctly causing it to bend (if flexible) creating geometrical distortion

• Failure to orientate the image receptor correctly and using it back to front

• Failure to align the X-ray tubehead correctly in the horizontal plane causing:

• Failure to align the X-ray tubehead correctly in the vertical plane causing:

• Failure to instruct the patient to remain still during the exposure with subsequent movement resulting in blurring

• Failure to set correct exposure settings (image too dark or too pale – see earlier)

• Careless inadvertent use of the image receptor twice.

Jan 12, 2015 | Posted by in Oral and Maxillofacial Radiology | Comments Off on 14: The quality of radiographic images and quality assurance
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