17: 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 depend 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. 9), bitewings (Ch. 10) and panoramic radiographs (Ch. 15).

The creation of the visual digital image was described in Chapter 7, 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 7. 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 (QA) and suggested quality control measures.

Film-based image quality

As mentioned in Chapter 1, image quality and the amount of detail shown on a radiographic film depends 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. 4). 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:

Resolution is measured in line pairs per mm.

Practical factors influencing film-based image quality

In practical terms, the various factors that can influence overall 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.

Clinicians using film 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 17.1.

Table 17.1

Summary of common film quality problems and their possible causes.

Reason for rejection Possible causes Remedy to each particular fault
General Particular
Film too dark Processing fault (overdevelopment) Developer concentration too high Dilute or change chemicals
    Development time too long Adjust as necessary
    Developer temperature too high Adjust as necessary
  Excessive X-ray exposure Incorrect exposure setting Adjust and repeat examination
    Faulty timer on X-ray set Arrange service and repair of X-ray set
    Thin patient tissues Decrease exposure and repeat
  Fogged film Light leak in darkroom Check and correct
    Faulty safelighting Inspect safelights visually, coin test, and correct any fault detected
    Old film stock Discard film
    Poor film storage Discard film and re-assess storage facilities
    Light leak in cassette Check hinges and catches and repair or replace if required
Film too pale Processing fault (underdevelopment) Overdiluted developer
Inadequate development time
Change chemicals
Adjust as necessary
    Developer temperature too low Adjust as necessary
    Exhausted developer Change chemicals
    Developer contaminated by fixer Change chemicals
  Inadequate X-ray exposure Incorrect exposure setting Adjust and repeat
    Faulty timer on X-ray set Arrange service and repair of X-ray set
    Excessive thickness of patient’s tissues Increase exposure and repeat
Inadequate or low contrast Technique error Film back to front Adjust and repeat
  Processing fault Overdevelopment (plus dark films) Check development and time/temperature relationship
    Underdevelopment (plus pale films) As above
    Developer contaminated by fixer Change chemicals
    Inadequate fixation time (films opaque; milky sheen) Adjust as necessary
    Fixer exhausted (films opaque; milky sheen) Change fixer solution
  Fogged film See above See above
Unsharp image Technique error Patient movement Assess and instruct patient carefully
    Excessive bending of the film packet during exposure Adjust and repeat
    Poor patient positioning (in panoramic radiography) Greater care in positioning and full use of positioning aids
  Cassette error Poor film/screen contact Check cassette and repair or replace if necessary
    Incorrect intensifying screen speed Change screens
  Excessive X-ray exposure Incorrect exposure setting for thin object causing burn-out Decrease exposure setting and repeat
Film marked Handling fault Film packet bent Careful handling
    Careless handling in darkroom As above
  Processing fault Chemical spots Careful chemical handling
    Insufficient chemicals to allow full immersion of film Check chemical tanks and adjust
    Automatic roller marks Clean processor
    Patient biting too hard on the film Instruct patient correctly and repeat
    Dirt on intensifying screens Clean screens regularly
Poor positioning (Chs 9, 10 and 15) Film packet incorrectly positioned Film back to front (plus pale film) Use film holders for intraoral radiography when possible
    Not covering area of interest As above
    Film used twice (plus dark film) Greater care in film handling
  X-ray tubehead incorrectly positioned Too steep an angle producing foreshortening Use beam-aiming devices when possible
    Too shallow an angle producing elongation As above
  Patient incorrectly positioned Patient incorrectly placed (in panoramic unit) Greater care in positioning and full use of positioning aids

image

Reproduced, with modifications, from Dental Update with kind permission of Professor K. Horner and George Warman Publications.

Film too dark (Figs 17.1 and 17.2)

Possible causes

Jan 12, 2015 | Posted by in Oral and Maxillofacial Radiology | Comments Off on 17: The quality of radiographic images and quality assurance
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