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:
Characteristics of the X-ray beam
The ideal X-ray beam used for imaging should be:
• Sufficiently penetrating to pass through the patient, to a varying degree, and react with the film emulsion to produce good contrast between the various black, white and grey shadows (see earlier)
• Parallel, i.e. non-diverging, to prevent magnification of the image (see Ch. 3)
• Produced from a point source to reduce blurring of the image margins and the penumbra effect (see Ch. 3).
Image sharpness and resolution
• 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 Ch. 20)
• Screen unsharpness, caused by the diffusion and spread of the light emitted from intensifying screens (see Ch. 4)
• 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
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.
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 | |
Reproduced, with modifications, from Dental Update with kind permission of Professor K. Horner and George Warman Publications.
Film too pale (Fig. 17.3)
Possible causes
– Faulty X-ray equipment, e.g. timer
– Incorrect exposure time setting by the operator
– Failure to keep timer switch depressed throughout the exposure
– Inadequate time in the developer solution
– Developer solution too dilute
