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:

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.

Film too dark (Figs 17.1 and 17.2)

Possible causes

• Overexposure owing to:

– Faulty X-ray equipment, e.g. timer

– Incorrect exposure time selected

• Overdevelopment owing to:

– Excessive time in the developer solution

– Developer solution too hot

– Developer solution too concentrated

• Fogging owing to:

– Poor storage conditions:

 Allowing exposure to stray radiation

 Too warm

– Old film stock used after expiry date

– Faulty cassettes allowing ingress of light

– Faulty darkroom/processing unit:

 Allowing leakage of stray light

 Faulty safe-light

• Thin patient tissues.

Film too pale (Fig. 17.3)

Possible causes

• Underexposure owing to:

– Faulty X-ray equipment, e.g. timer

– Incorrect exposure time setting by the operator

– Failure to keep timer switch depressed throughout the exposure

• Underdevelopment owing to:

– Inadequate time in the developer solution

– Developer solution too cold

– Developer solution too dilute

– Developer solution exhausted

– Developer contaminated by fixer

• Excessive thickness of patient’s tissues

• Film packet back to front (film also marked).

Image unsharp and blurred (Fig. 17.4)

Possible causes

• Movement of the patient during the exposure (see also Chs 9, 10 and 15)

• Excessive bending of the film packet during the exposure (see also Ch. 9)

• Poor film/screen contact within a cassette

• Film type – image definition is poorer with indirect-action film than with direct-action film

• Speed of intensifying screens – fast screens result in loss of detail

• Overexposure – causing burn-out of the edges of a thin object

• Poor positioning in panoramic radiography (see Ch. 15).

Film marked (Fig. 17.5)

Possible causes

• Film packet bent by the operator

• Careless handling of the film in the darkroom resulting in marks caused by:

– Finger prints

– Finger nails

– Bending

– Static discharge

• Processing errors owing to:

– Chemical spots

– Under fixation – residual silver halide emulsion remaining

– Roller marks

– Protective black paper becoming stuck to the film

– Insufficient chemicals to immerse films fully

• Patient biting too hard on the film packet

• Dirty intensifying screens in cassettes.

Patient preparation and positioning (radiographic technique) errors (Fig. 17.6)

These errors can happen whatever image receptor is being used and were described in detail and illustrated in Chapters 9, 10 and 15. They are summarized below and can be divided into intraoral and panoramic technique errors.

Quality assurance in dental radiology

The World Health Organization (WHO) has defined radiographic quality assurance (QA) programmes as ‘an organized effort by the staff operating a facility to ensure that the diagnostic images produced by the facility are of sufficiently high quality so that they consistently provide adequate diagnostic information at the lowest possible cost and with the least possible exposure of the patient to radiation’.

Quality control measures are therefore as essential in a general dental practice facility as they are in a specialized radiography department. This importance of quality is acknowledged in the UK in the Ionising Radiations Regulations 1999 which make quality assurance in dental radiography a mandatory requirement. This chapter is based broadly on the recommendations in the UK Guidance Notes for Dental Practitioners on the Safe Use of X-ray Equipment published in 2001.

Quality control procedures for film-based radiography

The essential quality control procedures relate to:

• Image quality and film reject analysis

• Patient dose and X-ray equipment

• Darkroom, image receptors and processing

• Working procedures

• Staff training and updating

• Audits.

Image quality and film reject analysis

Image quality assessment is an important test of the entire QA programme. Hence the need for clinicians to be aware of all the various factors, outlined earlier, that affect image quality and to monitor it on a regular basis. This assessment should include:

• A day-to-day comparison of the quality of every radiograph to a high standard reference film positioned permanently on the viewing screen and an investigation of any significant deterioration in quality.

• A formal analysis of film quality, either retrospective or prospective, approximately every six months. The 2001 Guidance Notes recommended the simple three-point subjective rating scale shown in Table 17.2, and shown previously in Chapters 9, 10 and 15, be used for film-based intraoral and extraoral radiography.

Table 17.2

Subjective quality rating criteria for film-captured images published in 2001 Guidance Notes for Dental Practitioners on the Safe Use of X-ray Equipment

Rating	Quality	Basis
1	Excellent	No errors of patient preparation, exposure, positioning, processing or film handling
2	Diagnostically acceptable	Some errors of patient preparation, exposure, positioning, processing or film handling, but which do not detract from the diagnostic utility of the radiograph
3	Unacceptable	Errors of patient preparation, exposure, positioning, processing or film handling, which render the radiograph diagnostically unacceptable

• Based on these quality ratings, performance targets can be set. Suitable targets recommended in the Guidance Notes are shown in Table 17.3 with the advice that practices should aim to achieve these targets within three years of implementing the QA programme. The ‘interim targets’ should be regarded as the minimum achievable standard in the shorter term.

Table 17.3

Minimum and interim targets for radiographic quality from the 2001 Guidance Notes*

Rating	Target	Interim target
1	Not less than 70%	Not less than 50%
2	Not greater than 20%	Not greater than 40%
3	Not greater than 10%	Not greater than 10%

^*Percentage of radiographs taken.

• Analysis of all unacceptable films given a rating of 3, sometimes referred to as film reject analysis (see below).

Film reject analysis

This is a simple method of identifying all film faults and sources of error and amounts to a register of reject radiographs. To do this, it is necessary to collect all rejected (grade 3) radiographs and record:

• Date

• Nature of the film fault/error, as shown earlier, e.g.:

– Film too dark

– Film too pale

– Low or poor contrast

– Unsharp image

– Poor positioning

• Known or suspected cause of the error or fault and corrective action taken (see Table 17.1)

• Number of repeat radiographs (if taken)

• Total number of radiographs taken during the same time period. This allows the percentage of faulty films to be calculated.

Regular review of film reject analysis records is an invaluable aid for identifying a range of problems, including a need for equipment maintenance, additional staff training as well as processing faults that could otherwise cause unnecessary radiation exposure of patients and staff.

Patient dose and X-ray equipment

One of the aims of quality assurance stated earlier is to ensure that radiation doses are kept as low as reasonably practicable. It is therefore necessary to measure patient doses on a regular basis and, if available, compare them against national diagnostic reference levels (DRLs). The latest DRLs published in the UK by the Health Protection Agency (HPA) in 2012 are 1.7 mGy for an adult lower molar periapical and 93 mGy cm² for an adult panoramic examination. To try to ensure compliance, typical equipment QA measures include:

• An initial critical examination and report – carried out by the installer

• An acceptance test – carried out by a radiation protection adviser/medical physicist before equipment is brought into clinical use, which should include measurement of patient dose

• A re-examination report following any relocation, repair or modification of equipment that may have radiation protection implications

• Regular checks of important features that could affect radiation protection including:

– Correct functioning of warning lights and audible alarms

– Correct operation of safety devices

– Satisfactory performance of the counterbalance for maintaining the correct position of the tubehead

– Correct alignment of the beam collimation

– X-ray equipment output. This can be performed in dental practice using one of the recently developed devices shown in Fig. 17.7.

• Checks on panoramic equipment regarding reproducibility, uniformity, beam alignment and synchronization of the exposure with tube motion. For example, the following simple test could be carried out in dental practice, based broadly on the IPEM Report 91(2005)

1. On installation of the equipment a baseline image is created by placing a 1 mm thick copper strip over the X-ray port (to filter the beam), and exposing a cassette containing a film using a standard adult panoramic exposure

2. The test is repeated every 1–3 months and the subsequent images compared to the baseline image and assessed for reproducibility, uniformity beam alignment and synchronization:

• Reproducibility and uniformity – the image should be of similar density to the baseline image and there should be no areas of non-uniformity. There will always be a vertical dark central band present on a panoramic image, due to higher exposure factors being used at this point in the panoramic cycle for extra penetration of the cervical spine. Any other vertical banding may indicate a fault in the exposure or the rotation system, as shown in Fig. 17.8.

• Beam alignment and synchronization – the radiation field area should lie completely within the edges of the image receptor.

• Written records and an equipment log should be maintained and include:

– All installer’s formal written reports describing the checks made, the results obtained and action taken

– Results of all equipment checks in chronological order

– Details of all routine or special maintenance

• An up-to-date inventory of each item of X-ray equipment should be maintained, and available, at each practice and contains:

– Name of the manufacturer

– Model number

– Serial number or other unique identifier

– Year of manufacture

– Year of installation

• Compliance with any national medical physics recommendations, for example in the UK the Recommended Standards for the Routine Performance Testing of Diagnostic X-ray Imaging Systems, Report 91 of the Institute of Physics and Engineering in Medicine (IPEM), published in 2005.

Darkroom, image receptors and processing

Darkroom

The QA programme should include instructions on all the regular checks that should be made, and how frequently, with all results recorded in a log. Important areas include:

• General cleanliness (daily), but particularly of work surfaces and film hangers (if used)

• Light-tightness (yearly), by standing in the darkroom in total darkness with the door closed and safelights switched off and visually inspecting for light leakage

• Safelights (yearly), to ensure that these do not cause fogging of films. Checks are required on:

– Type of filter – this should be compatible with the colour sensitivity of film used, i.e. blue, green or ultraviolet (see Ch. 6)

– Condition of filters – scratched filters should be replaced

– Wattage of the bulb – ideally it should be no more than 25W

– Their distance from the work surface – ideally they should be at least 1.2 m (4 ft) away

– Overall safety (i.e. their fogging effect on film) – the simple quality control measure for doing this is known as the coin test:

1. Expose a piece of screen film in a cassette to a very small even exposure of X-rays (so-called flash exposure) to make the emulsion ultra-sensitive to subsequent light exposure

2. In the darkroom, remove the film from the cassette and place on the worksurface underneath the turned-off safelight

3. Place a series of coins (e.g. nine) in a row on the film and cover them all with a piece of card

4. Turn on the safelight and then slide the card to reveal the first coin and leave for approximately 30 seconds

5. Slide the card along to reveal the second coin and leave again for approximately 30 seconds

6. Repeat until all the coins have been revealed

7. Process the film in the normal way.

An example of the coin test is shown in Fig. 17.9. Fogging (blackening) of the film owing to the safelight will then be obvious when compared to the clear area protected by the coin. The part of the film adjacent to the first coin will have been exposed to the safelight for the longest time and will be the darkest. In practice, the normal film-handling time under the safelight can be measured and the effect of safelight fogging established.

Note. The coin test can also be used to assess the amount of light transmission through the safety glass of automatic processors by performing the test within the processor under the safety glass under normal daylight loading conditions.

Image receptors

The QA programme requires written information, usually obtained from the suppliers, on film speed, expiry date and storage conditions as well as details regarding the maintenance and cleaning instructions of cassettes and intensifying screens. Typical requirements could include:

X-ray film

This requires:

• Ideal storage conditions – cool, dry and away from all sources of ionizing radiation – as recommended by the manufacturers

• Strict stock control with records to ensure usage before the expiry date

• Careful handling.

Cassettes

These require:

• Regular cleaning of intensifying screens with a proprietary cleaner

• Regular checks for light-tightness, as follows:

1. Load a cassette with an unexposed film and place the cassette on a window sill in the daylight for a few minutes

2. Process the film – any ingress of light will have fogged (darkened) the film (see Fig. 17.10A)

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• Regular checks for film/screen contact, as follows:

1. Load a cassette with an unexposed film and a similar-sized piece of graph paper

2. Expose the cassette to X-rays using a very short exposure time

3. Process the film – any areas of poor film/screen contact will be demonstrated by loss of definition of the image of the graph paper (see Fig. 17.10B)

• A simple method of identification of films taken in similar-looking cassettes, e.g. a Letraset letter on one screen.

Processing

The QA programme should contain written instructions about each of the following:

Chemical solutions

These should be:

• Always made up to the manufacturers’ instructions taking special precautions to avoid even trace amounts of contamination of the developer by the fixer, e.g. always fill the fixer tank first so that any splashes into the developer tank can be washed away before pouring in the developer

• Always at the correct temperature

• Changed or replenished regularly – ideally every 2 weeks – and records should be kept to control and validate these changes

• Monitored for deterioration. This can be done easily using radiographs of a step-wedge phantom:

1. Make a simple step-wedge phantom using the lead foil from inside intraoral film packets, as shown in Fig. 17.11).

2. Radiograph the step-wedge using known exposure factors

3. Process the film in fresh solutions to produce a standard reference film

4. Repeat, using the same exposure factors, every day as the solutions become exhausted

5. Compare each day’s film with the standard reference film to determine objectively any decrease in blackening of the processed film, which would indicate deterioration of the developer (see Fig. 17.12)

6. Record the results.

Processing equipment

• Manual processing requires the use of accurate timers, thermometers and immersion heaters. Instructions on their proper use should be provided.

• Automatic processors require regular replenishment of chemical solutions and regular cleaning, especially of the rollers. All cleaning procedures should be written down, including how often they should be carried out.

• A record log confirming that all cleaning procedures have been carried out should be kept.

Working procedures

These include:

• Local rules – required in the UK under the Ionising Radiations Regulations 1999. These rules should contain the procedural and operational elements that are essential to the safe use of X-ray equipment, including guidance on exposure times, and as such should contain much of what is relevant to the maintenance of good standards in QA.

• Employers’ written procedures – required in the UK under the Ionising Radiation (Medical Exposure) Regulations 2000.

• Operational procedures or systems of work – these include written procedures that provide for all actions that indirectly affect radiation safety and diagnostic quality, e.g. instructions for the correct preparation and subsequent use of processing chemicals (as explained earlier).

• Procedures log – the QA programme should include the maintenance of a procedures log to record the existence of appropriate Local Rules and Employers’ Written Procedures, together with a record of each occasion on which they are reviewed or modified (ideally every 12 months).

Staff training and updating

It is a legal requirement in the UK and in many other countries that all practitioners and operators are adequately trained and that continuing professional development (CPD) is undertaken. The QA programme should incorporate a register of all staff involved with any aspect of radiography and should include the following information:

• Name

• Responsibility

• Date, nature and details of training received

• Recommended date for a review of training needs.

Audits

Each procedure within the QA programme will include a requirement for written records to be made by the responsible person at varying intervals. In addition, the person with overall responsibility for the QA programme should check the full programme at intervals not exceeding 12 months. This is an essential feature of demonstrating effective implementation of the programme. Clinical audits may include:

• The QA programme and associated records

• The justification and authorization of radiographs

• The appropriateness of requests/investigations

• The clinical evaluation of radiographs.

Digital image quality

As with film-based image quality described earlier, digital image quality and the amount of detail shown on a digital radiographic image depends on several factors, including:

Practical factors influencing digital image quality

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

As a result of all these variables, digital image faults and alterations in digital image quality are inevitable and not all these faults can be corrected by manipulating the image using image enhancement software, as described in Chapter 5. QA programmes are still required.

Clinicians using digital imaging need to be able to recognize the cause of the various image faults so that appropriate corrective action can be taken.

Typical digital image faults

Examples of some typical digital image faults, other than those as a result of poor radiographic technique described in Chapters 9, 10 and 15, are shown below.

Solid-state detectors

Faults include:

• Blooming – caused by overexposure and overloading/flooding the CCDs

• Cracked sensor

• Sensor used back to front

• Loss of uniformity.

(See Figs 17.13–17.15.)

Photostimulable phosphor plates

Faults include:

• Phosphor plate used back-to-front

• Light leakage/fogging

• Debris on the plate

• Scratched plate

• Loss of uniformity.

See Figs 17.16–17.19.

Computer software malfunction

As with any computer software, occasional glitches and malfunctions happen. When they do, the digital image can be completely distorted, as shown in Fig. 17.20.

Quality control procedures for digital radiography

The overall quality control procedures for digital radiography are similar, and in some instances identical, to those required for film-based radiography. They relate to:

Those QA procedures specifically relevant to digital imaging, including digital image quality assessment and digital equipment, are described below.

Digital equipment

Solid-state sensors

These require:

• Regular checks to ensure no evidence of cracks or damage to the cable and sensor casing (see Fig. 17.21)

• Regular assessment for non-uniformity of receptor.

Phosphor plates

These require:

• Regular checks for visible scratches and dirt (see Figs 17.22)

• Being passed daily through the reader to detect scratches (see Fig. 17.22)

• Regular cleaning following the manufacturer’s instructions

• Regular assessment for non-uniformity of receptor (see Fig. 17.22).

Monitors

These require

• Regular cleaning

• Regular QA calibration/checks for distortion, grey scale reproduction, limiting resolution (at both high and low contrast) and uniformity. This can be done using specific test patterns designed for this purpose such as the Technical Group 18 QC (TG18-QC) test pattern produced by the American Association of Physicists in Medicine, as shown in Fig. 17.23A or the SMPTE test pattern designed by the Society of Motion Pictures and Television Engineers, as shown in Fig. 17.23B. Alternatively, specifically designed QA calibration tools and software can be used, as shown in Fig. 17.23C.

Footnote

The requirement for quality assurance and quality control measures, for both film-based and digital imaging, in general dental practice applies equally to specialized hospital radiography departments. However, in view of the cost implications, the expensive, sophisticated equipment, available for precise quality assurance measurements and monitoring used in hospital X-ray departments, is often inappropriate to general dental practice. The practical suggestions in this chapter are designed to satisfy the WHO definition of QA by bringing an element of objectivity to quality assurance in practice, but at the same time being simple, easily done and inexpensive.

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