LECTURE 4: Quality Assurance


Quality Assurance is defined as the planned and organised actions necessary to provide adequate confidence that dental X-ray equipment will produce quality radiograms reliably with minimal doses to patients and staff. A Quality Assurance program includes quality control procedures for the monitoring and testing of dental X-ray equipment and related components, and administrative methodology to ensure that monitoring, evaluation and corrective actions are properly performed. A Quality Assurance program will include all practices established by the owner or dentist to ensure that:

  1. every radiographic examination is necessary and appropriate, based on prior clinical evaluation;
  2. the radiograms produced contain sufficient information to aid proper clinical assessment;
  3. the radiograms produced are correctly interpreted; and
  4. the radiographic examination is performed with the lowest possible radiation dose to the patient, consistent with clinical diagnostic requirements.

Goals of Quality Assurance Program

The principal goal of a Quality Assurance program is to provide accurate and timely diagnosis. The secondary goal is to minimize radiation exposure to the patient while the principal goal is achieved. Two aspects will affect the operation of a Quality Assurance program; one dealing with the equipment and the other with its operation. It is essential that the equipment be in proper working condition if any Quality Assurance program is to achieve its goals. It is necessary that all staff members participate fully in the implementation and operation of the Quality Assurance program. All staff members must understand the goals of the program and must be committed to the concept. When equipment functions properly, the majority of problems can be traced to human errors such as improper selection of loading factors and poor patient positioning.

Any program initiated only to comply with regulatory requirements is not likely to provide maximum possible benefit to the patient. It is, therefore, essential that all dental staff understand, support and participate in the operation of the Quality Assurance program. Some provincial/territorial jurisdictions require facilities to implement and participate in a Quality Assurance program.

To provide accurate and timely diagnosis while minimizing radiation exposure to the patient, the radiogram must contain all critical information necessary for accurate interpretation. If critical elements are missing on the radiogram, the film is considered of poor quality. The results of poor quality radiograms are the potential for incorrect interpretation, and the possibility of a repeat radiographic procedure resulting in unnecessary radiation exposure to the patient, and increased cost for films, chemicals and time.

In addition to improved diagnostic quality of radiograms, some of the benefits associated to the Quality Assurance program are as follows.

  1. Film and processing chemicals - The reduction in the number of radiographic examinations resulting from a decrease in repeated films. More appropriate radiographic examination prescriptions will save both films and film processing chemicals.
  2. Equipment - The reduction in the number of radiographic examinations will lead to a reduction in workload, which in turn will put less stress on dental X-ray equipment and film processors. This will make critical components such as the X-ray tube last longer and require less frequent servicing. Furthermore, any problems with equipment will be diagnosed earlier before more serious and costly problems occur.
  3. Patient flow - The reduction in the number of repeated films, and better diagnostic quality of the radiograms will allow better and more efficient use of time for the dental team. This will result in better predictability of scheduling and possibly greater patient flow.
  4. Stress - The reduction in the number of repeated films will reduced stress levels for staff by knowing that film quality will be consistent, and that they are less likely to fall behind schedule.

Establishment of Quality Control Procedures

The three following steps are needed for establishing quality control procedures.

  1. Equipment operation - Ensure that the dental X-ray equipment and film processing equipment function properly. This means the replacement, repair, upgrading or calibration of the equipment, if necessary.
  2. Baseline performance - Establish baseline performance values such as tube voltage, and timer accuracy, for each X-ray unit. This baseline performance will be used to detect any changes in equipment performance.
  3. Reference test image - Obtain a reference test image using the dental X-ray equipment and an attenuation step wedge. This image will be used for comparison of daily test films. A reference film using an anthropomorphic phantom is also recommended.

It is important to keep records of equipment operation data and baseline performance measurement. These records will be needed to diagnose any changes in film quality.

Many dental X-ray equipment supply companies distribute Quality Assurance kits which include an attenuation step wedge, a thermometer, record keeping forms, etc. Such kits may be useful in setting up Quality Assurance programs.

Radiographic Imaging Quality Control

Radiograms are the final products of dental radiographic procedures. Proper processing of radiographic films is essential to achieve the goals of the Quality Assurance program. The following are general guidelines on the conditions of storage areas, darkroom, and film processing.

  1. Film and chemical storage -- Since radiographic films are sensitive to light, heat, humidity, chemical contamination, mechanical stress and X-radiation, they should be stored at temperatures in the range of 10°C to 21°C with humidity between30%to 60%. It is best to follow the film manufacturer's instruction. Film storage areas should be free of chemical fumes and X-radiation. Shielding of storage areas should follow guidelines set in section 6.3. Processing chemicals should be protected from freezing. Manufacturer's recommendations should be followed in storing chemicals to avoid oxidization and any chemicals showing sign of oxidization or sedimentation must not be used.
  2. Darkroom conditions - The darkroom must be clean of dirt, dust, and spilled chemical residues. The darkroom must be light-tight and that proper darkroom lighting used. Guidelines set in section 6.2 should be followed.
  3. Manual processing - Film manufacturers' recommendations regarding film chemicals, processing temperature and processing time must be adhered to during manual processing.Athermometer and timer must always be used. Guidelines set in section 6.1 should be followed. A schedule for periodic replenishment of film chemicals based on the workload and on the type of film used should be prepared.
  4. Automated processing - One of the best methods to monitor the operation of an automated dental film processor is to process a film exposed with an attenuation step wedge and compare the processed film with a reference test image. Temperature and levels of chemicals should be monitored regularly. A schedule for periodic replenishment of film chemicals based on the workload and on the type of film used should be prepared.

Procedures during implementation

During the implementation phase of the Quality Assurance program, the establishment of baseline information and equipment evaluation is necessary. The more technical evaluation should be performed by an organization or individual specializing in this type of evaluation.

  1. Performance of X-ray equipment

    1. Calibration of equipment
    2. Stability of equipment
    3. Proper radiation beam alignment
    4. Mechanical and electrical performance
    5. Inspection and replacement of worn or broken components
    6. Manufacturer's preventive maintenance schedule

  2. Handling of image receptor

    1. Proper handling of films, cassettes, screens, and chemicals

  3. Establishment of charts

    1. Loading factors charts
    2. X-ray exposure values charts
    3. Time-temperature development charts

  4. Optimization of processing equipment

    1. Conditions of tanks
    2. Condition of processing equipment
    3. Posting of maintenance schedule

  5. Darkroom

    1. Light tightness
    2. Adequate safelighting
    3. Cleanliness
    4. Adequate temperature control of water supply

  6. Condition of protective devices

    1. Protective clothing
    2. Protective thyroid shield
    3. Protective barriers
    4. Film holders

Procedures During Operation

The following list presents the tests to be done during the operation of a Quality Assurance program.

  1. Test films and film processing - Test films are needed to monitor the performance of the dental X-ray system and film processing. Generally, different types of test films are needed, such as ones using a step wedge, a step wedge is irradiated using loading factors comparable to a radiographic dental procedure (i.e., bitewing exam). The produced film is processed and compared with the reference film. The film is then evaluated visually or with a densitometer. A reference film is used to evaluate any changes in film density, contrast, resolution, or other features in image quality.
  2. Retake record - A record of every retake should be made, including the reason for the retake along with and any corrective actions. Any trends or repeated errors should be identified and corrected.
  3. Darkroom operation - Light tightness and correct safelighting should be assessed on a regular basis. 
  4. A daily inspection of film processing solutions levels, and the temp of the developer taken.
  5. 6. Dental X-ray equipment - Dental X-ray equipment should be checked on a regular basis and after servicing. The testing of dental X-ray equipment should be done by qualified individuals.



Performance Criteria

Minimum Frequency

Test film and film processing

± 1 step (stepwedge) < ± 0.1 optical density


Test radiogram



Retake record



Operation of darkroom



Cassettes and screens




There must be radiation-absorbing filters that provide a degree of attenuation such that the first half-value layer of aluminum is not less than the value shown in Table 1 for a selected X-ray tube voltage. For other X-ray tube voltages, the half-value layer of the radiation beam must not be less than the value obtained by linear interpolation from that table.

Annually and after equipment service

Controlling timer

An electronic timing device must be provided to automatically terminate the irradiation. Mechanical timers must not be used. The timer must be designed and constructed in such a way that
  1. it is not possible to energize the X-ray tube without automatic or manual resetting of the timer after each loading;
  2. irradiation cannot be started with the timer set at its zero or OFF position; and
  3. the production of X-rays is automatically terminated after a preset time, preset milliampere-second value, a preset exposure or air kerma value.

Annually and after equipment service

X-ray tube shielding

The X-ray tube must be enclosed within a shielded housing. The housing must be constructed so that the leakage radiation, measured at a distance of one metre in any direction from the focal spot of the X-ray tube, does not exceed 0.87 mGy (100 mR) in one hour for any specified rating of the tube.

Annually and after X-ray tube housing service

X-ray tube voltage

The actual peak X-ray tube voltage should not deviate from the indicated or selected value by more than 7%, or by the value specified by the manufacturer. It must not be possible to set or operate the X-ray tube with the tube voltage below 50 kilovolts (peak).

Annually and after equipment service

Irradiation switch

There must be an irradiation switch to start and terminate X-ray production. This switch must be of a type that requires continuous pressure by the operator to produce X-rays. Where the irradiation switch is a foot switch it must be so constructed that operation of the X-ray tube cannot occur inadvertently should the foot switch be overturned. Where the irradiation switch is mounted at the end of a cable, the cable must be of sufficient length to enable the operator to stand at least 3 metres from the tube housing and the patient. If the switch is in a fixed location, it must be at least 3 metres from the tube housing.

Annually and after equipment service

Focal spot to skin distance

A position-indicating device must be provided to limit the minimum focal spot to skin distance to not less than 18 cm. The applicator must be an open-ended type. Pointed cone or close-ended applicators must not be used

Annually and after equipment service

Beam alignment and collimation

The primary radiation beam must be collimated in size at the end of the applicator to a circle not more than 7 cm in diameter

Annually and after equipment service