CBCT Quality Assurance Plan

What is Quality Assurance in radiology?
Quality Assurance (QA) is a plan of action to ensure that a diagnostic x-ray facility will produce consistent, high-quality images with a minimum of exposure to patients and personnel.

Why do we need quality assurance?
The QA process is essential in order to provide confidence in the suitability of an imaging technique for its intended purpose and to ensure its safe use in clinical practice. The advantage of conducting QA is that we can ensure that radiological images are of the best quality and are produced at the lowest practicable dose of radiation. This results in better outcomes for the patient and makes clinical practice more rewarding.

What constitutes a QA programme?

A well-designed QA programme should be comprehensive but inexpensive to operate and maintain for the dentist and staff.
QA should address the following:
·         Image quality assessment
·         Practical imaging technique
·         Patient dose and equipment checks
·         Image viewing checks

The QA programme should entail surveys and checks that are performed according to a regular timetable. A written log of this programme should be maintained by staff to ensure adherence to the programme and to raise its importance among staff. A specific person should be named as leader for the QA programme.

1. Image quality assessment

For any dentist it is important to get the best image quality to ensure the best outcomes for patients. On a daily basis, image quality can be monitored subjectively, looking for such features as:
·         Image sharpness
·         Adequacy of contrast
·         Adequate coverage of the area of interest
·         The obtrusive presence of artefacts
Often, any deterioration can be picked up without too much difficulty. There is always a risk though that image quality can slowly “drift” from ideal to a level that may affect diagnosis. As such, audit of image quality needs to be incorporated into any QA programme.
The first element of this is a “Reject Analysis”. If a CBCT examination has to be repeated, the reason for this must be recorded and the overall rate at which repeat examinations are performed should be calculated. The European Guidelines on Radiation Protection in Dental Radiology (2004) adopted a simple system for assessing image quality of conventional dental radiographs: ‘Excellent’ (no faults), ‘Acceptable’(some faults but not affecting image interpretation) and ‘Unacceptable’ (faults leading to the radiograph being unsuitable for interpretation). It is recommended that the same three point scale is used for assessing image quality of CBCT examinations.  In terms of targets, no greater than 10% of radiographic examinations should be in the “unacceptable” category. In view of the dose implications of CBCT, the UK Health Protection Agency has recently recommended a more rigorous target of 5% for CBCT examinations.
2. Practical imaging technique

In many ways, CBCT should be more straightforward to perform than intraoral radiography; careful adherence to the manufacturer’s instructions and use of the patient immobilisation and positioning aids should minimize the potential risk of technique errors.
Nonetheless, problems that can occur due to incorrect technique include:
  • Mismatch between the area of clinical interest and the Field of View.
  • Artefacts overlying the area of clinical interest that could have been avoided by appropriate head positioning.
  • Artefacts overlying the area of clinical interest that could have been avoided by removal prior to imaging.
  • Movement artefacts due to a failure to inform the patient about remaining still.
  • Failure to correctly position imaging stents/ surgical guides.
Of course, failure to correctly set the correct exposure factors can also be seen as an error of imaging technique, but this is best considered below as part of patient dose and equipment checks.
3. Patient dose and equipment checks

It is usually performed by using a test phantom in conjuction with software routines that help in the interpretation of the results [see also 3.2 CBCT Quality Control Test Tools].
Phantoms designed for medical CT Quality Assurance results in images with worse low-contrast resolution than the medical CT scan. Furthermore, discrimination between objects with different density was not always successful. It is speculated that this is due to the fact that dental CBCT units are optimized for imaging of hard tissues and is also related to the low dose delivered compared with medical CT. Therefore, the use of a specifically designed phantom, with a size and densities resembling those of dental interest is necessary for the testing of the imaging performance characteristics, using special software tools for the interpretation of the results and the evaluation of image quality. [see 3.2 CBCT Quality Control Test Tools]
An important aspect of the Quality Assurance process is calculations of the delivered dose, in relation to the resulting image quality. [see 2.1 Radiation doses and risks of CBCT and 2.4 Dose Maps for CBCT].

4. Image viewing checks