Optimization in dental radiology

FAQs for health professionals

» Should the manufacturer of the X-ray set provide me with anything to help with routine QA checks of X-ray equipment?

Yes. 

Professional societies in collaboration with national authorities often recommend that users make regular image quality performance checks on X-ray equipment (and viewing screens where relevant). This is particularly important for dental cone-beam CT systems and panoramic X-ray equipment. To enable users to do this, manufacturers should provide details of the test procedures and the expected results in the equipment’s instruction manual. Any test objects or phantoms that are necessary for these tests and specific to individual equipment models or manufacturers should be provided with the equipment as standard.

» What are the most important features of dental X-ray examinations that contribute to dose reduction?

Professional societies, in collaboration with national authorities should publish guidance for users of dental X-ray equipment on how to optimize the radiation exposure of patients during justified X-ray examinations. For each imaging modality, there are many actions that can be taken to achieve a significant reduction in dose. These are listed below for intra oral; panoramic and cephalometric, dental CBCT. In addition, ensuring high quality clinical images a significant means of protecting patients by maximizing the benefits of the X-ray examination.

For intra oral equipment:

  • Rectangular collimation which approximates the size and shape of the receptor reduces dose significantly in comparison to circular collimation; a dose reduction exceeding 60 % can be achieved in dental radiology by using rectangular collimation;
  • The fastest available film consistent with achieving satisfactory diagnostic results should be used. E-speed and F-speed films reduce dose by more than 50% compared with D-speed films;
  • Digital detectors have the potential for further dose reduction, even compared with F-speed film, provided the repeat rate and use of higher exposure factors than necessary are controlled;
  • Using tube voltage in the range 60 to 70 kV; 
  • The X-ray tube filtration should be sufficient to reduce entrance skin dose to the patient consistent with producing satisfactory image quality; 
  • A position indication device which ensures a minimum focus-to-skin distance of 20 cm should be attached to the tube head (eg. by use of a long collimator/cone as opposed to a short conical one); 
  • Exposure settings used should be the minimum consistent with the speed of the imaging system used. Advice on exposure settings should be provided in the manual for the X-ray equipment, which should be available in the user’s native language and written in easily understood terminology. 

Where old X-ray equipment is used, it may be possible to take immediate action to achieve a significant reduction in patient dose:

For panoramic and cephalometric equipment:

  • Only the fastest screen-film combinations (at least 400) that are compatible with imaging requirements should be used for panoramic and cephalometric imaging.Note that the intensifying screen and film must be spectrally matched, for example, if the screen emits light in the green region of the spectrum, the film used should be one that is sensitive to green light. Furthermore, the physical condition of screens deteriorates over time and it is important that their condition is monitored and that badly damaged screens are replaced;
  • The X-ray beam for cephalometric imaging should be collimated to the area of clinical interest; 
  • The inclusion of wedge filters in cephalometric equipment reduces exposure to the soft-tissue facial profile and allows optimal imaging, while the provision of asymmetric collimation allows the exposed area to be confined to the area of clinical interest; 
  • Modern panoramic systems also allow the field to be limited to the area of clinical interest, thereby offering a significant potential for dose reduction. If available, limitation of field size to the area required for diagnosis should be used for panoramic radiography;
  • Where available, paediatric examination modes should always be used for examinations of children. If not available, the exposure factors (such as kV, mA, exposure time) should be suitably adjusted. This may result in a dose saving to the patient of 50% or more. 

For dental CBCT equipment:

  • Volume of patient imaged. The smallest volume compatible with the clinical situation should be used where this provides lower radiation dose to patients; 
  • kiloVoltage and mAs. These should be optimized for each clinical application and patient; 
  • Choice of voxel size. Some CBCT systems offer a choice of “resolution”. The voxel size compatible with the clinical situation should be used where this provides lower radiation dose to the patient; 
  • Number of projections and reconstruction algorithm. Some CBCT systems allow the operator to opt for imaging based on a reduced number of basis projections. Such options should be used where the resulting image quality is acceptable for the clinical situation. 

When considering buying a CBCT unit, you should check to see whether it is able to comply with national reference doses for dental CBCT where available.

» How does a digital image receptor affect patient dose in dental radiology?
 

  • Two types of digital system are used in intraoral, panoramic and cephalometric imaging. One involves imaging sensors based on charge-couple devices (CCD) and another uses photo-stimulable storage phosphor (PSP) plates (see the image of a PSP in its plastic cover below);
  • Radiographic technique for digital imaging should be adjusted for the minimum patient doses required to provide the required image quality for each examination type; 
  • Intraoral digital radiography offers a potential for significant dose reduction; some studies report that, depending on the diagnostic task, a lower exposure may be used when density and contrast is adjusted using the software features.This is one of the benefits of digital radiography where image quality can be optimized after the image has been taken;
  • Although digital radiography offers possibility of significant dose reduction, it can, in practice, lead to increased patient dose. This can arise from, for example: using an image quality higher than is necessary; use of unduly long exposure times; retakes by staff (e.g. due to bad positioning) that may go undetected; and lack of concern for collimation. Furthermore, due to smaller sensor size, more than one exposure may be required to cover the anatomical area imaged using a single conventional film. 

Optimization-radiographic quality
If a patient is exposed to X-rays for the purpose of producing a radiograph, but the resulting image is not of adequate quality for clinical use, then the patient has been put at risk for no benefit. Ensuring adequate quality is, therefore, a fundamental part of radiation protection. 

» How do I know if my radiograph is of a good standard?

You need to compare your performance by reviewing your radiographic quality against a recognized standard. Such quality standards for clinical images, and guidance on the audit process, are available. As a minimum target, your aim should be to ensure that no greater than 10% of radiographs are of unacceptable quality. If you fail this test, then actions can be taken to reduce the proportion of unacceptable radiographs, with a target of a 50% reduction at each successive audit cycle.

» How can I ensure that I get high quality intraoral radiographs?

Choosing the correct exposure factors, ensuring accurate patient and X-ray source position (using film holders), along with careful processing should together contribute to achieving excellent results in radiography. For intraoral radiography a simple test tool – an image of a step wedge is useful for maintaining high image quality (see the image - Sensitometric steps) . During installation, a reference standard radiograph of the step wedge should be made using the optimized exposure setting for an adult/child. Subsequent radiographs of the step wedge/phantom should be made during clinical use and compared with the reference one to ensure that image quality is maintained.

» How can I ensure that I get high quality panoramic radiographs?

By achieving accurate patient positioning and by good processing of the film. These are the two commonest causes of poor panoramic radiographic quality. Accurate positioning is helped by using all the positioning aids correctly and by adequate training. Test tools for panoramic radiography are available.

» How can I ensure that I get high quality cephalometric radiographs?

By using a cephalostat and a fixed X-ray source/patient/image receptor relationship. This is achieved using a dedicated cephalometric attachment to panoramic X-ray equipment. In cases where there is no alternative to using a dental X-ray set as the source, it is very important to to ensure correct collimation of the beam and alignment with the cephalostat.

» Can I expect an increased number of rejected films when switching from film to a digital imaging receptor for intraoral radiography?

Yes, this may happen. 

When switching to a digital image receptor, the retake rate can increase, mainly due to wrong positioning of the X-ray tube and small image receptor with respect to region of interest (ROI). Furthermore, repeating the exposure is much easier when using digital receptors and this has been reported to lead to increased reject rates. Careful positioning using sensor-holders with a beam-aiming device and audit of clinical image quality will avoid and or reduce retakes.

» I suspect that my intra oral X-ray equipment delivers very high patient doses. Can I do anything to immediately reduce doses while still obtaining good quality images?

Yes.

Older models are more likely to operate below 60 kV, either by design or due to deterioration of the X-ray tube head over its working life. Older models are also more likely to have low values of total filtration. Both low operating potential (kV) and low filtration are strongly associated with high patient doses, as is the use of speed group D, which is often observed to be used with older X-ray equipment. Therefore, an immediate saving in dose can be achieved by taking the following steps, pending the future replacement of the X-ray equipment:

  • Move to the use of E-speed film;
  • To improve the effective X-ray beam quality and provide a lower radiation output rate consistent with the use of E-speed film, a further 1.0 mm of aluminium beam filtration should be added to the X-ray tube head, as close as possible to the X-ray beam window in the tube head. This may require the help of a technician;
  • Continue to use the exposure settings you were using before, unless image quality is severely affected. In this case, the help of a medical physics expert should be sought. The above steps should provide a reduction in dose of at least 70%;
  • Poor film processing conditions may have as great an impact on patient doses as the X-ray equipment, and so attention should also be paid to ensure that all aspects of processing are carried out in accordance with the advice provided in (see European Guidelines on Radiation Protection in Dental Radiology, RP 136, published by European Commission, Luxembourg (2004)) and that proper quality assurance methods are in place.