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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:

  • Using tube voltage in the range 60 (minimal) to 70 kV.
  • Nominal focal spot size should range between 0.4 and 0.7.
  • Tube current usually ranges between 3.5 to 8 mA, the exposure time should be below 1s in every exposition.
  • The X ray tube filtration should be sufficient to reduce entrance skin dose to the patient consistent with producing satisfactory image quality. 
  • Rectangular collimation is strongly recommended, it 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. 
  • 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. 
  • 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. 
  • Portable intra-oral X ray units should only be selected in specific situations (Berkhout et al. 2015; UK PHE 2016; HERCA).

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:

  • The X-ray beam for cephalometric imaging should be collimated 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 [Lecomber et al.1993]. 
  • 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. 
  • Only the fastest screen-film combinations (at least 400) that are compatible with imaging requirements should be used for conventional 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 use of PSP receptors should be discouraged due to inferior image quality (Benediktsdottir et al. 2003)

For dental CBCT equipment:

  • The Field of View (FOV) should be adapted to the clinical indication ensuring that a region of interest can be covered with a reasonable margin of error, without exposing areas which are not needed for diagnostics [EC, 2012].
  • CBCT units should at least offer a small-FOV option (not larger than 6x6 cm), but do not necessarily need a large-FOV option.
  • Exposure parameters (kV and mAs) should be optimized for each clinical application and patient. Specifically, high- medium- and low-mA settings should be available in order to optimize scans for patients with different head sizes. 
  • Regarding scan/exposure time, a high-speed scan option (10 s scan time or faster, regardless of the exposure time) should be available for patients at risk for movement (e.g. small children).
  • Users should be aware that the voxel size is one of many parameters determining image sharpness, and not compare units based on this parameter. While smaller voxel sizes do not always yield a diagnostic benefit [Uzun et al. 2015, Kamburoğlu et al. 2015], it is recommended that CBCT units have a high-resolution mode with a voxel size below 0.2 mm, in order to properly visualize trabecular bone [Pauwels et al. 2015a] as well as other anatomical details and small pathologies [Kolsuz et al. 2015, Lukat et al. 2015]. 
  • 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 in European guidelines on radiation protection in dental radiology. 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. 

Read more:

•    UNITED NATIONS SCIENTIFIC COMMITTEE ON THE EFFECTS OF ATOMIC RADIATION, Sources and effects of ionizing radiation, UNSCEAR Report Volume 1, New York, USA (2000). 
•    European Commission, European Guidelines on Radiation Protection in Dental Radiology, RP 136, Luxembourg (2004). 
•    SEDENTEXCT. Radiation Protection; Cone Beam CT for Dental and Maxillofacial Radiology. Provisional Guidelines (v1.1 May 2009). 
•    Espelid, I., Mejàre, I., Weerheijm, K., EAPD., European Association of Paediatric Dentistry guidelines for the use of radiographs in children. European Journal of Paediatric Dentistry 4 (2003) 40-48. 
•    Harris, D., Buser, D., Dula, K., Gröndahl, K., Jacobs, R., Lekholm, U., Nakielny, R., van, Steenberghe, D., van, der, Stelt, P., E.A.O. Guidelines for the use of diagnostic imaging in implant dentistry. Clinical Oral Implants Research 13 (2002) 566-570. 
•    Isaacson, K.G., Thom, A.R., Horner, K., Whaites, E., Orthodontic Radiographs. Guidelines for the use of radiographs in orthodontics. British Orthodontic Society: London, (2008). 
•    Pendlebury, M.E., Horner, K., Eaton, K.A., (eds). Selection Criteria for Dental Radiography. Faculty of General Dental Practitioners (UK) Royal College of Surgeons of England, London, (2004). 
•    Lecomber, A.R., Faulkner, k., Dose reduction in panoramic radiography. Dentomaxillofac Radiol. 22 (1993) 69-73. 
•    American Academy on Pediatric Dentistry ad hoc Committee on Pedodontic Radiology: American Academy on Pediatric Dentistry Council on Clinical Affairs. Guideline on prescribing dental radiographs for infants, children, adolescents, and persons with special health care needs. Pediatric Dentistry 30 (7 Suppl): 236-7 2008. 
•    LOOE, H.K. et al., Radiation exposure to children in intraoral dental radiology, Rad. Prot. Dosim. 121 (2006) 461-465. 
•    WAGNER et al. Exposure of the pregnant patient to diagnostic radiations: A Guide to Medical Management, 2nd Edition, Publisher: Medical Physics Publishing, Madison, (WI 1997). 
•    NATIONAL RADIOLOGICAL PROTECTION BOARD, Radiation exposure of the UK population, Review NRPB R263, Chilton, UK (1993). 
•    NATIONAL COMMISSION FOR RADIATION PROTECTION, Implementation of the principle as low as reasonable achievable (ALARA) for medical and dental personnel, NCRP Report 107, NCRP (1990). 
•    AUSTRALIAN RADIATION PROTECTION AND NUCLEAR SAFETY AGENCY, Radiation Protection in Dentistry, Code of Practice and Safety Guide, RPS 10, Canberra, Australia (2005).