Dental radiography and three-dimensional imaging

Dental radiography and imaging play an important role in all aspects of dentistry, including orthodontic treatment planning. Orthodontic radiography has traditionally included intra- and extra-oral images, comprising primarily of occlusal, dental panoramic and lateral cephalometric radiographs. Well taken intra-oral radiography provides the highest imaging resolution for diagnosis in dentistry and panoramic radiographs image both jaws, providing the practitioner with a magnified overview of the dental arches. However, two dimensional (2D) imaging is susceptible to superimposition by normal anatomical structures, which can hinder its diagnostic use in certain situations and therefore, more information may be required in the third plane to aid diagnosis. Further exploration with three-dimensional (3D) dental imaging originally involved the use of cross-sectional tomographs, using spiral multi-directional tomography.1 This provided slice information of the jaws; however, these images appeared blurry and distorted due to relatively thick slices and a highly operator-dependant technique. This lead to only limited use in orthodontics and many patients had conventional medical computed tomography (CT) scans instead.

Cone beam computed tomography (CBCT) was originally developed for use in angiography and planning radiotherapy, this technique utilizes a cheaper imaging detector, a quicker single revolution scan and lower radiation dose for the patient compared to conventional CT.2 This technology was further developed for use in dento-maxillofacial imaging, where it can provide high quality images of the bony structures of the facial skeleton and has been available in Europe since 1998.3 CBCT was originally adopted by those placing dental implants, where it was used to visualize the quantity and volume of alveolar bone and to assess the relationships with surrounding anatomical structures. The first generation of CBCT scanners imaged large fields of view within the facial skeleton, including the base of the skull. However, with the further development of CBCT imaging, machines are now able to image much smaller sections of the facial skeleton4 and further limit the radiation dose to the patient, where the current standard of care is to only image what one needs to assess.

Until the landmark studies by John Ludlow et al,5^–7 we knew CBCT was supposed to deliver a lower radiation dose than medical CT; however, few would have realized that some of the early generation CBCT machines could actually impart a radiation dose equivalent to a medical CT scanner! These studies also reaffirmed that the radiation dose from traditional intra- and extra-oral imaging was much less than CBCT and that the use of 2D imaging should not be discounted as a major diagnostic tool. Early case reports demonstrated the wide and varied use of CBCT imaging; however, much of what was reported would have been seen in traditional 2D radiographs. Therefore, in the 2008 publication of the British Orthodontic Society's Orthodontic Radiography guidelines,8 the recommendation was to use CBCT with caution, not to use CBCT imaging as a routine procedure and to develop evidence-based guidelines. Since orthodontics generally includes a younger cohort of patients who are actively growing and developing, the relative risk from radiation is much higher that that for adult patients. Therefore, the use of CBCT should be restricted for use in those who will benefit most from the information gained. The European Commission-funded Sedentex-ct (Safety and Efficacy of a New and Emerging Dental X-ray Modality) developed evidence-based guidelines for the use of CBCT in dentistry and have published the first selection criteria for dental CBCT imaging.9 With over 40 CBCT scanners available on the market and the manufacturers targeting dental practices, the production of selection criteria can only benefit those who use this imaging modality.

The Sedentex-ct guidelines have reinforced the need to perform a thorough clinical examination, with the routine use of CBCT imaging positively discouraged. The use of CBCT in orthodontics has been recommended for investigating impacted teeth, teeth associated with resorption, cleft palate, temporomandibular joints and orthognathic surgical planning. However, significant issues still arise with regard to who should perform image interpretation because not every dentist will understand the complex anatomy of the maxillofacial skeleton and may only concentrate on the teeth, thereby potentially missing important clinical findings.10 Once again, further training and research will be needed to obtain maximum benefit from CBCT imaging.

CBCT has equipped the dentist with 3D imaging; however, not every patient warrants a 3D scan. Patients should only be imaged if the CBCT scan will aid treatment where the benefit to the patient outweighs the radiation risk and the imaging will be properly conducted and interpreted to maximize the diagnostic yield.