Cone beam computed tomography: an update for general dental practitioners

Legislation

Under IR(ME)R 2000, justification for any medical ionising radiation exposure is required and should be documented.⁴ In compliance with IR(ME)R regulations, dose optimization should be employed to keep scan FOV to the absolute minimum; this is particularly important for children and young adults.⁶ Justification for the exposure should always be assessed by the IR(ME)R practitioner on an individual, case by case basis.⁴

Within Europe, the SEDENTEXCT consortium produced guidelines in 2012 (European Guideline: Radiation Protection No 172). These are evidence-based guidelines covering CBCT justification, radiation protection, quality assurance and staff training.⁷ Within the UK, the Health Protection Agency (HPA) have published guidelines, The Safe Use of Dental Cone Beam Computed Tomography, 2010, which provide practical insight into the acquisition and setting up of a CBCT machine, with standards for best practice clearly outlined.^8,9

Types of CBCT scanners

An enormous range of CBCT equipment is available to purchase, many with different properties or features which can affect image quality, size or user experience. Examples of these properties include grayscale (bit) depth, field of view, focal spot size, voxel size range, post processing technology and patient positioning.¹⁰ For the purposes of this paper, CBCT scanners can be divided into three main groups:

Some small FOV scanners can be used to record different overlapping areas and stitch the images together to create a larger FOV scan.³ Many machines combine dental panoramic tomography (DPT) and lateral cephalometric (lat ceph) settings with CBCT (CS 9300, Carestream Dental, 150 Verona St, Rochester, NY 14608, USA), which can reduce the need for multiple large machines. The choice of CBCT size is dependent on the clinical indication for imaging, for example, in endodontics the region of interest is often a single tooth meaning that the smallest scan size of 4 cm x 4 cm can be used. In comparison, assessment of bone for multiple implants may require the full height and width of the jaws.

When purchasing a CBCT machine, theoretical and practical training using generic software for data reconstruction is provided by the manufacturer. Viewing software enables multi planar reconstruction, allowing images to be viewed in axial, sagittal and coronal planes, and often includes a series of sections through the bone in the area of interest such as an implant screen or TMJ screen (Figure 3). Reconstruction can also be used to simulate plain film views such as pseudo panoramic or lateral cephalometric, which in turn allows the operator to choose the focal trough/points to allow maximum detail, however, due to the increased dose of a CBCT, this is not recommended as a routine method for acquiring a plain film view. Reformatting can also create 3D volume rendering, which may be used to provide a useful visualization of disease for the clinician and an informative teaching aid for the patient (Figures 4 and 5).¹¹

The radiographic technique for CBCT acquisition is similar to the dental panoramic technique. The patient is positioned with the occlusal plane (maxillary or mandibular depending on the region of interest) horizontal to the floor and held in position with a combination of a chin rest, headband, bite peg or head rods, depending on the equipment used. Light guides aid correct positioning and usually a scout view is recorded to enable a customized field of view to be chosen, to minimize the radiation dose to the patient (Figure 6).

Additional software can provide more detailed specialized reconstruction algorithms. Examples of this include dental implant planning, for bony measurements and fabrication of computer-aided surgical implant guides, and orthodontic treatment planning, which can be used to postulate the aesthetic outcome of treatment. CBCT scan be used for production of CAD-CAM surgical models and subsequent formation of personalized plates for orthognathic surgery and trauma.¹²

Advantages and disadvantages of CBCT

It is important for the general dental practitioner to understand the advantages and disadvantages of CBCT scanning in order to make an informed decision on the justification for a scan, including whether or not further imaging will change patient care or treatment outcomes. For a comprehensive list of advantages and disadvantages of CBCT see Table 1.^2,13

The most disruptive artefact within a reconstructed CBCT scan volume is a streaking/beam hardening artefact which is caused by objects with a high atomic number, specifically metal.^5,14 Titanium dental implants cause a less streaking artefact than other dental metals, such as cobalt chrome, as titanium has a lower atomic number than many other metals, but dental implants may be close to, or have a metal suprastructure. A metalwork artefact can often be close to, and may obscure, the region of interest, rendering the exam non-diagnostic. This is particularly associated with post crowns (Figure 7). Potential improvements to a streaking artefact include separating the jaws with a cotton wool roll or spatula, ensuring the occlusal plane is parallel to the floor, customizing the field of view to the absolute smallest practical and making sure any piercings or dental appliances are removed prior to scanning. Since a CBCT dataset is compiled during a single rotation to cover the full image volume, a streaking artefact affects the whole field of view rather than just the area local to the metalwork. Post processing algorithms applied to the dataset help to minimize the effect on the whole scan.

Patient movement causes blurring and loss of sharpness. This can be reduced by fully explaining the procedure to the patient prior to scanning, doing a practice run and confirming the chin rest is fully engaged with head fixation.

Indications

Indications for CBCT scans can be broadly split into the dental sub-specialties: restorative, oral surgery, orthodontics, paediatrics and the developing dentition, endodontics and dental and maxillofacial trauma. CBCT is also utilized within medicine for temporal bone imaging, sinus imaging, radiotherapy planning, angiography and orthopaedic imaging.^15,16 The SEDENTEXCT guidelines provide a specific framework for the indications of CBCT scanning in the oral and maxillofacial region (Table 2).⁷

Cross-sectional CBCT assessment for dental implants can give information on bone quality, height and volume and can also be used for treatment planning using surgical stents and guides, and assessment of sinus anatomy and pathology prior to sinus grafting and bone augmentation. Voxel size and resolution can be reduced when imaging for implant planning, as the information required is less detailed than, for example, root canal anatomy. The authors' establishment generally uses a 0.3 mm voxel size for implant scans, which provides adequate bony detail for treatment planning, whilst reducing dose. Implant planning software, such as Nobel guide (Nobel Biocare, Klozen, Zurich, Switzerland) and Simplant OMS (Materialise, Leuven, Belgium) offer an implant planning software collision programme which allows the clinician to plan surgery by mapping out the inferior dental nerve canal, selecting an appropriate implant (type and size) and advising if there is a collision between the two structures.

CBCT is especially useful for dental implant planning if there is a suspicion of an anatomical variant, such as a pronounced anterior loop of the ID nerve canal at the mental foramen, pronounced lingual concavity, or buccal ID nerve branching in the posterior body of the mandible.¹⁷ Care should be taken not to over prescribe the use of CBCT for dental implants as CBCT is not required for all cases. Current advances in implantology mean that zygomatic and pterygoid implants are now being placed and conventional plain film imaging is simply not clear or detailed enough for such complex treatment planning.¹⁸

CBCT can aid surgical treatment planning for extraction and surgical removal of teeth, assess inferior alveolar nerve intimacy to wisdom teeth, characterize cysts, tumours and other bony abnormalities, and investigate sinus disease (Figures 8 and 9).

The use of CBCT within orthodontic and paediatrics is primarily due to the manifestation of developmental abnormalities, for example, unerupted tooth localization, supernumeraries, assessment of impacted teeth and external resorption, cleft palate and complex skeletal abnormalities.^19,20 The use of higher radiation dose imaging, such as CBCT in younger patients, should be carefully considered and used only where there is clear patient benefit and justification. The main advantage of CBCT within dental and maxillofacial imaging is that 3D imaging has the ability to detect subtle changes in the teeth when this cannot be confirmed by two-dimensional (2D) digital or film-captured images. For example, resorption of a lateral incisor by an impacted canine; 3D representation can show this well in multi planar reconstruction and volumetric rendering, enabling accurate detailed treatment planning.

There have been significant recent advances in endodontic therapy, both in treatment planning, imaging and tooth preparation. CBCT can provide additional information on unusual root canal anatomy, such as identification of accessory canals, dens invaginatus, external and internal root resorption and surgical endodontics with complicating factors.^7,21 CBCT should only be considered for endodontic indications when conventional imaging techniques are inadequate. To visualize root canal anatomy, a small FOV scan would be indicated with a corresponding small voxel size ranging from 0.075 mm–0.2 mm, which shows finer detail. A balance needs to be maintained between better resolution (decreased voxel size) of the image and increased noise and dose to the patient.⁵

The use of CBCT for trauma is now more widely recognized as a low radiation dose alternative to conventional spiral CT for facial and mandibular fractures. It is also useful for assessment of dental trauma, including luxation and horizontal and vertical root fractures.

Contra-indications

There are specific clinical situations where a CBCT scan may not be the best imaging modality of choice due to individual patient factors. Examples of this include patients who cannot be accurately positioned in the machine, such as kyphotic patients, patients who cannot stay still, such as patients with Parkinson's disease or those with learning difficulties. Depending on the type of CBCT machine used, there can be difficulties related to the physical characteristics of the patient. CBCT should not be routinely used for orthodontic assessment, caries diagnosis, assessment of periodontal bone support or standard uncomplicated endodontics for root canal anatomy.⁷

Other situations where CBCT may not be appropriate include acute spreading of infection and neoplasia, where another imaging modality may be more appropriate, such as ultrasound, MRI or conventional CT. CBCT should not be used purely to obtain a reconstructed panoramic or cephalometric image alone.

Image reporting and interpretation training

Current guidance from the Health Protection Agency 2010 and the SEDENTEXCT Consortium recommends that all CBCT images are reviewed by an appropriately trained dentist or Dental and Maxillofacial Radiologist. The Health Protection Agency guidelines on CBCT scanning suggest a minimum of 3 hours of theoretical training, 2 hours of interpretation training and 6 hours of practical training, in order to operate equipment and report dental CBCT scans.⁸ The SEDENTEXCT guidelines have also made recommendations as to the appropriate training for dentists using CBCT following a similar framework of theoretical, interpretation and practical training and the guidelines website provides a free interactive e-learning environment online.⁷ These guidelines have evolved from the Basic Principles for Use of Dental Cone Beam Computed Tomography: Consensus Guidelines of the European Academy of Dental and Maxillofacial Radiology (EADMFR). EADMFR have also published a position paper on the basic training requirements for the use of CBCT for dentists. These guidelines broadly spilt CBCT training into two levels of education:

These guidelines also provide learning outcomes for both levels 1 and 2, covering knowledge and understanding, skills and ability and judgement, which overall provide a reference of CBCT competencies for the general dental practitioner.^22,23

An imaging centre performing a CBCT has a legal responsibility under IR(ME)R to ensure that the scan will be reviewed and reported. An appropriately trained general dental practitioner can assess and report a small volume CBCT scan (covering only the dental bearing area) if he/she is familiar with the normal appearance, anatomical variants and radiological appearance of disease. The imaging centre performing the scan has a responsibility to ensure that the scan will be reviewed and reported appropriately. This reporting may be carried out within the imaging centre itself (eg at a private clinic or secondary care hospital setting) or there may be written protocols or a service level agreement in place to transfer this responsibility to the referrer or to a third party. However, all radiological examinations are required under IR(ME)R to undergo an evaluation and report by a competent person. CBCT scans can be reported by a dental and maxillofacial radiologist or head and neck radiologist, particularly in the case of large volume CBCT scans, which encompass unfamiliar areas such as skull base and petrous temporal bone.²² Large scan volume is defined within the SEDENTEXCT guidelines as over 10 cm height and therefore defined as craniofacial scan size; 10 cm and below falls within the category of dento-alveolar or limited volume. Scan diameter is not defined within the SEDENTEXCT guidelines, however, this parameter can be changed, depending on the machine, and should also follow the ALARA principle of dose reduction techniques.⁴ However, many CBCT scans that are recorded within both primary and secondary care are much smaller than this and, for the purposes of reporting, a large field of view scan can be considered which includes structures unfamiliar to the clinician.

Courses provided online, through conferences or Section 63 events, are available to assist the general dental practitioner in CBCT interpretation. Currently, within the UK, there is no standardized assessment for competency in CBCT interpretation and it is not mandatory within the undergraduate curriculum. There is a future opportunity for service level agreements for CBCT reporting initiated by clinical commissioning groups bridging the gap between primary and secondary NHS care. However, within the current and future contract, changes within both primary and secondary care within the UK means that it is difficult to predict how this will develop in the future.

Conclusion

Cone Beam Computed Tomography is becoming more widespread within both the general practice and the hospital setting, however, careful case selection needs to be applied to ensure that the resulting examination is dose optimized, diagnostic, and results in a net benefit to the patient by influencing its management. Achieving this requires a thorough understanding of the practical use of CBCT and associated software, the legislation and guidelines, and the clinical interpretation of images recorded. This may require appropriate further training for all relevant members of the dental team involved in this imaging modality and local advice from the Radiation Protection Adviser and Medical Physics Expert.