Tips for viewing and interpreting cone beam computed tomography radiographs in orthodontics

In accordance with the established guidelines, including the Ionising Radiation (Medical Exposure) Regulations,¹ SEDENTEXCT² and British Orthodontic Society guidelines,³ the decision to take a cone beam computed tomography (CBCT) image should be case-specific, indicationorientated and clinically justified.⁴ In cases where plain films fail to provide the necessary information required for diagnosis and treatment planning, additional information provided by a CBCT can improve diagnostic yield and anatomical accuracy.^5,6

There are several recognized indications in orthodontics where CBCT may be appropriate. These include:

• Unerupted tooth localization
• Assessment of external resorption in relation to unerupted teeth
• Assessment prior to implant
• Cleft palate assessment
• Orthognathic planning.²

If the decision is made to take a CBCT, the ALARP (as low as reasonably practicable) principle of keeping the patient's radiation dose as low as possible should be followed, owing to the potentially significantly higher effective dose administered by CBCT than by conventional radiographic techniques.

In addition to a formal report by an appropriately trained individual,² clinicians who request CBCT imaging should undergo additional formal training to enable them to interpret CBCTs. The following sections and accompanying images address the three main key points for viewing and interpreting CBCT images.

View CBCT in all three spatial planes

Ensure the system for which radiographs are reviewed permits the CBCT to be viewed in all planes of space: axial, coronal and sagittal.

Images should always be reviewed in the three planes of space because misinterpretation of CBCT images may negatively affect orthodontic treatment planning.⁴ For example, there may be times when apical root resorption is less apparent on the axial view than it is on the coronal or sagittal view. In the axial view, it is relatively easy to detect circumferential root resorption compared to apical resorption, as it is difficult to determine the root length in axial views.

Use of oblique reference lines

The use of oblique reference lines is recommended in all three planes, as these allow clinicians to clearly orientate the CBCT and optimise their assessment of individual teeth and their location in relation to other hard and soft tissues. The key to using the reference lines is to ensure that you are always viewing the CBCT centred through the long axis of the individual tooth in question. If the reference lines are not adjusted, the resulting view shows only a part of the tooth in each slice, which would inaccurately reflect the slice from an oblique angle.

Ideally, in the sagittal and coronal view, the whole tooth (from tip to apex) should be visible in a single slice.

For the tooth in question:

• View the CBCT image in all three planes (Figure 1);
• Select the tab for the oblique reference lines (Figure 2);
• Repeat this process for the sagittal (Figure 5) and coronal views (Figure 6).

If the cursor is orientated on the incorrect tooth (for example, over the canine instead of the lateral incisor when aiming to assess root resorption of the latter), or at the incorrect inclination, incomplete or warped views of other teeth can occur, which could lead to misinterpretation. For example, in Figure 7, the CBCT image is centred on the canine, which gives the appearance of a severely resorbed lateral incisor in the sagittal view.

Use of 3D reformatting tool (3D volume render)

Diagnosis of root resorption from 3D images should not be used, owing to the risk of artefacts and inaccuracies. This is because when bone is ‘wiped away’ to reveal the roots of the teeth, the software is only choosing to wipe away the tissue that is less dense. There are times, for example when the bone and root density is so similar, that root is ‘wiped away’ at the same time as bone when the image is manipulated (Figure 8).

Three-dimensional manipulation of the CBCT image can, however, be useful for patient education and surgical planning, particularly highlighting ideal surfaces for placement of attachments for surgical colleagues (Figure 9). Patients can greatly benefit from seeing their malocclusion in a virtual environment. Not only can this visual feature aid communication with patients, involving patients in their care and the decision-making process can improve the informed consent process.

Conclusions

In addition to using the formal CBCT report, clinicians who request CBCTs should have a clear understanding of how to correctly view CBCT images. This includes having an understanding and demonstrating the correct use of the software tools available.

Viewing the tooth in question, by manipulation of the reference lines, in all three planes of space is essential. Improving patient education and surgical planning are key benefits of the 3D volume render tool. However, diagnosis of root resorption from the 3D render is not accurate or reliable.