Cone beam computed tomography (CBCT)

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Cone beam computed tomography (CBCT)

Cone beam computed tomography (CBCT) has been developed in recent years specifically for use in the dental and maxillofacial regions and is gradually establishing itself as the imaging modality of choice in certain clinical situations. It is also referred to as digital volume tomography or cone beam volumetric imaging. As described later, the size of the volume or field of view (FOV) of the maxillofacial skeleton imaged varies; as a result CBCT scans are often described by their field of view:

• Small, limited or dento-alveolar (approx. 4 cm³)

• Medium or maxillofacial (approx. 8 cm³)

• Large or craniofacial including the cranial base ± the skull vault.

Main indications

Considerable controversy exists as to the main indications for CBCT and robust clinical research justifying its use is limited. Some authorities argue that CBCT should be regarded as almost routine for all dental and maxillofacial applications, while others argue that CBCT should not be used unless the results of the examination are going to alter patient management as the technique delivers a higher dose than conventional two-dimensional imaging. In 2011 the European-wide Safety and Efficacy of a New and Emerging Dental X-ray Modality SEDENTEXCT project published guidelines based on what evidence was available at that time. These guidelines, and a more cautious approach to the justified use of CBCT, are endorsed by the authors. Hence the indications summarised below are based broadly on the SEDENTEXCT recommendations.

Clinical examples illustrating many of these indications are included in later chapters.

Equipment and theory

Multiple CBCT machines are currently available with new, upgraded models launched regularly by most manufacturers of X-ray equipment. Almost all modern machines resemble panoramic units, as shown in Fig. 16.1.

Fig. 16.1 Examples of two CBCT machines specially designed for imaging the maxillofacial skeleton. A The i-CAT (imaging Sciences International, Inc., USA) and B the 3-D Accuitomo (J. Morita, Japan).

All equipment employs a cone-shaped X-ray beam (rather than the flat fan-shaped beam used in conventional CT as described in Chapter 18) and a special detector (e.g. an image intensifier linked to a charge-coupled device (CCD) or, more commonly, an amorphous silicon flat panel). The scanning/image creation process divides into three stages:

• Data acquisition

• Primary reconstruction

• Secondary or multiplanar reconstruction

Stage 1 – Data acquisition

The patient is positioned with the unit (as described later). The equipment orbits around the patient in a 180°, 270° or 360° rotation, taking approximately 5–40 seconds, and in one cycle or scan, images a cylindrical or spherical volume referred to as the field of view (FOV). As all the information/data is obtained in the single scan, the patient must remain stationary throughout the exposure. As described earlier, the FOV can vary in size enabling small, medium or large volumes of a patient to be imaged. Using a large field of view (e.g. 15 cm³) most of the maxillofacial skeleton fits within the cylindrical or spherical shape and is imaged in the one scan as shown in Fig. 16.2.

Stage 2 – Primary reconstruction

Having obtained data from the one scan, the computer then divides the volume into tiny cubes or voxels (ranging in size between 0.076 mm³ and 0.4 mm³) and calculates the X-ray absorption in each voxel. As with pixels in two-dimensional digital imaging, described in Chapter 5, each voxel is allocated a number and then allocated a colour from the grey scale from black through to white. Typically one scan contains over 100 million voxels. The overall image resolution clinically of hard tissues (teeth and bones) is generally very good in CBCT imaging, although measured spatial resolution (3–4 line pairs/mm) is not yet as good as two-dimensional film-based or digital imaging (10–25 lp/mm) (see Chapter 4). Using a smaller voxel size potentially increases the spatial resolution but increases the radiation dose. Even so CBCT cannot be used to examine the soft tissues in detail because of the size of the kV and types of detectors used and the amount of scatter. Essentially all that can be seen is the outline of the soft tissues where it interfaces with air.

Stage 3 – Secondary or multiplanar reconstruction

Following the primary reconstruction, the computer software then allows the operator to select voxels in the three anatomical orthogonal planes to create sagittal, coronal or axial images – as shown in Fig. 16.2. A set of sagittal, coronal and axial images appear simultaneously on the computer monitor. The software then enables these image data sets to be scrolled through in real time. For example, by selecting and moving the horizontal cursor up and down on the coronal image, all the axial images can be scrolled through from top to bottom.

Using a small field of view (e.g. 4 cm³) just two or three teeth and their supporting structures fit within the cylindrical or spherical shape. The same three stages of data acquisition, primary reconstruction and secondary or multiplanar reconstruction are employed to create images in the sagittal, coronal and axial planes, as shown in Fig. 16.3.

Fig. 16.3 A Diagram showing the basic concept of *data acquisition* for a small field of view CBCT. A cone-shaped X-ray beam orbits once round the patient acquiring data within a small cylinder containing two or three teeth. B An example of a small volume CBCT scan showing axial, coronal and sagittal images and a 3-D reconstructed image following *secondary* or *multiplanar reconstruction*.

Multiplanar reconstruction also allows voxels in other planes to be selected. For example, it is possible to plot the curvature and shape of the dental arch to enable the computer to construct a panoramic image made up of the voxels that coincide with the plotted arch shape – either mandibular or maxillary (see Fig. 16.4).

Fig. 16.4 A Diagram showing how plotting the shape of the arch identifies the voxels that are required for the computer to generate a panoramic image. B Panoramic image based on the curved shape of the mandibular arch. C Panoramic image based on the curve of the maxillary arch.

In addition, it is also possible to reconstruct cross-sectional (also referred to as transaxial) images of any part of the jaw, and with appropriate software to produce so-called volume rendered or surface rendered images, as shown in Fig. 16.5.

Fig. 16.5 An example of the i-CAT *implant planning screen*, showing an axial slice allowing the arch shape to be defined resulting in the panoramic image, together with 10 transaxial/cross-sectional images through the midline of the mandible and a volume rendered 3-D image.

Technique and positioning

As in panoramic radiography, described in Chapter 15, the exact patient positioning techniques vary from one machine to another, but whatever the machine, written protocols for each CBCT examination should be provided which include details of patient positioning, exposure parameters and volume size. There are, however, some general requirements that are common to all machines and these can be summarized as follows.

Patient preparation

• Patients should be asked to remove any earrings, jewellery, hair pins, spectacles, dentures or orthodontic appliances.

• The procedure and equipment movements should be explained to reassure patients and the importance of remaining stationary throughout the scan should be stressed.

Equipment preparation

• The smallest volume size needed to answer the clinical question should be used to reduce the radiation dose to the patient. Using a smaller volume reduces scatter and potentially improves image quality.

• Optimal exposure factors should be selected to satisfy the diagnostic requirements of the examination. Higher exposure factors may be chosen if a higher spatial resolution is required.

• Optimal reconstructed voxel size should be selected. If choosing a larger voxel size results in a reduced patient dose (due to lower exposure factors being used) then this should be considered as long as the lower resolution is compatible with the aims of the radiographic examination.

• Some machines offer a ‘quick scan’ where the rotation arc is reduced. This feature reduces the number of projections taken and therefore reduces the dose. If the required diagnostic information can be obtained using this scan protocol then it should be selected.

Patient positioning

• The patient should be positioned using the manufacturer’s guidelines to ensure that the correct region of interest is captured. A scout view may be useful to ensure the right part of the jaw is imaged, as shown in Fig. 16.6.

Fig. 16.6 Examples of *scout* views taken on the i-CAT Classic. A The first scout view of the mandible shows that the lower part of the mandible has been missed. The set-up was altered and the scout view repeated. B The second scout view following alteration of the set-up. The whole body of the mandible is now included, and the full scan can now be carried out in the knowledge the complete region of interest will be imaged.

• Once positioned correctly, using the light beam markers, immobilization chin cups and head straps must be used to prevent any patient movement as shown in Fig. 16.7.

Fig. 16.7 A Patient positioned in the i-CAT Next Generation and B in the 3-D Accuitomo. Note the use of the light beam markers and immobilizing chin cup and head strap to prevent movement. Both children have protective thyroid collars in place.

• There is no need for the routine use of a protective lead apron.

• There is no need for the routine use of a protective thyroid collar as the thyroid gland does not normally lie in the primary beam, however its use should be considered on a case by case basis particularly in children. If used, it must be positioned so that it does not interfere with the primary beam since this could lead to significant artefacts.

Normal anatomy (Figs 16.8–16.12)

Fig. 16.8 Examples of large volume CBCT images with the main anatomical features indicated. A An axial image at the level of the condyle. B An axial image at the level of the ramus of the mandible.

Fig. 16.9 Examples of CBCT images of the mandible showing the main anatomical features. A A 3-D surface rendered reconstruction of the mandible viewed from the lingual side (© Materialise Dental NV – SimPlant®). B An axial image through roots of the mandibular teeth. C A sagittal image through the midline of the mandible.

Fig. 16.10 Further examples of CBCT images of the mandible showing the main anatomical features. A A 3-D surface rendered reconstruction of the mandible viewed from the front (© Materialise Dental NV – SimPlant®). B An axial image at the level of the mental foramina. C A coronal image through the molar teeth. D A panoramic type reconstruction along the course of the inferior dental canal.

Fig. 16.11 Examples of CBCT images of the maxilla showing the main anatomical features. A A 3-D surface rendered reconstruction of the left side of the maxilla (© Materialise Dental NV – SimPlant®). B An axial image through the base of the maxillary antra. C A coronal image through the incisive canal. D A coronal image through the pterygoid plates.

Fig. 16.12 Further examples of CBCT images of the maxilla showing the main anatomical features. A A 3-D surface rendered reconstruction of the maxilla viewed from below (© Materialise Dental NV – SimPlant®). B An axial image through the roots of the maxillary teeth. C A coronal image through the maxillary antra. D A sagittal image through the midline of the maxilla.

Radiation dose

The exposure factors on some CBCT machines are fixed by the manufacturer. However, on other units operators are able to adjust the exposure factors – typically in the range 60–120 kV and in the range 1–20 mA – allowing optimization of the dose. As mentioned the scan time varies between 5 and 40 seconds, but the equipment may use a pulsed beam rather than a continuous beam. As a result, during a scan lasting 20 seconds the patient may be exposed to ionizing radiation for about 3.5 seconds only.

The effective dose from different CBCT machines varies considerably. It depends on a number of factors including:

• The exposure factors (kV, mA and time of exposure)

• The volume size – field of view

• The type of equipment used

• The part of the jaw/maxillofacial skeleton being imaged.

Typically doses are lower than medical CT but higher than conventional dental radiography. However, some new CBCT units are producing very small field of view, high resolution images with doses equivalent to that of a few periapical radiographs. The effective doses from CBCT imaging, initially shown in Chapter 6, are shown again in Table 16.1.

Table 16.1

Table showing the effective dose from a range of different radiographic examinations including CBCT

X-ray examination	Effective dose (E) mSv
Bitewing/periapical radiograph	0.0003–0.022
Panoramic radiograph	0.0027–0.038
Upper standard occlusal	0.008
Lateral cephalometric radiograph	0.0022–0.0056
Skull radiograph (PA)	0.02
Skull radiograph (lateral)	0.016
Chest (PA)	0.014
Chest (lateral)	0.038
CT head	1.4
CT chest	6.6
CT abdomen	5.6
CT mandible and maxilla	0.25–1.4
Barium swallow	1.5
Barium enema	2.2
Dento-alveolar CBCT	0.01–0.67
Craniofacial CBCT	0.03–1.1

Advantages and disadvantages

Advantages

• Multi-planar reformatting and data manipulation allowing anatomy/pathological conditions to be viewed in different planes

• Lower radiation dose than medical CT

• Geometrically accurate images

• Very good spatial resolution

• Fast scanning time

• Compatible with implant and cephalometric planning software.

Disadvantages

• The patient has to remain absolutely stationary throughout the scan to avoid movement artefacts (see Fig. 16.13)

Fig. 16.13 Sagittal section through the upper incisor region resulting from severe patient movement. The scan is non-diagnostic and needs to be retaken.

• Soft tissues not imaged in detail

• Computer constructed panoramic type images are not directly comparable with conventional panoramic radiographs – particular care is needed in their interpretation

• Radiodense objects such as restorations and root filling materials can produce so-called beam hardening artefacts as well as the streak or star artefacts shown in Fig. 16.14.

Fig. 16.14 Examples showing beam hardening and streak or star artefacts caused by radiodense (metallic) objects.

Assessment of image quality

In the UK the 2010 Health Protection Agency’s guidelines included slightly modified image quality criteria and ratings from those recommended for conventional two-dimensional dental radiographs and shown in several earlier chapters. The recommended subjective image quality ratings and minimum targets for CBCT are shown in Table 16.2.

Table 16.2

Subjective image quality ratings and targets for CBCT recommended by the Health Protection Agency in the UK in 2010

Quality rating	Basis	Target
Grade 1 — Diagnostically acceptable	No errors or minimal errors in either patient preparation, exposure, positioning or image reconstruction and of sufficient image quality to answer the clinical question	Not less than 95%
Grade 2 — Diagnostically unacceptable	Errors in either patient preparation, exposure, positioning or image reconstruction which render the image diagnostically unacceptable	Not greater than 5%