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Cone-beam computed tomography – A technology that is here to stay

There are a few things in dentistry that can dramatically change the way we diagnose and treat our patients. This has been seen with the introduction of cone-beam computed tomography (CBCT). We are at the beginning of a new paradigm in dental radiology that will aid dental professionals in making the most accurate diagnosis and will get patients more involved in the decision-making processes that will affect both the treatment and the long-term success of their dental health. Various imaging modalities have been used in the dentomaxillofacial fields over the past few decades, none of them with entirely satisfactory results. This is particularly true for more demanding imaging tasks, such as implant planning, temporomandibular joint imaging, detection of facial fractures, lesions and diseases of soft tissue in the head, and neck and reconstructive facial surgery. The first CBCT scanner ever to be built was for angiography among other tasks at Mayo in 1982. Fahrig, et al. developed a CBCT system based on an image intensifier and C-arm for use in angiography.

Although CBCT has existed for over two decades, its true potential has not yet been fully tapped. The major innovation compared with intraoral and panoramic imaging is that it provides high-quality, thin-slice images. Cone-beam machines emit an X-ray beam shaped liked a cone, rather than a fan, as in conventional CT machines. The beam exiting the patient is captured on a 2D planar detector, usually an amorphous silicon flat panel or sometimes an image intensifier/CCD detector. The beam diameter ranges from 4 cm to 30 cm. As the X-ray source goes around the patient’s head, the sensor captures from 160 to 599 basis images. These images are used to compute a spherical or cylindrical volume including all, or a portion of, the face. In this volume, the densities at all locations (voxels) are calculated from the basis images. Voxels are cuboids and can be as small as 0.125 mm. Serial cross-sectional views are made in the axial, sagittal, and coronal planes. From this data set, the operator can also extract thick or thin, planar, or curved and 3D reconstructions in any orientation.

Many third-party software have been developed for assisting in implants treatment planning and for orthodontics, for display of relationships between hard and soft tissues, and for making measurements of true distances and angles. The data set generated by cone-beam imaging can also be used to produce rapid prototyping models for treatment planning, such as in orthognathic surgery cases, for forensic uses such as legal cases, or to build surgical guides for implant placement.

Unlike CT, the contrast resolution is limited to the densities of calcified structures, such as bone. Although the interface between soft tissues and air is readily identified, no soft-tissue window images exist, as in CT that enable differentiation of various soft tissues. Another limitation is the presence of metallic artefacts caused by metallic restorations, root canal filling material, and implants. These artifacts appear as bright or dark streaks in the image plane containing these structures and they degrade image quality. Such artifacts may also appear as dark bands around amalgam restorations simulating recurrent caries, or as dark zones or streaks around endodontic materials, simulating root fractures.

Four technological and application-specific factors have converged to make this technology possible. First, compact and high-quality flat-panel detector arrays were developed. Second, the computer power necessary for cone-beam image reconstruction has become widely available. Third, X-ray tubes necessary for cone-beam scanning are less expensive than those required for conventional CT. Fourth, by focusing on head and neck scanning only, one can eliminate the need for sub-second gantry rotation speeds that are needed for cardiac and thoracic imaging. This significantly reduces the complexity and cost of the gantry. In short, cone-beam CT is ideally suited for high-quality and affordable scanning of the head and neck in dentomaxillofacial applications.

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