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3D CBCT Feature James Mah, DDS, MSc, MRCD, DMSc

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Limitations of Conventional Imaging
Conventional imaging for orthodontic treatment is, by and large, comprised of a lateral cephalogram, a panoramic and occasionally selected intra-oral or other plane films. Cephalometry, or the measurement of the head, was developed as an anthropological technique to quantify shape and sizes of skulls. The discovery of X-rays by Röntgen in 1895 revolutionized medicine and dentistry. Almost 40 years later, traditional cephalometry in two dimensions, known as roentgenographic cephalometry, was introduced to the dental profession (Broadbent, 1931) and has since remained relatively unchanged. Since these early years, cephalograms have been used widely as a clinical and research tool for the study of craniofacial growth, development, and treatment effects and outcomes. Despite widespread utilization, as a diagnostic instrument, errors in cephalometry and its subsequent analysis are well documented (Mah and Hatcher, 2006, Macri and Athanasious 1997, Bookstein, 1983, Carlsson, 1967). Significant error is associated with ambiguity in locating anatomic landmarks because of the lack of well-defined anatomic features, outlines, hard edges and shadows, and variation in patient position. In addition, manual data collection and processing in cephalometric analysis have been shown to have low accuracy and precision (Macri and Athanasious, 1997). Specific landmarks such as Porion and Condylion cannot be accurately and consistently located on lateral cephalograms and deemed to be highly unreliable (Adwalla et. al., 1988).

Similar to lateral cephalograms, panoramic imaging is commonly used despite significant shortcomings. Panoramic radiographs provide some information about mandibular symmetry; present, missing, or supernumerary teeth; dental age; eruption sequence; and limited information about gross periodontal health, sinuses, root parallelism, and the temporomandibular joints. A panoramic projection also can reveal to some degree the presence of pathologic conditions and variations from normal. A point to stress, however, is that panoramic radiography has many shortcomings related to the reliability and accuracy of size, location, and form of the images created. These discrepancies arise because the panoramic image is made by creating a focal trough or region of focus to conform with the generic jaw form and size. Panoramic projection provides the best images when the anatomy being imaged approximates this generic jaw. However, any deviations from this generic jaw form result in a structure that is not centered within the focal trough, and the resultant image shows differences in size, location, and form compared with the actual object (Hatcher, 1997). In addition to the mismatch between panoramic focal trough and the imaged anatomy, the variations in horizontal and vertical X-ray beam angulations can lead to a false perception of the anatomic truth. A relevant clinical example of this phenomenon can occur when the panoramic projection is used to evaluate mesiodistal angulations or alignment of adjacent roots. The dental areas most susceptible to false interpretation of root alignment include the regions between the canine and first premolars in both arches and between the mandibular canines and the adjacent lateral incisors (McKee et. al., 2002).

Clinical measures of the effectiveness of any diagnostic test can be expressed as its sensitivity and specificity (Akobeng, 2007). Quite simply, sensitivity is the ability to determine the proportion of people with the disease as a positive result and specificity is the ability to determine the proportion of people without the disease as a negative result. Using these expressions of the effectiveness of diagnostic imaging, conventional imaging techniques do not fare very well. For TMJ evaluation, panoramic imaging showed a sensitivity of 0.64±0.11, while the panoramic-tomography mode was 0.55±0.11 and conventional tomograms were 0.58±0.15. In contrast the diagnostic accuracy of CBCT was 0.95±0.05 (Honey et. al., 2007). Localization of impacted maxillary canines using conventional imaging techniques is mixed. The parallax technique has a sensitivity of 89 percent for horizontal tube shift and a sensitivity of 46 percent for a vertical tube shift if the canine is buccally impacted, however if the canine is palatally impacted, both horizontal and vertical tube shift techniques have a sensitivity of only 63 percent (Armstrong, 2003). In situations where the mandibular third molar is in close proximity to the inferior dental canal, panoramic imaging shows 66 percent sensitivity, 74 percent specificity for the relationship of the tooth to the canal (Bell, 2003). The need for advancement in dental imaging is found in other aspects of dentistry as well. In endodontics, over a range of all tooth types, endodontist evaluators failed to identify at least 1 RCS in four out of 10 teeth using conventional imaging (Matherne et al, 2008). In contrast, a comparison of periapical films and CBCT of posterior maxillary teeth referred for apical surgery, in 156 roots, CBCT showed significantly more lesions (34 percent, p < 0.001) than periapicals (Low et. al., 2008).

CBCT Imaging
Three-dimensional imaging was primarily introduced to orthodontics with multi-slice computed tomography (MSCT) used in medical imaging (Bodner et. al., 2001). However, due to reasons of cost, access and radiation exposure (Mah et al, 2003) to the patient, MSCT is not generally used for routine orthodontic treatment. In recent years, a radiographic imaging modality was introduced to dentistry, named Cone Beam Computed Tomography (CBCT). CBCT was introduced for dentistry in 1998 with the NewTom QR-DVT 9000 (NIM s.r.l., Verona, Italy). After an initial period of slow adoption and the emergence of other CBCT manufacturers, this technology has become widely accepted in recent years with the number of CBCT units installed in the United States almost doubling each year since 2005.

Many different names have been suggested for this technology. Although functional nomenclature such as digital volume tomography (DVT) or cone-beam volumetric tomography (CBVT) (or simply volumetric tomography or cone-beam imaging) have been proposed in an effort to differentiate it from its high radiation conventional medical computed tomography (CT) counterpart, the original cone-beam computed tomography (CBCT) label seems to have been largely adopted by most users. Typically, many single 2D images are captured from predefined angles during a single isocentric rotation of the X-ray source/sensor unit. These raw images are then compiled into a 3D dataset using specialized reconstruction algorithms. The volume is often referred to as the “3D image,” although technically this is still a misnomer since the views on the computer screen are in reality still planar and not holographic projections. Nevertheless, the resultant “3D image” still offers many advantages over standard 2D X-ray radiographs:
  • 3D representation of dental and craniofacial structures
  • Custom image reformatting to provide optimal perspectives for visualization
  • Orthogonal images which do not contain magnification errors or projection artifacts
  • Management of superimpositions
  • Interoperability in DICOM format
  • Data can be utilized in other diagnostic, modeling and manufacturing applications
  • Radiation exposure within a similar range of other dental radiographic imaging devices, which is generally an order of magnitude lower than that of medical CT devices
There are numerous CBCT systems currently on the market, with an estimate of more than 30 CBCT device manufacturers worldwide as of early 2009. Configurations vary from system to system, with differences in: 1) patient position during image acquisition [supine position similar to medical CT devices, stand-up configurations patterned after common panoramic machines, seated units, or portable systems developed for intraoperative examination and mobile scanning centers], 2) image capture sensor type, 3) field of view, 4) X-ray generator, and 5) reconstruction algorithm and visualization software.

With the introduction of CBCT imaging to dentistry, 3D imaging has enjoyed widespread research and clinical interest. Like MSCT, CBCT does offer the advantages of volumetric imaging and 3D data that are free of projection errors, including magnification. Clinicians are learning how to take full advantage of the entire volume for comprehensive 3D analysis in orthodontics (Huang et. al., 2005). This includes detailed review of the craniofacial skeleton, dentition, soft tissues of the face, the temporomandibular joints, sinuses and airway space. Inter-structural relationships such as condylar position relative to occlusion or the dentition support of the lips have also been identified. For more specific situations such as an impacted canine, a detailed description of the maxillofacial complex with the use of CBCT as a diagnostic aid will enhance the understanding of the situation and help the clinician plan accordingly in the presence of any abnormal findings (Walker et. al., 2005). More recent CBCT devices offer smaller voxel volumes, 12- or 14-bit grayscale resolution, and improved software allowing for enhanced visualization. New software tools are available for clipping and other volume operations to remove superimposing and other structures, leading to better visibility of the structures of interest. In addition, new approaches to treatment simulation and prediction are in development. One of the more recent and exciting developments is the ability to superimpose two data volumes. This feature could provide new insights into growth, development, treatment effects and outcomes. In addition, it stands to conclusively answer some of the controversies in orthodontics regarding specific treatment claims and effects.

References
  1. Mozzo P, Procacci C, Tacconi A, Martini PT, Andreis IA. A new volumetric CT machine for dental imaging based on cone-beam technique: preliminary results. Eur Radiol 8:1558–64, 1998.
  2. Araki K, Maki K, Sekil K, Sakamakil K, Haratal Y, Sakaino R, Okano T, Seo K. Characteristics of a newly developed dentomaxillofacial X-ray cone beam CT scanner (CB MercuRayTM): system configuration and physical properties Dentomaxillofac Radiol 33:51-59, 2004.
  3. Sukovic P, Brooks S, Perez L, Clinthorne NH. DentoCAT - a novel design of a cone-beam CT scanner for dentomaxillofacial imaging: introduction and preliminary results. Int Congr Ser 1230: 700-705, 2001.
  4. Molteni R. The so-called cone beam computed tomography technology (or CB3D, rather!). Dentomaxillofac Radiol 37:477-478, 2008.
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  7. Mah JK, Danforth RA, Bumann A, Hatcher D. Radiation absorbed in maxillofacial imaging with a new dental computed tomography device. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 96:508-13, 2003.
  8. Ludlow, J.B.; Davies-Ludlow, L.E.; and Brooks, S.L.: Dosimetry of two extraoral direct digital imaging devices: NewTom cone beam CT and Orthophos Plus DS panoramic unit, Dentomaxillofac Radiol 32:229-234, 2003.
Source References
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Author’s Bio
James Mah, DDS, MSc, MRCD, DMSc, is an associate clinical professor at University of Southern California and University of Nevada, Las Vegas. He is the associate editor of Harvard Society for the Advancement of Orthodontics Bulletin and the technology editor for the Journal of Clinical Orthodontics. Dr. Mah is a member of the Pacific Coast Society of Orthodontists; the American Cleft Palate- Craniofacial Association; the American Society of Bone and Mineral Research; the International Association for Dental Research; and the World Federation of Orthodontists. His 3D work has been featured by various media outlets including the Los Angeles Times, The National Post, Tech TV and other magazines and journals. Mah is the recipient of numerous awards including the Moyers Symposium Edison Honor, University of Michigan (2002); the American Association of Orthodontists Foundation Corporate Center Award (2001); and the Cleft Palate Foundation Research Award (1998).
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