Advanced cross-sectional imaging techniques such as CT are used in dentomaxillofacial imaging to solve complex diagnostic and treatment-planning problems, such as those encountered in craniofacial fractures, endosseous dental-implant planning, and orthodontics, among others. With the advent of CBCT technology, cross-sectional imaging that had previously been outsourced to medical CT scanners has begun to take place in dental offices. Early dedicated CBCT scanners for dental use were characterized by Mozzo et al and Arai et al in the late 1990s. Since then, more commercial models have become available, inciting research in many fields of dentistry and oral and maxillofacial surgery. To date, multiple ex vivo studies have attempted to establish the ability of CBCT images to accurately reproduce the geometric dimensions of the maxillodental structures and the mandible. A relatively low patient dose for dedicated dentomaxillofacial scans is a potentially attractive feature of CBCT imaging. An effective dose in the broad range of 13–498 Sv can be expected, with most scans falling between 30 and 80 Sv, depending on exposure parameters and the selected FOV size. In comparison, standard panoramic radiography delivers 13.3 Sv and multi-detector CT with a similar FOV delivers 860 Sv. Image quality can vary considerably with dose; images acquired with higher radiation exposure often produce superior image quality. The discussion below reviews potential CBCT applications in the dentomaxillofacial regions. Most of this research remains preliminary; further prospective and outcomes-based research is required to make informed recommendations on the appropriate use of CBCT in dentomaxillofacial imaging.
Obstructive sleep apnea (OSA) is characterized by the episodic cessation of breathing during sleep and is frequently unidentified and undiagnosed. Conventionally, the most common tool for diagnosis of OSA is in-laboratory polysomnography; however, this technique is expensive, requires specialized resources, and is time-consuming for patients (Pack 2004; Flemons, Douglas, Kuna et al 2004). Although imaging is not generally used for OSA diagnosis, it can help identify airways at risk for obstruction and also patients whose airway anatomies may contribute to OSA (Hatcher 2010).
Cephalometric radiographs are advantageous to clinicians as an adjunctive diagnostic tool for OSA patients. CBCT presents an opportunity with three-dimensional images of the airway to serially examine individuals, acquire airway patency information, and improve the evaluation of sites of airway obstruction.
Cross-sectional imaging techniques can be an invaluable tool during preoperative planning for complicated endosseous dental implantation procedures. Conventional linear tomography and CT have traditionally been used in pre-surgical imaging, though the former has overlain ghosting artifacts and the latter has a higher high radiation exposure and cost. Practitioners have begun using office-based CBCT scanners in preoperative imaging for implant procedures, capitalizing on availability and low dosing requirements. A review by Guerrero et al outlines the clinical and technical aspects of CBCT, which have popularized this new technique. Preliminary evidence addresses the ability of CBCT images to characterize mandibular and alveolar bone morphology, as well as to visualize the maxillary sinuses, incisive canal, mandibular canal, and mental foramina, all structures particularly important in surgical planning for dental implantology. Several studies have described the 3D geometric accuracy of CBCT imaging in the maxillodental and mandibular regions as well.
Cross-sectional imaging affords overlay-free visualization of structural and anatomic relationships important for addressing many radiologic questions in orthodontics. The current standard of care for overlay-free imaging in orthodontics is conventional CT. Low-cost office-based CBCT imaging has recently been explored for orthodontic applications, including assessment of palatal bone thickness, skeletal growth patterns, dental age estimation, upper airway evaluation, and visualization of impacted teeth. Although preliminary results are encouraging, established cross-sectional techniques such as conventional CT provide superior image quality of dental and surrounding structures for advanced orthodontic treatment planning. Low dosing requirements appear to remain a benefit of CBCT when compared with conventional CT, with a routine orthodontic CBCT study delivering an effective dose of 61.1Sv compared with 429.7 Sv for multi-section CT. Lateral cephalograms deliver 10.4 Sv in comparison, though without the benefit of 3D structural visualization.
Morphologic changes of the temporomandibular joint (TMJ) as depicted with conventional MR imaging, CT, and radiographic imaging are often useful in diagnosing pathologic processes such as degenerative changes and ankylosis, joint remodeling after diskectomy, malocclusion, and congenital and developmental malformations. CBCT is a technique that has recently inspired research in TMJ imaging, though preliminary experiments have yet to translate into clinical studies. Several cadaver studies have explored the use of TMJ CBCT to assess peri-articular bony defects, flattenings, osteophytes, and sclerotic changes. Preliminary studies have also directly compared CBCT with radiography, multidetector row CT (MDCT), and linear tomography for detection of osseous abnormalities of the TMJ. A recent systematic review by Hussain et al suggests that axially corrected sagittal tomography is still the method of choice in the detection of periarticular erosions and osteophytes.
CBCT has been explored for applications in endodontics, including peri-radicular surgical planning, assessment of periapical pathology, and dentoalveolar trauma evaluation. The diagnostic properties of CBCT at the root apices and peri-radicular region have been reported in several studies. In retrospective cohorts and case reports, CBCT has been suggested as superior to periapical radiographs in the characterization of periapical lucent lesions, reliably demonstrating lesion proximity to the maxillary sinus, sinus membrane involvement, and lesion location relative to the mandibular canal. There may also eventually be a role for CBCT in early detection of periapical disease, which could lead to better endodontic treatment outcomes. Promising results have been demonstrated in studies characterizing CBCT images for endodontic surgical planning purposes as well.
To optimize the diagnostic value of the CBCT, the dentist must have mastery of human anatomy as seen in the axial, sagittal, and coronal views radiologically, as well as three-dimensionally.
The first reported applications of CBCT in periodontology were for diagnostic and treatment-outcome evaluations of periodontitis. Ex vivo studies later characterized the ability of CBCT to accurately reconstruct periodontal intrabony and fenestration defects, dehiscences, and root furcation involvements in comparison with radiography, MDCT, and histologic measurements. CBCT 3D geometric accuracy has been suggested to be equal to radiography and MDCT but with better observer-rated image quality than MDCT as well as superior periodontal-defect detection than radiography. Although periodontal bony defects are well visualized with CBCT, conventional radiography still affords higher quality bony contrast and delineation of the lamina dura. CBCT ex vivo visualization of the periodontal ligament and periodontal ligament space has been evaluated in comparison with radiography with mixed results, a more recent study suggesting that CBCT visualization is still inferior to that of radiography.
Head and Neck
As CBCT imaging systems have become more widely available, interest in the intraoperative and diagnostic CBCT applications in the extracranial head and neck regions has intensified. The reported high isotropic spatial resolution and relatively low dose requirements of CBCT are characteristics that have made it particularly attractive. In the head and neck region, a premium is placed on discriminating fine anatomic detail in territories where the vascular and bony structural anatomy is particularly complex. Potential applications in sinus, temporal bone, and skull base imaging have been explored. Head and neck CBCT studies visualizing the paranasal sinuses; temporal bones; maxillary sinus floor and alveolar process of the maxilla; and orbital floors respectively.
Sinus Imaging/Frontal Recess
Comparatively low dosing requirements, high-quality bony definition, and the compact design afforded by CBCT scanners have made them attractive for office-based and intraoperative scanning of the paranasal sinuses. To date, there have been few studies comparing image quality in paranasal sinus CBCT scans with that in MDCT. Alspaugh et al did directly compare the spatial resolution obtained with CBCT scans of the paranasal sinuses with that of 16- and 64-section MDCT scanners. They concluded that 12 line pairs per centimeter (lp/cm) isotropic spatial resolution could be obtained with an effective dose of 0.17 mSv compared with a dose requirement of 0.87 mSv for 11-lp/cm spatial resolution in a 64-section MDCT scanner.
To a large degree, evidence supporting sinus CBCT imaging has emerged from exploration of intraoperative CBCT applications in endoscopic sinus surgery (ESS). Both spatial and soft-tissue contrast was sufficient to aid surgical navigation in the frontal recess. More recent clinical studies have also provided qualitative evidence that intraoperative CBCT provides high-quality definition of bony anatomy, which can lead to refinement of surgical strategy. In a series of twenty-five patients undergoing ESS, Batra et al found that residual bony partitions and stent locations could be visualized with intraoperative CBCT scans, leading to surgical revision. CBCT has also been used recently to evaluate contrast delivery during sinus irrigation after ESS.
Preliminary evidence suggests that CBCT may be suited for specific imaging tasks in the context of intraoperative and perioperative bony structural evaluations, enabling low-dose assessment of individualized paranasal sinus anatomy, surgical outcomes, and stent placements. To our knowledge, there is no current evidence, however, supporting CBCT use in general diagnostic sinus imaging owing to lack of soft-tissue contrast resolution. Furthermore, significant complications of ESS, including encephalocele, subarachnoid hemorrhage, and meningitis are unlikely to be evaluated adequately with current CBCT image quality.
Temporal Bone/Lateral Skull Base
The temporal bone was one of the earliest targets for head and neck CBCT imaging. Specific applications have been explored, including post-procedural middle and inner ear implant evaluation, visualization of the reuniting duct in the inner ear, and intraoperative temporal bone surgical guidance.
Visions for future development
Further technical improvements to CBCT devices can be anticipated in the future. Advances in FP CBCT relate to detector design (Kalender and Kyriakou 2007, Gupta et al. 2008) and will consequently expand the applicability of FP CBCT. FP CSI detectors have a slower response than the proprietary ceramic detectors used in MDCT systems and the quantum efficiency of CSI detectors is also slightly slower. These two characteristics limit the temporal resolution and dynamic range, respectively, of FPDs compared with standard MDCT detectors (Orth et al. 2008). Possible improvements are multiple FPs to increase the volumetric coverage, or dual-source CT to provide faster scanning times and double the amount of spectral information (Gupta et al. 2008). Reducing the detector read-out time would decrease scanning time. An increased dynamic range would allow higher dosages and thus better soft tissue contrast (Bartling et al. 2007).
X-ray scatter in CBCT limits image quality significantly by reducing contrast and creating image artefacts (Siewerdsen et al. 2006). Physical modifications to the image acquisition equipment such as anti-scatter grids (Siewerdsen et al. 2004, Gupta et al.2006), scatter reduction algorithms (Ning et al. 2004, Gupta et al. 2006), beam filters (Gupta et al. 2006, Mail et al. 2009) and object-to-detector distance (i.e. air-gap), have been investigated as potential ways to minimize scatter in CBCT. Image reconstruction from cone-beam projections collected along a circle source trajectory is commonly done using the Feldkamp algorithm, which performs well only with a small cone angle. For that reason, variants of the Feldkamp algorithm have been developed for practical applications that involve large cone angles (Zhuang et al. 2008).
Dental CBCT unit manufacturers have already introduced artefact reduction algorithms within the reconstruction process. For example, instead of the Feldkamp back projection, an iterative reconstruction called algebraic reconstruction technique (ART) has been used (Scanora 3D). It requires fewer projections to perform the reconstruction. These algorithms reduce image-, noise-, metal-, and motion-related artefacts (Scarfe and Farman 2008).
The current literature on CT metal artefact reduction can be divided into iterative and projection modification methods (Zhang et al. 2007). The iterative method involves reconstruction of the CT image using only non-corrupted projections while discarding those projections affected by metal objects. In the projection modification method the metal shadows in the raw projection data are first segmented and then replaced using some estimated values. This latter method has been increasingly favored because of its simplicity and has also been used in CBCT applications.
Devices which allow variation of FOV and resolution, thus making possible task-specific protocols, are indicated in dental and maxillofacial imaging (Scarfe and Farman 2008). The so-called region of interest (ROI) imaging technique reduces radiation exposure to the patient, causes less scattering to the detector, and has the potential to increase the spatial resolution of the reconstructed images (Wiegert et al. 2005, Cho et al. 2007). Standards for image quality and dose for the various diagnostic tasks should be developed. Furthermore, multimodal imaging devices, including conventional panoramic and cephalometric options in addition to CBCT, will most probably be a future trend (Scarfe and Farman 2008).
The applications of CBCT technology in dentistry are seemingly endless. Using CBCT, subjective identification of anatomy and pathology relevant in dental practice can be readily achieved. To optimize the diagnostic value of the CBCT, the dentist must have mastery of human anatomy as seen in the axial, sagittal, and coronal views radiologically, as well as three-dimensionally. While dentists are traditionally accustomed to interpreting imaging made in their offices, the complexity of the multi-planar CBCT datasets exceed that of the two-dimensional flat film. It is likely that the standard of care for image interpretation may shift with the availability of CBCT technology in dental offices. Because dentists are legally responsible for the content of an entire image, and not just the teeth, dentists are relying upon the expertise of specialists in the area of oral and maxillofacial radiology for interpretation assistance.