Cone-Beam Computed Tomography for Oral and Maxillofacial Imaging

The invention of computed tomography (CT) technique revolutionized diagnostic imag- ing. Compared to conventional X-ray imaging procedures, CT involves higher radiation doses. Recently, cone-beam CT (CBCT) specifically designed for maxillofacial imaging was introduced. CBCT technique is based on a cone-shaped X-ray beam centered on a two-dimensional (2D) detector. The detector system performs one rotation around the patient, producing a series of 2D images which are then reconstructed in a 3D data set. The contem porary knowledge regarding CBCT and its proper application guides the practitioner for improvement in diagnostic purposes and treatment planning. The aim of this chapter is to focus on the details, advantages, drawbacks, and clinical applications of CBCT as a headmost CT imaging technique in the oral and maxillofacial (OMF) region. The main clinical applica tions of CBCT in the OMF region are dentistry including dentoalveolar and maxillofacial surgery, orthodontics, endodontics, and periodontics; and otolaryngology. The aforementioned clinical use of CBCT was described in detail with illustrated sample cases. In most of the cases in OMF region, CBCT takes the place of multi-slice CT. Thus, clinicians should know the clinical applications and capabilities of CBCT technique with its drawbacks.


Introduction
Accurate diagnostic imaging is a key factor for diagnosis and treatment planning. The invention of computed tomography (CT) technique revolutionized diagnostic imaging. Since the inception of CT in the 1970s, it has become one of the commonly used imaging methods [1]. Threedimensional (3D) imaging provided by CT technology gives the opportunity to the clinician to examine the oral and maxillofacial (OMF) region without superimposition and distortion of the CBCT imaging is performed by using a rotating gantry to which an X-ray source and detector are fixed. Cone-beam machines radiate an X-ray beam shaped liked a cone or pyramid, rather than a fan, as in conventional CT machines. The X-ray source and detector rotate around a rotation abutment fixed within the center of the region of interest. The beam exiting the patient is captured on a 2D planar detector, usually an amorphous silicon flat panel or sometimes an image intensifier/charge coupled device (CCD) detector. During the rotation, multiple consecutive planar projection images of the field of view (FOV) are acquired in a complete, or sometimes partial, arc. This procedure varies from a traditional medical CT, which uses a fanshaped X-ray beam in a helical progression to obtain individual image slices of the FOV and then heaps the slices to get a 3D representation. Each slice requires a separate scan and separate 2D reconstruction. Because CBCT exposure combines the whole FOV, only one rotational turn of the gantry is necessary to acquire enough data for image reconstruction [6,7]. A cone-beam reconstruction process creates a 3D matrix that can be viewed as a series of 2D cross-sectional images-axial, sagittal, and coronal views. From this data set, the operator can also extract thick or thin, planar or curved reconstructions in any orientation. Axial planes are a series of slices from top to bottom in the volume. Sagittal planes are a series of 2D slices from left to right, and coronal planes are a series of 2D slices from anterior to posterior. In a multi-planar reformation (MPR) window, these three orthogonal planar views are related through intersection lines or crosshairs, allowing for straightforward orientation and navigation (Figure 1).
The dose associated with each scan is affected by a number of scan parameters selected by the practitioner, either manually or through preset exposure protocols. For most CBCT systems, the kVp is fixed, and the tube current (mA) and exposure time (s) can be varied depending on the desired image quality and patient size. After reconstruction, CBCT images can be manipulated in different ways to optimize the visualization of anatomical structures and lesions and to isolate certain parts of the image. Basic filtering can also be applied, both during and after reconstruction, in order to smooth or sharpen the image [8]. CBCT has some advantages and disadvantages over conventional CT. These items are summarized below [9][10][11][12][13][14].

Advantages of CBCT over conventional CT
• CBCT is less expensive and involves a smaller system, thus providing in-office imaging.
• The X-ray beam is limited, and lower radiation dose is used.
• CBCT has higher spatial resolution, providing better visualization of mineralized structures.
• Accurate images are obtained.
• The scan time is more rapid.
• The display modes are exclusive for oral and maxillofacial imaging.
• The imaging artifacts are fewer.
• CBCT has potential for vertical scanning in a natural seated position.

Disadvantages of CBCT over conventional CT
• Scattered radiation.
• Limited dynamic range of the X-ray detectors.
• Owing to the small detector size, the FOV, and scanned volume are limited.
• Limited soft tissue contrast.  • Bone density values without a linear correlation between different devices.
• Reduced image quality in regions close to high-density neighboring structures such as dental restorations and implants.
According to literature, the subjective image quality was lower on older CBCT units compared to multi-slice CT [15]. However, recent CBCT units with higher resolution showed opposite results [16]. Although new CBCT scanners with flat panel detectors seem to be less prone to metal artifacts, an important problem including susceptibility to movement artifacts still remains [9]. The effective radiation doses for various CBCT devices range from 52 to 1025 micro-sieverts [5]. It was reported that 3D volumetric images obtained with CBCT technology involved up to four times less radiation than conventional CT [17]. Although CBCT requires lower radiation doses compared to CT imaging, the aforementioned radiation doses are still higher than 2D imaging. In order to protect patients and staff during acquisition of images, the selection criteria of CBCT examination should weigh potential benefits against the risks associated with radiation dose. For this purpose, appropriate clinical usage, protective shielding, and the smallest FOV for diagnostic purposes should be obtained (Figure 2).

Applications of CBCT in oral and maxillofacial region
CBCT is used in all medical and dental disciplines practicing in OMF region, whereas early CBCT devices were dedicated to implantology and dental imaging; nowadays, applications include the whole face and skull. Image analysis function obtains quantifiable accurate data from the image for diagnostic and scientific purposes. Linear, curved, and angular measurements, area and volume calculation, and densitometric analysis can be performed [13,[18][19][20]. The main clinical applications of CBCT in the OMF region are: dentoalveolar and maxillofacial surgery, maxillofacial pathology, orthodontics, implantology, endodontics, periodontics, and otolaryngology. The CBCT technology can also be used in forensic medicine. The clinical use of CBCT will be reviewed below with illustrated sample cases:

Oral and maxillofacial surgery
The OMF surgery is the main discipline which uses the CBCT technology. In a literature review, it was reported that 41% of the scientific papers related to the clinical applications of CBCT dealt with the use for maxillofacial surgery [9]. The main topics may be summarized as dental implantology, impacted and supernumerary tooth, OMF pathology, maxillofacial trauma, temporomandibular joint (TMJ) disorders, dentofacial discrepancies, and cleft palate.

Dental implantology
During the preoperative planning of dental implant treatment, radiographic methods for assessment of bone quality and quantity are frequently used. For 2D assessment, orthopantomography (OPG) is frequently used. The American Association of Oral and Maxillofacial Radiology recommends cross-sectional views for evaluation of a potential implant site [14]. CBCT provides the 3D visualization of the alveolar bone height, width, and thickness, and spatial proximity of inferior alveolar and incisive canals, maxillary sinus, and nasal cavity (Figure 3). It is the contemporary method used for planning dental implant surgery and bone augmentation procedures ( Figure 4). As shown in our previous clinical study, accurate measurements can be performed on CBCT images for diagnosis, treatment planning, and evaluation of treatment results of bone augmentation and dental implants [21]. The CBCT images are free from distortion; however, errors in patient positioning may lead to inaccurate measurements which may cause damage to anatomical structures. The success of dental implant treatment is influenced by both the quality and quantity of available bone for implant placement. During the past few years, the concept of using CT-derived Hounsfield unit (HU) values had increasing popularity for quantitative assessment of bone density [22,23]. However, patients are exposed to high radiation during CT scanning. Besides quantitative assessment of bone, CBCT can also be used for evaluation of bone quality ( Figure 5). In recent studies, significant correlations between the density values of CBCT and HU values of CT were reported [24,25]. In contrast, controversial results concerning the assessment of bone density using CBCT were also reported in literature [13,26]. In our previous clinical studies, we observed significant correlation between CBCTderived bone density and dental implant stability parameters including insertion torque value and resonance frequency analysis [27,28]. Thus, it seems to be possible to predict bone density, initial implant stability, and possibility of immediate or early loading using CBCT scan prior to implant surgery.
Moreover, it is also possible to perform navigation-guided implant surgery by using specific software and CBCT imaging. The 3D specific navigation system provides surgeons an additional planning tool during implant surgery, offering instant and continuous visualization of drill tip and angle and their relations with neighboring vital structures that have to be respected in the three spatial planes in CBCT images. With the guidance of the dynamic navigation system, surgeon can monitor the 360° view of the relationship among implant drill, inferior alveolar nerve, maxillary sinus, nasal cavity, and buccal and lingual alveolar bone plates during surgery ( Figure 6).

Impacted and supernumerary tooth
The CBCT is also used for presurgical evaluation of impacted teeth, supernumerary teeth, and their relations with neighboring anatomic structures (adjacent teeth, inferior alveolar nerve,  mental nerve, and maxillary sinus). Thus, clinician is able to plan the surgery and inform the patients about possible risks (Figure 7). In a recent clinical study, Jawad et al. reported that CBCT provided improved detection rates (63% versus 45% for plain radiographs) of root resorption associated with impacted canines. The authors also introduced a new root resorption scale for CBCT imaging [29].

Oral and maxillofacial pathology
All kinds of pathologic lesions which affect bone tissue in the OMF region including infection, cysts, tumors, and osteonecrosis can be monitored by CBCT imaging. The CBCT assessment provides maxillofacial surgeon to visualize the accurate localization of pathologic entity and its relation with adjacent vital structures in multi-planar view (Figure 8). For medication-related osteonecrosis of the jaws (MRONJ) lesions, the innovative use of routine tumor-surveillance imaging in combination with CBCT imaging was described to provide a high-resolution 3D analysis [30]. The authors interpreted functional imaging information by fusing positron emission tomography/computed tomography (PET/CT) and single-photon emission computed tomography/computed tomography (SPECT/CT) data with CBCT data. Hence, the authors stated that this new composite image analysis, if validated, will facilitate surgical planning by demarcating MRONJ area. The 3D model preparation and adaptation of the reconstruction plates to the jawbone before surgery can also be possible with CBCT imaging for maxillofacial reconstruction patients (Figure 9).

Maxillofacial traumatology
Maxillofacial trauma patients can be assessed with CBCT to plan appropriate treatment method (Figure 10). The diagnostic performance of CBCT in detecting orbital floor fractures was reported to be better than ultrasonography. Moreover, it was also reported that CBCT could be used in detecting fractures as a reliable surrogate to CT [31]. The CBCT technology  can also be used in combination with specific computer software for preoperative virtual planning and fabrication of patient-specific reconstruction plate for mandibular fractures [32]. When CBCT and multi-detector CT were compared in diagnostic imaging of midface, it was concluded that CBCT provided better image quality at lower doses, comparable image quality at higher doses, and superior spatial resolution in standard-and reduced-dose settings [33]. However, in another recent study, it was concluded that CBCT was not optimal for postoperative facial imaging compared to multi-slice CT in terms of visualization of maxillofacial bony structures in the vicinity of osteosynthesis materials [34].

Temporomandibular joint disorders
Temporomandibular disorders associated with degenerative pathologies or abnormalities in the bony structures of condyle, glenoid fossa, and articular eminence such as cortical erosion, articular surface flattening, osteophytes, condiylar hyper-, hypo-, or aplasia, ankylosis, and coronoid process hyperplasia can be visualized with CBCT [9][10][11]. A CBCT imaging for TMJ complex requires less time and lower radiation doses, it provides the multi-planar views for both TMJs from a single 360° rotation scan, and it simplifies positioning of patient [11]. The linear, angular, and volumetric measurements can also be performed with CBCT imaging software for research purposes. The image-guided puncture technique for TMJ using CBCT can also be used to determine the optimum angle and distance in order to prevent middle cranial fossa damage [35]. These measurements can be used to produce stereographic models

Dentomaxillofacial discrepancies and cleft palate
The growth abnormalities of maxillofacial bones can be assessed by CBCT imaging (Figure 14). The treatment planning and success of the surgical treatments of alveolar cleft patients can be assessed with CBCT imaging (Figure 15). As reported in our previous study, it is possible to compare the bone density values of the cleft and non-cleft sites to evaluate the success of alveolar cleft repair using CBCT technology [36]. Moreover, 3D measurement of cleft area including volume of bone defect and outcomes after alveolar grafting in cleft lip and palate patients can also be assessed by CBCT imaging. The dentofacial discrepancies can be visualized, and virtual planning of orthognathic surgeries can be performed with CBCT imaging and special software. Recently, a novel technique for splintless orthognathic surgery, using CBCT imaging with computer-aided design/computer-aided manufacturing (CAD/CAM) technology and virtual planning software, was introduced [37].
The growth abnormalities of head and neck bones can also be scanned using CBCT technology ( Figure 16). As reported in our previous study, visualization of the styloid process elongation in detail including linear and angular measurements can also be performed with CBCT imaging [18].     After bone augmentation surgery.

Orthodontics
Radiographic analysis is an important aspect for diagnosis and treatment planning in orthodontics. CBCT imaging allows the radiographic assessments in detail with lower radiation doses and without any distortion and superimposition of the other structures. The airway analysis before and after orthognathic surgeries, growth assessment, accurate measurements of cleft area in cleft lip and palate patients, assessment of skeletal and dental structures, assessment of TMJ complex, treatment planning for orthognathic surgery, accurate estimation of space requirement for unerupted or impacted teeth, assessment of orthodontics-induced root resorption, determination of possible regions for mini screw placement, and linear and angular measurements for severe skeletal discrepancies can be performed with CBCT imaging.
The CBCT imaging can be used to assess the amount of interradicular bone, root proximity, the localization of maxillary sinus and inferior alveolar nerve, and density of the available bone, all of which are important in determining the stability and success of orthodontic mini screws. Due to the considerable variation of available bone thickness between individuals, a CBCT imaging is recommended in order to determine the maximum screw length [38]. A CBCT Figure 16. The 3D CBCT imaging of a patient with bilateral elongated styloid process (Eagle syndrome). Note that the length and medial angulations of the styloid process can be measured by using CBCT software.
imaging study indicated that vertical facial pattern of the patients should be taken into consideration when adjusting the insertion angle of mini screws at the maxillary buccal site [39].
CBCT can also be used in association with CAD/CAM technology for production of custommade orthodontic appliances [40].
In terms of evaluation of impacted teeth, small volume of CBCT can be used as a supplement to panoramic imaging in the following cases: when canine inclination in the panoramic X-ray exceeds 30°, when root resorption is suspected at adjacent teeth, and when the canine apex is not clearly seen in the panoramic X-ray [41].
The CBCT scans can be used to evaluate the outcomes of orthodontic treatments and orthognathic surgery. The 3D overlays of superimposed models and 3D color-coded displacement maps provided assessments of treatment changes, displacements of soft and hard tissue during postsurgical follow-up, and amount of relapse [42,43].
The landmark identification is greatly enhanced in CBCT images. It was reported that reproducibility of cephalometric measurements obtained from CBCT scans was better than that obtained from conventional cephalograms [44]. One of the application areas of CBCT for orthodontics is the growth assessment of patients. The cervical vertebra maturity assessment with CBCT provides reliable assessment of pubertal growth; thus, CBCT can be used to evaluate skeletal maturity for orthodontic treatment [45]. For airway analysis, lateral cephalograms have been routinely used. Axial cuts of 3D CBCT scans provide soft tissue points which are more clearly visible in CBCT sections compared with conventional radiography, thereby enhancing airway assessment [46]. The CBCT-assisted airway analysis also facilitates the diagnosis and treatment planning of obstructive sleep apnea (OSA) [47]. A recent CBCT imaging study concluded that 3D image reconstruction accurately confirmed morphological changes in the upper airway during oral appliance therapy of patients with OSA [48]. In terms of airway analysis with CBCT technology, there are controversial conclusions in literature. A recent systematic review shows that upper pharyngeal airway analysis using CBCT is a reliable method; however, there are some significant limitations including lack of manual orientation of images and selection of threshold sensitivity. Thus, further researches are necessary to adequately establish the reliability of airway analysis with CBCT imaging [49]. A CBCT image used for airway analysis is shown in Figure 17.

Periodontology
The role of CBCT in the diagnosis of periodontal diseases was studied in literature. CBCT displays 2D and 3D images that are necessary for the diagnosis and treatment planning of intrabony defects, furcation involvements, and buccal/lingual bone destructions [50]. In a recent experimental animal study, the diagnostic value of CBCT and digital intraoral radiography for detection of periodontal defects including furcation involvements, one-, two-, three-wall and trough-like intrabony defects, fenestration, and dehiscence were compared. It was concluded that CBCT was superior to digital radiography for detection of grade 1 furcation involvements, three-wall defects, fenestrations, and dehiscences [51]. It is also clinically reported that CBCT imaging provides detailed information about furcation involvement and reliable basis for decision of periodontal treatment [52]. In literature, most of the studies concerning the accuracy of CBCT in periodontal diagnosis assessed the efficiency of CBCT in bone defects. However, in our recent study, we concluded that gingival soft tissue thickness and acellular dermal grafts can be consistently evaluated with CBCT technique [53]. The detailed diagnostic imaging of periodontal diseases as well as peri-implantitis may be performed with CBCT technology (Figures 18 and 19). Literature shows that optimal detection of peri-implant bone loss is achieved using the smallest FOV, the highest number of acquisition frames, and the smallest voxel [54]. When the CBCT-derived features of peri-implantitis defects compared to the corresponding histomorphometric findings, it is concluded that CBCT represents an accurate diagnostic tool to estimate the histological extent of peri-implantitis [55]. When the performances of different radiographic techniques (intraoral radiography, OPG, CBCT, and CT) in detecting peri-implant bone defects were compared, the highest sensitivity was found with intraoral radiography and CBCT, and the highest specificity was found with intraoral radiography, while CT demonstrated the lowest performance [56].

Endodontics
The CBCT imaging for endodontic purposes provides the clinicians' wide view in the visualization of periapical lesions, internal or external root resorption, vertical root fractures, and accessory root canals. It was found that CBCT assessment of changes in periapical lesion and mucosal thickening dimensions may reveal useful information regarding endodontic treatment success [57]. The CBCT imaging also guides the clinicians for planning endodontic surgery and in elucidation of causes of fail after endodontic treatment [10,58]. However, there are two major disadvantages concerning the utilization of CBCT in endodontics: The increased radiation doses, compared to 2D imaging methods, limit its routine usage. Thus, benefits    getting with CBCT imaging should be carefully evaluated on an individual basis to protect the patients. The other disadvantage is that the radiopaque filling materials and posts crate artifacts which may compromise the diagnosis.
In a recent clinical study, it was reported that CBCT scans had 93% sensitivity, 78% specificity, and 88% accuracy for detection of vertical root fractures in endodontically treated teeth [59]. When CBCT and digital periapical radiography were compared in detecting mandibular molar root perforations, in the non-obturated root canals, the sensitivity and specificity of CBCT scans in perforation detection were better than those of three-angled periapical radiographs. However, in obturated root canals, periapical radiography was reported to be more trustworthy than CBCT for perforation detection [60]. In a recent study, periapical radiographs and CBCT were compared in detecting fractured instruments in root canals with and without filling. The results showed that in the absence of filling, accuracy values were similar in all imaging techniques. In the presence of filling, CBCT had low accuracy [61]. In our recent clinical study, we observed that preoperative CBCT examination demonstrated positive contributions to the endodontic surgery of maxillary first molar teeth [62]. Maxillary posterior teeth have close relationship with maxillary sinus. This may cause the peri-radicular infection to destroy cortical border of the maxillary sinus and spread into the sinus. Such cases may make the clinician to do false or missing diagnosis. In such cases, CBCT imaging allows the practitioner to do appropriate diagnosis of the peri-radicular lesion and its relationship with the adjacent anatomic structures [63]. A CBCT image acquired for planning of endodontic surgery is shown in Figure 20. In conclusion, according to the contemporary literature, the decision to perform a CBCT examination in endodontics should be kept in limited due to its low accuracy in diagnosis. However, utilization of CBCT may give more benefits in planning endodontic surgery.

Otolaryngology
Depending on the FOV used, CBCT images may show partial or the entire nasal cavity, paranasal sinuses, airway, cervical vertebrae, and temporal bone. In fact, specific ear, nose, and throat imaging programs have been increasingly included in CBCT systems, suggesting that CBCT may at some point entirely replace medical CT imaging in certain otolaryngologyrelated applications [64]. In terms of otolaryngology, CBCT imaging can be used to assess airway, paranasal sinus pathologies, nasal polyps, temporal and frontal bone anatomy, middle ear, and cochlear implantation [9,[65][66][67][68]. As an imaging guidance, CBCT can be used to treat lymphatic leakage after thyroidectomy [69]. The CBCT-based percutaneous image-guided technique may provide mini invasivity and identification of the anatomy and site of the leakage. An example of CBCT imaging used for detailed assessment of maxillary sinus volume is shown in Figure 21.

Forensic medicine
One of the contemporary application fields of CBCT for maxillofacial imaging is forensic medicine. Age estimation of individuals is an important aspect of forensic science. The CBCT can also be used as new method of age estimation by measuring the pulp-to-tooth area ratio  in 3D images in living individuals [70,71]. The forensic age estimation can also be performed using CBCT-derived analysis of spheno-occipital synchondrosis [72].
Moreover, the CBCT-derived anthropometric measurements on mandibular images can be used for sex estimation in forensic settings [73]. Besides age and sex estimation, CBCT can be used in forensic science for identification of unknown human bodies through frontal sinus 3D superimposition technique [74].

Conclusion
In the present chapter, a review of literature related to the clinical applications of CBCT technique for oral and maxillofacial imaging was undertaken with illustrated sample cases. Tremendous advancements have been acquired after the introduction of CBCT imaging technology especially for oral and maxillofacial practice. The contributions of CBCT for maxillofacial imaging have been demonstrated in several studies for diagnosis, treatment planning, evaluation of treatment outcome, and research purposes. The widespread use of CBCT in maxillofacial region represents the most important advance in diagnostic radiology without disadvantages of multi-slice CT especially including high radiation dose and increased cost. In most of the cases in OMF region, CBCT takes the place of multi-slice CT. Dentists and clinicians dealing with this field should have the knowledge of working principles, requirements, appropriate indications, clinical benefits, drawbacks, and hazardous effects of CBCT technology for proper utilization. In literature, there were inconsistencies and discrepancies about the CBCT device settings, properties, radiation doses, image acquisition protocol, and estimation of bone density which confuse the readers. The most common clinical applications of CBCT were in OMF surgery including implantology and impacted teeth. The subjective image quality was higher in multi-slice CT than in older CBCT units. However, the recent CBCT units showed opposite results. Moreover, new CBCT units with flat panel detectors seem to be less prone to metal artifacts. The CBCT provides less radiation than multi-slice CT but more than panoramic X-ray. Thus, it is crucial that the ALARA principle ("As Low As Reasonably Achievable" radiation dose) should be respected.