Dental Cone Beam Computed Tomography

Function

Initially, medical computed tomography was used for 3D imaging in dental applications, but dental CBCT quickly became the preferred option. This is because of CBCT's ability to produce volumetric images of the jawbone at a reasonable cost and radiation dose, as well as its compact size, affordability, and ability to be located nearby or in the office.[2] It was first used to evaluate impacted teeth, apical lesions, and mandibular and maxillary diseases.[1] There have been several models since the first, and they continue to improve. Now they can rotate around the head in a single scan and get 360° pictures using only 17 seconds of accumulated exposure time.[1]

CBCT has numerous advantages over other imaging techniques, including computed tomography, panoramic imaging, and intraoral imaging. These benefits have contributed to its widespread use in dentistry. Specifically, CBCT has been effectively utilized in areas such as implantology, endodontics, orthodontics, and pathology assessment because it provides horizontal, vertical, and axial views of structures. CBCT is also advantageous because it is less expensive, requires less space, has a limited field of view, and has a shorter scanning time. Most importantly, CBCT provides a lower average radiation dose than computed tomography scans. Despite its numerous advantages, it is essential to acknowledge the limitations of CBCT. For instance, it may have lower contrast and higher radiation doses than traditional radiographic techniques such as intraoral and panoramic radiography. Therefore, CBCT should be restricted to situations where its benefits outweigh the potential risks. Its usage should be limited to cases where it is justifiable.[3]

How Does Cone Beam Computed Tomography Work?

CBCT uses an imaging scanner designed for imaging the head and neck and can produce 3D scans of the maxillofacial skeleton. The CBCT machine is similar in size to the one used for panoramic radiography. Instead of a linear array of detectors, CBCT machines use a 2D planar sensor. x-rays are emitted as a large cone covering the area of the head being examined. Since the cone beam irradiates a large volume rather than a thin slice, the machine does not need to rotate as often as a computed tomography scanner; it turns once, providing all the necessary information to reconstruct the region of interest. This technique allows dentists to obtain 2D reconstructed images in all planes and 3D reconstructions with minimal exposure to x-radiation.[1] Dental CBCT technology is specifically designed to produce high-quality images of the teeth, jaws, and face by capturing tomographic data from multiple angles. An x-ray tube and 2D sensor rotate around the patient's head from 180 degrees to 360 degrees to gather imaging data. These images are then reconstructed into tomographic images using a computer.[1][4]

Issues of Concern

Differences Between Cone Beam Computed Tomography Scanners and Medical Computed Tomography Scanners

• CBCT scanners are less expensive than computed tomography scanners. They cost approximately 3 to 5 times less.
• CBCT scanner equipment is smaller and lighter.
• CBCT scanners have a better spatial resolution (smaller pixels).
• The room does not have to be at a particular temperature (cold) for CBCT scanners. They can be installed in a dental office.
• CBCT scanners do not require the electrical power that computed tomography scanners do.
• No floor strengthening is required for CBCT scanners.
• CBCT scanners are easy to operate.[1]

Patient Positioning

Depending on the CBCT machine, patients may stand, sit, or lie on a table. In-office 2D imaging usually involves sitting or standing. For 3D cone-beam imaging, minimizing patient movement is instrumental in obtaining high-quality images and reducing image blur and motion artifacts. Dentists should consider this when choosing the appropriate patient positioning for CBCT imaging.[1] In most cases, CBCT is taken while the patient is sitting or standing. However, some devices may require the patient to lie supine for imaging. Nowadays, most devices can take both panoramic radiographs and CBCTs.[4]

Exposure Dose

Voxels in dental CBCT imaging are rectangular cuboids with side lengths ranging from 0.08 to 0.4 mm. The field of view (FOV) width can range between 4 to 20 cm, and its height can range from 3 to 20 cm. The tube voltage can be set between 60 and 120 kV, and the current can range from 1 to 10 mA. The duration of each imaging session can vary between 5 to 40 seconds.[5] These parameters differ considerably depending on the device and its release date.

The exposure dose during dental CBCT imaging can vary significantly depending on the imaging conditions. The effective dose from a single imaging session can range between 10 to 1000 μSv. The exposure dose primarily depends on the lateral area of the FOV, which is the product of its height and width. Therefore, selecting the smallest FOV that meets the imaging objective is crucial for reducing the exposure dose. It is essential to exercise caution when using large-diameter FOVs, as the exposure dose can exceed that of computed tomography under low-dose conditions.[4]

The FOV should be adjustable to select an optimal value that meets the imaging objective. A small FOV can be set to visualize a few teeth, and a much larger FOV can be chosen when imaging the entire head.[4] In principle, dental CBCT voxel values can be unstable because the FOV diameter is generally smaller than the patient's head. As a result, a complete set of image data cannot be acquired, making it impossible to calculate mathematically correct computed tomography values for image reconstruction. Additionally, while the effective doses for conventional intraoral, panoramic, and cephalometric radiography range from 1-8 µSv, the exposure dose from dental CBCT can exceed this amount by more than 10 times, even under low-dose conditions. Therefore, it is essential to exercise caution when using dental CBCT imaging.[4]

Scanning Time

The scanning time in CBCT goes from 5 to 40 seconds. The exposure times are shorter because of the pulsing of the x-ray beam, ranging from 1 second to 40 seconds.The times differ between scanners from a few seconds to several minutes, depending on the model.[6]

Procedure

The healthcare provider must conduct a thorough interview and document the patient's medical history. If the lesions appear restricted to teeth, jaw, or other dental hard tissues, intraoral or panoramic radiography should be done to obtain the necessary information for an accurate diagnosis. If the knowledge gained from radiography is insufficient to diagnose the problem and the patient does not require urgent irreversible surgery, such as tooth extraction, palliative treatment should be provided while monitoring the patient's condition.

Dental CBCT should not be performed at this stage unless the results would change the treatment plan. However, if vague symptoms persist or irreversible treatments are necessary, dental CBCT may be appropriate to provide safe and dependable care with 3D anatomic information. Neglecting to perform dental CBCT when required would harm the patient, even if there's a risk of radiation exposure.

Other imaging techniques, such as medical-grade computed tomography or MRI, should be used to diagnose soft-tissue pathology rather than dental CBCT. The smallest possible FOV should be chosen during imaging to minimize radiation exposure. A dental radiologist may be consulted to increase diagnostic accuracy when imaging a large area and when a lesion suspected to be a tumor is detected on small-field imaging.[4]

Cone Beam Computed Tomography General Recommendations

• 2D radiography or plain radiography is the imaging modality of choice. However, CBCT should be considered when the diagnosis cannot be adequately made with 2D imaging. When utilizing CBCT, it is vital to use established criteria to select an appropriate field of view (FOV).
• Conducting a comprehensive clinical assessment is crucial before utilizing CBCT or any other radiation-based examination. CBCT, in particular, is associated with a higher dose of x-rays, and therefore, it is essential to exercise caution while determining the appropriate FOV to scan. When a small or medium FOV is sufficient for the intended purpose, it is advisable to avoid using a large FOV.
• Imaging with CBCT before implant surgery is more useful than post-implant imaging.
• The effective doses for dentoalveolar CBCT range from 11 to 674 μSv. The effective doses for craniofacial CBCT range from 30 to 1,073 μSv.
• CBCT is appropriate when a tooth is impacted, infected, or missing and 2D radiography fails to detect the underlying pathology. CBCT can be used for pre-implant planning, preoperative evaluation, and postsurgical assessment in various oral-surgical, periodontal, endodontic, restorative, and prosthodontic scenarios.
• To reduce radiation exposure in children and adolescents, it is crucial to use dose-sparing techniques that adhere to the ALARA (As Low As Reasonably Achievable) principle. This principle involves minimizing radiation exposure to the lowest possible level while achieving the intended diagnostic result. Using such techniques can significantly reduce effective radiation doses, thereby minimizing the potential long-term health risks associated with excessive exposure.[7]

Clinical Significance

Caries and Periodontal Disease

Bitewing radiographs are the most appropriate method for evaluating dental caries and should be considered even in preschool children. CBCTs should not be used as a routine method for detecting caries; however, if a CBCT is performed for another reason, the presence of caries must also be evaluated.[8] Notably, CBCT provides information in 3 dimensions but has a lower resolution than intraoral radiographs. Furthermore, metallic restorations in the path of the X-ray beam produce dark stains, creating caries-like radiolucencies on the crowns of other teeth or even masking true carious lesions.[8]

The diagnosis of periodontal disease is based mainly on clinical examination, supplemented by radiographic evaluation, which may provide additional information that could influence the prognosis and treatment of the disease. Bitewings provide accurate geometry and details. Bitewing radiographs that have already been indicated for caries detection can be used to assess bone levels around teeth without additional radiation exposure.[9] Digital radiographs may provide better definition than conventional radiographs for evaluating alveolar bone levels.[10] Full-mouth periapical and panoramic radiographs are used to visualize the periapical tissues and the entire length of the roots and stage periodontal disease. CBCT is not recommended as a routine method to evaluate periodontal bone support. However, CBCT is more accurate in assessing bone defects and furcation lesions than conventional 2-dimensional intraoral radiographs.[11]

Endodontics

A small-FOV CBCT should be considered in endodontics when lower-dose conventional radiography does not provide sufficient information and when the use of CBCT is likely to change the diagnosis and treatment plan.[12] The potential benefits of CBCT over conventional radiography must justify the higher radiation exposure.[12]

According to the European Society of Endodontology, small-FOV CBCT is indicated in the following situations when conventional radiographic evaluation is inconclusive or inadequate:

• Assessment and management of dentoalveolar trauma.
• Evaluation of complex root canal systems before endodontic therapy.
• Inspection of complex root canal anatomy.
• Evaluation of endodontic complications, such as post-perforation.
• Evaluation of root resorption.
• Identification of obliterated root canals.
• Detection of periradicular bone changes that suggest root fractures.
• Presurgical review before endodontic surgery.[12]

Implantology

Dental implants are placed to replace missing teeth. Currently, CBCT is the imaging modality of choice before dental implant placement.[5] It can be used for comprehensive digital treatment planning and constructing surgical guides for guided surgery.[5] A radiographic examination is required to assess the quantity and quality of the remaining bone and ensure the correct implant position in the alveolar bone without compromising important anatomical structures, eg, neurovascular structures, maxillary sinus, and adjacent teeth.[8] As with all radiological examinations, the decision to order a CBCT should be based strictly on diagnostic and treatment-planning needs, with a conscious effort to minimize patient radiation exposure.[13]

Bone quality and reports on implant success and failure are used in implant treatment. The quality and quantity of bone available at the implant site are critical local patient factors in determining the success of dental implants.[14] Factors such as bone density, skeletal size, bone architecture, the 3D orientation of the trabeculae, and matrix properties contribute to bone quality. It is not only a matter of mineral content but also of structure. 

Local patient factors, such as the quality and quantity of bone available at the implant site, are crucial in determining the success of dental implants. Bone quality is categorized into 4 groups. An implant placed in type 4 bone (very thin cortical bone with low-density trabecular bone with poor strength) has a higher chance of failure. This type of bone is often found in the posterior maxilla, and some studies report higher implant failure rates in this region.[14]

Bone density can be obtained from computed tomography units and expressed in Hounsfield units (HU). This is not part of the system international (SI) system but is a practical unit representing the relative deviation of the measured linear attenuation of material from that of water. Unlike computed tomography scans, CBCT does not allow for the measurement of bone density in Hounsfield units. Methods have been proposed to convert computed tomography numbers measured on CBCT scans to HU.[14]

For HUs used in CBCT, the accuracy of the HU should be known. With more advanced CBCT software and methods, it should be possible to improve the accuracy of CBCT HU values when determining bone densities at implant sites. CBCT provides a subjective assessment of bone quality, not an objective one.[14] Computer-generated surgical guides can be fabricated by integrating CBCT scans and computer-aided design and manufacturing technology. The planned implant's type and size, position within the bone, relationship to the planned restoration and adjacent teeth or implants, and proximity to vital structures can be determined before surgery.[14]

There are 3 types of computer-generated surgical guides available:

• Tooth-supported: they are used in partially edentulous cases.
• Mucosa-supported: used primarily in fully edentulous cases and designed to rest on the mucosa.
• Bone supported: can be used in partially or fully edentulous cases, but they are used mainly in fully edentulous instances in which significant ridge atrophy is present, and good seating of a mucosa-supported guide is questionable.[14]

Maxillary Sinus Floor Elevation

In the posterior maxilla, tooth replacement with dental implants requires sinus lifting surgery when the maxillary sinus is pneumatized and extends toward the alveolar process. This surgery increases bone quality and quantity in the maxilla's posterior region. If the amount of bone between the ridge crest and the maxillary sinus floor is inadequate, <5 mm, an open sinus lift procedure is indicated. Preoperative CBCT or computed tomography before open sinus lift surgery is recommended to evaluate factors such as membrane thickness, the presence of sinus septa, the alveolar antral artery trajectory, and residual bone height.[15]

Extraction of Teeth

Preoperative CBCT before tooth extraction is mainly indicated for impacted mandibular third molars to reduce the risk of injuring the inferior alveolar nerve (IAN) during surgery.[8] However, the radiograph of choice before removing mandibular third molars is orthopantomography. This x-ray provides information about the proximity of the tooth to the IAN. The most common radiographic signs associated with IAN injury are the darkening of the roots of the third molar, interruption of the cortical lines of the canal, and diversion of the canal.[16] CBCT imaging must only be indicated in specific cases where the operator has a clinical question that panoramic or intraoral radiographs cannot answer.[17] Up-to-date data shows that using CBCT before removing mandibular third molars does not reduce nerve injury or lead to better patient outcomes than panoramic radiographs.[17]

Orthodontics

Small FOV CBCT may be indicated in the following cases as part of orthodontic treatment planning when conventional radiographs do not provide enough information:[8]

• Unerupted permanent maxillary canines.
• Dilacerated teeth.
• Unerupted or supernumerary teeth that are close to the inferior alveolar canal.
• Clefts that may require grafting.

Large FOV CBCT is sometimes indicated in orthognathic surgery planning where 3D data are required.[8]

Pathological Lesions of The Jaws

CBCT may help evaluate large odontogenic and non-odontogenic cysts and benign tumors of the jaws. CBCT can show the extent of the lesion and the closeness to essential structures, such as the maxillary sinus; it may also help plan the surgical approach.[8] It is noteworthy that because CBCT provides very little information about soft tissues, it should not be used if there is suspicion of malignancy.[8]

Dental and Facial Trauma

The radiographic modality of choice for evaluating traumatic dental injuries (TDIs) is intraoral radiography. CBCT may help diagnose root fractures, alveolar bone fractures, and displaced teeth.[8] CBCT has shown superior results for detecting root fractures compared to conventional radiographs.[18] However, the accuracy of CBCT may be compromised if the suspected tooth has a metallic post in its root, due to radiographic artifacts.[8] CBCT should be reserved for TDIs where the clinical findings and the information provided by conventional radiographs do not is insufficient to allow for correct management.[19] CBCT can be implemented to evaluate facial trauma when soft tissue detail is not required, eg, fractures of the condyle, zygomatic arch, and some zygomatic complex fractures.[8]

Sinus Disease

CBCT can be an alternative to multi-detector computed tomography (MDCT) for evaluating chronic rhinosinusitis.[8] However, it is not recommended when malignancy or fungal infection is a concern, as it provides little information on soft tissues.[8]

Temporomandibular Joint (TMJ)

Most patients with TMJ symptoms suffer from internal disc derangement or myofascial pain, where radiography does not tend to provide extra helpful information.[8] CBCT is effective in detecting condylar osteoarthritis and rheumatoid arthritis.[8] MRI is the method of choice for evaluating the soft tissues of the TMJ.[20]

Enhancing Healthcare Team Outcomes

The application of CBCT has grown exponentially across dentistry, impacting different specialties and fields. CBCT imaging provides accurate measurements, improves the localization of impacted teeth, provides visualization of airway abnormalities, helps identify and quantify asymmetry, assesses periodontal structures, identifies endodontic problems, and is beneficial for viewing condylar positions and temporomandibular joint (TMJ) bony structures.[21] CBCT is a valuable imaging modality and one of the most important recent advances in dentistry. Dental professionals should understand its appropriate indications and applications. Its use should be reserved for cases in which the expected diagnostic or therapeutic benefit outweighs the associated risks. When applied judiciously, CBCT can support improved patient outcomes.