Dental Cone Beam Computed Tomography

Function

Initially, medical computed tomography (CT) was used for 3D imaging in dental applications, but dental cone beam computed tomography (CBCT) quickly became the preferred option. This is because of CBCT's ability to produce volumetric images of the jaw bone at a reasonable cost and radiation dose, as well as its compact size, affordability, and ability to be located nearby or in-office.[2]

It was first used to evaluate impacted teeth, apical lesions, and mandibular and maxillary diseases.[1]

There are several models since the first one that continues 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]

Cone beam computed tomography has numerous advantages compared to other imaging techniques, such as CT scans, panoramic imaging, and intraoral imaging. These benefits have contributed to its widespread usage in the field of dentistry. Specifically, cone beam computed tomography has been effectively utilized in areas such as implantology, endodontics, orthodontics, and pathology assessment because it provides horizontal, vertical, and axial views of structures.

Cone beam computed tomography 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, Cone beam computed tomography provides a reduced average radiation dosage compared to CT scans.

Despite its numerous advantages, it is essential to acknowledge the limitations of cone beam computed tomography. For instance, it may have lower contrast and higher radiation dosages when compared to traditional radiographic techniques such as intraoral and panoramic radiography. Therefore, cone beam computed tomography 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 (CBCT) Work?

Cone beam computed tomography uses an imaging scanner designed for imaging the head and neck and can produce 3D scans of the maxillofacial skeleton. The machine used in cone beam computed tomography is similar in size to the one used for panoramic radiography. Instead of a linear array of detectors, cone beam computed tomography 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 area instead of a thin slice, the machine does not need to rotate as often as CT; it turns once, providing all the necessary information to reconstruct the region of interest (ROI). This technique allows dentists to obtain 2D reconstructed images in all planes and 3D reconstructions with minimal exposure to X-radiation.[1]The dental CBCT technology is specifically designed to produce high-quality images of the teeth, jaws, and face by capturing tomographic images from different 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 with the help of a computer.[1][4]

Issues of Concern

Differences Between Cone Beam Computed Tomography Scanners (CBCT) and Medical Computed Tomography Scanners (CT)

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

Patient Positioning

Depending on the cone beam computed tomography 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 picture blur and motion. Dentists should consider this when choosing the appropriate patient positioning for cone beam computed tomography 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 cone beam computed tomography imaging are rectangular cuboids with sides varying in length 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 amount during dental cone beam computed tomography imaging can significantly differ based 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 fulfills the imaging objective is crucial to reduce the exposure dose. It is essential to exercise caution when dealing with large-diameter FOVs, as the exposure dose can be higher than that of CT under low-dose conditions.[4]

The FOV should be adjustable to select an optimal FOV to meet 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 cone beam computed tomography voxel values can be unstable because the diameter of the FOV is generally smaller than that of the patient's head. As a result, a complete set of image data cannot be acquired, making it impossible to calculate mathematically correct CT 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 cone beam computed tomography can exceed this amount by more than ten times, even under low-dose conditions. Therefore, it is essential to exercise caution when using dental cone beam computed tomography imaging.[4]

Scanning Time

The scanning time in cone beam computed tomography goes from 5 to 40 seconds. The exposure times are less because of the pulsing of the X-ray beam, ranging from 1 second up 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 the radiography is insufficient to diagnose the problem and the patient does not require urgent irreversible surgery like tooth extraction, palliative treatment should be given while monitoring the patient's condition.

Dental cone beam computed tomography 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 cone beam computed tomography may be appropriate to provide safe and dependable care with 3D anatomic information. Neglecting to perform dental cone beam computed tomography when required would harm the patient, even if there's a risk of radiation exposure.

Other imaging techniques, like medical-grade CT or MRI, should be used to diagnose soft tissue pathology instead of dental cone beam computed tomography. The smallest possible FOV should be chosen during imaging to minimize radiation exposure.

A dental radiologist may be consulted to increase the diagnostic accuracy when imaging a large area and when a lesion suspected to be a tumor is found with small-field imaging.[4]

Cone Beam Computed Tomography General Recommendations

• 2D radiography or plain radiography is the imaging modality of choice. However, cone beam computed tomography should be considered when the diagnosis cannot be adequately made with 2D imaging. When utilizing cone beam computed tomography, it is vital to use established criteria to select an appropriate field of view (FOV).
• Conducting a comprehensive clinical assessment is crucial before utilizing cone beam computed tomography or any other radiation-based examination. Cone beam computed tomography, 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 cone beam computed tomography before implant surgery is more useful than post-implant imaging.
• The effective doses for dentoalveolar cone beam computed tomography range from 11 to 674 μSv. The effective doses for craniofacial cone beam computed tomography range from 30 to 1,073 μSv.
• Cone beam computed tomography is appropriate in cases where a tooth is impacted, infected, or missing, and 2D radiography fails to detect the underlying pathology. Cone beam computed tomography 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 utilize dose-sparing techniques that follow 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 the effective doses of radiation, thereby minimizing the potential long-term health risks associated with excessive radiation exposure.[7]

Clinical Significance

Caries and Periodontal Disease

Bitewings radiographs are the most appropriate method to evaluate for 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 done for another reason, the presence of caries must be evaluated too.[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, further supplemented by radiographic evaluation, where the latter may give more information that could influence the prognosis and treatment of the disease. Bitewings provide accurate geometry and details. Bitewings radiographs that have already been indicated for detecting caries can be used to assess bone levels around teeth without further radiation exposure.[9]

Digital radiographs may provide a better definition than conventional radiographs to evaluate 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 two-dimensional intraoral radiographs.[11]

Endodontics

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

The European Society of Endodontology recommends a small FOV CBCT in the following cases if evaluation with conventional radiographs is inconclusive or insufficient.[12]

• 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 like 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.

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, e.g., neurovascular structures, maxillary sinus, and adjacent teeth.[8] As with all radiological examinations, the decision to order a CBCT should be based strictly on the 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 like bone density, skeletal size, bone architecture, the 3D orientation of the trabecula, and matrix properties conform bone quality. It is not only a matter of mineral content but also of structure. 

Local patient factors like 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 four 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 CT 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 CT scans, CBCT does not allow for the measurement of bone density in Hounsfield units. Methods have been proposed to convert CT numbers measured on cone beam computed tomography scans to HU.[14]

For HUs on CBCT to be used, 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. Cone beam computed tomography provides a subjective assessment of bone quality, not an objective one.[14]

Computer-generated surgical guides can be fabricated by integrating cone beam computed tomography scans and computer-aided design 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 three types of computer-generated surgical guides available:

• Tooth-supported: they are used in partially edentulous cases.
• Mucosa supported: they are used primarily in fully edentulous cases and are 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, extending 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 CT before open sinus lift surgery is recommended to evaluate certain factors like membrane thickness, presence of sinus septa, alveolar antral artery trajectory, and residual bone height.[15]

Extraction of Teeth

Preoperative CBCT before tooth extraction is mainly related to impacted mandibular third molars due to 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 will provide information about the proximity of the tooth with 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

Cone beam computed tomography 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 on soft tissues, it should not be implemented if there is a suspicion of malignancy.[8]

Dental and Facial Trauma

The radiographic modality of choice to evaluate traumatic dental injuries (TDIs) are intraoral radiographs. Cone beam computed tomography may help diagnose root fractures, fractures of the alveolar bone, and displaced teeth.[8] CBCT has shown superior results for detecting root fractures compared to conventional radiographs.[18] However, the accuracy of the CBCT may be compromised if the suspected tooth has a metallic post in the root due to the formation of 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, e.g., fractures of the condyle, zygomatic arch, and some zygomatic complex fractures.[8]

Sinus Disease

Cone beam computed tomography can be an alternative to multi-detector computed tomography (MDCT) to evaluate chronic rhinosinusitis.[8] However, it is not recommended when malignancy or fungal infection is a concern as it provides small 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 when the soft tissues of the TMJ are to be evaluated.[20]

Enhancing Healthcare Team Outcomes

The application of cone beam computed tomography (CBCT) has grown exponentially across dentistry, impacting different specialties and fields. Cone beam computed tomography imaging provides accurate measurements, improves the localization of impacted teeth, gives us visualization of airway abnormalities, can be helpful to identify and quantify asymmetry, assess periodontal structures, identify endodontic problems, and is beneficial to view condylar positions and temporomandibular joint (TMJ) bony structures.[21]

Cone beam computed tomography is an excellent option when indicated and one of the greatest new technologies that have been introduced into dentistry in recent years. Dental healthcare workers need to be aware of its indications and applications. Therefore, the utilization of CBCT should be restricted to situations where its benefits outweigh the potential risks, and its usage should be limited to cases where it is justifiable. This will lead to better patient outcomes.