Cone beam computed tomography (CBCT) scanners are increasingly utilized in dental imaging. Potential uses include planning dental implant treatment, endodontics, orthodontics, and maxillofacial surgery. The scanner allows the clinician to visualize abnormal teeth and to evaluate the jaws and facial structure.
Development of specialized CBCT scanners for dentistry began in the late 1990s, and the scanners have been available in the United States since early this century. In 2015 it was estimated that 13% of dental practices in the United States had already purchased CBCT scanners. Worldwide, sales of CBCT scanners account for approximately 18% of the revenue for dental imaging equipment and are predicted to increase by approximately 5% year-on-year.
CBCT scanners are a fantastic tool when utilized correctly. CBCT is highly accurate, providing the clinician with detailed three-dimensional data in coronal, sagittal, and axial planes. Radiation exposure from CBCT is ten times less compared to conventional CT scans. The dimensional accuracy provides the clinician with an imaging modality that has minimal distortion.
How Do They Work?
The fundamental principle for a CBCT scanner is to generate x-rays in a tube containing two oppositely charged electrodes (an anode and a cathode) and an electrical circuit and separate them by a vacuum. When an electrical current is applied to the cathode, the filament heats up, inducing the electron release. The high voltage generated between the electrodes causes released electrons to accelerate toward the anode, colliding with high speed at the focal point. In a CBCT scanner, the focal point is typically 0.5 mm wide and determines the image sharpness. Most of the energy generated through this action is lost as heat, but a small part is converted into x-rays. The x-ray beam is concentrated by being passed through a lead alloy collimator. Most collimators have predefined field of view (FOV) openings. Collimation is one of the main differences between a cone beam CT scan and an ordinary two-dimensional x-ray. The x-ray detector in a CBCT scanner converts the x-ray photons into an electrical signal.
Most CBCT scanners have a rotatable C-shaped arm connecting the x-ray source and a detector. The arm usually rotates horizontally around the seated or standing patient’s head, which is comfortably stabilized in a holder. Single images are taken at specific intervals, like lateral cephalometric images.
The images are quick to capture. Typical rotation times vary between 10 and 40 seconds. During rotation, the cone-shaped x-ray beam produces several hundred 2-D x-ray projections that the detector acquires for reconstruction into a three-dimensional image. The series of images is called the projection data and is utilized by software programs that use algorithms to generate 3-D volumetric data. Scanners designed so that the patient can stand usually accommodate wheelchairs as well, taking up no more space than a standard panoramic x-ray device. Scanners that have a built-in table or chair require a slightly larger area.
Determining If the Patient Requires a 3-D Cone Beam CT Scan
The radiation produced by CBCT scanners is minimal, but the FDA recommends that this imaging modality only be performed when clinically necessary. Factors to consider include:
Determining if the information provided cannot be obtained using other imaging modalities.
Discussing the benefits and potential risks with the patient before taking a CBCT scan to make sure they have a clear understanding of the examination.
Optimizing exposure settings during the scan to provide the lowest radiation dose that will still give an adequate image quality.
Points to Consider When Taking a CBCT
When capturing data, make sure the correct field of view (FOV) is selected before scanning. Ideally, the FOV should be adjustable in width and height to reduce radiation exposure to the patient. When selecting a wider FOV, it’s important to remember that the scan may cover the skull base and areas of the spine. Choosing a smaller FOV reduces radiation exposure to the patient, but also reduces the amount of data collected.
The resolution of the scan depends on the resolution and quality of the flat panel detector, the number and spacing of the basis images from which the data is generated, and the sophistication of the software used to reconstruct the images. Another factor that affects the resolution is the power of the x-ray source; increased resolution results in increased radiation dose to the patient because longer exposure times are required to collect additional data for a more detailed reconstruction. Although a higher resolution scan may be desirable, it might be possible to achieve the objectives of the scan by using a lower resolution setting, thus reducing the patient’s radiation exposure. Matching the resolution and FOV to the intended usage for the scan minimizes exposure of radio-sensitive organs, including the salivary and thyroid glands.
Generally, occupational exposure from CBCT should not be a problem, provided the equipment has been correctly installed. The scanner must be housed with the appropriate shielding. A dedicated room with a lead or brick screen for the operator to stand behind is usually required. When taking conventional 2-D dental x-rays, many clinicians stand a safe distance of more than 1.5 m away from the machine, but with a CBCT machine it’s necessary to stand at least 8 m away.
Optimizing Images Obtained during a CBCT
The patient must be positioned correctly during a CBCT to eliminate motion artifacts. The acquisition time of up to 40 seconds may make it impossible for some patients to remain perfectly immobilized, especially if they are a young child or elderly patient. Patient movement causes artifacts like double contours or black and white stripes, and these compromise the image quality. Other factors that may affect patient movement include:
Incorrect positioning of the patient’s head
Incorrect fitting of the patient’s chin into the chin support or chin cup
The unit arm touching the patient’s hair during the scan
The position of the cotton roll stabilizing the patient’s jaws
Even when patient movements are minor and difficult to perceive with the naked eye, they cause discrepancies between the image dimensions and reality. A computer error can also occur, but frequently poor imaging is attributed to FOV adjustments and inadequate imaging of the region of interest.
Factors that make a massive difference to the quality of CBCT scans include choosing the correct FOV and thoroughly discussing the procedure with the patient. Ensure that the operator is appropriately trained in positioning the patient and operating the equipment. Continuing education and training for clinicians and dental professionals performing CBCT exams improve radiation safety significantly and improve the probability of collecting high-quality data.
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