- Continuing Education
Cone-beam computed tomography (CBCT), also referred to as cone-beam volumetric tomography (CBVT) or cone-beam volumetric imaging (CBVI), was introduced in Europe in the late 1990s and approved by the FDA for use in the United States in 2001. 1 Because of the increasing applications of CBCT in dental practice over the past two decades, clinicians should seek to understand the basic concepts of this technology in order to provide appropriate treatment options for their patients.2
The main advantage of CBCT technology is the ability to view and analyze in three dimensions (3D) the patient’s osseous oral and maxillofacial structures, overcoming the magnification and superimposition of structures found with 2D imaging modalities like panoramic imaging.3,4 It is important to realize that with current CBCT technology only the bone and calcified structures are visualized along with airway spaces; soft tissue structures including muscles, glands, vasculature, and soft tissue tumors cannot be completely identified or assessed in a CBCT image. CBCT imaging provides high quality, accurate 3D representation of the osseous elements of the maxillofacial region.
When the patient is exposed to ionizing radiation, the beam passes through different tissues of varying densities. Structures with the highest density will be metallic materials, followed by enamel and bone. High density objects attenuate the x-ray beam, resulting in a reduction in the number of x-ray photons which strike the detector or sensor. Air absorbs very little of the x-ray beam. The difference in densities is registered by the x-ray detector, and the computer applies a formula/algorithm for each attenuation. The result is a numeric matrix of an assigned value to each location, displayed on the computer monitor as an image of varying shades of gray. High density structures display as white/light in the image, while low density structures that absorb very little of the x-ray beam, such as air, display as black/dark.4,5
How much ionizing radiation is the patient receiving during the CBCT acquisition? A CBCT study emits more ionizing radiation than an intraoral series or panoramic imaging but approximately 10 times less than a medical MDCT. The amount of ionizing radiation will be different according to each manufacturer or CBCT unit. Some CBCT machines emit continuous radiation exposure, which means that during the entire acquisition process the unit is continually emitting ionizing radiation. Other CBCT units utilize pulsated radiation or intermittent radiation with less total radiation emitted.6
Radiation dose produced by CBCT is dependent on various machine parameters, such as field of view (FOV), peak kilovoltage (kVp), milliamperage (mA), and a continuous or intermittent beam, number of basis images, scan time and the degree of rotation.5 These are factors that will influence the amount of radiation absorbed by the patient. Some of these factors are specific to each machine and some are clinician dependent, especially the field of view selected. A leaded apron should be utilized for patient protection as long as it is not positioned between the x-ray source and the area of interest and may be required depending upon state laws.
CBCT is a type of imaging that offers many advantages over conventional or multidetector computed tomography (MDCT) used in medicine.4,5 In CBCT imaging the x-ray beam is cone- or pyramidal-shaped, as compared to the narrow fan-shaped x-ray beam in MDCT imaging.1,4,7 The CBCT unit typically makes a single rotation of 180 or 360 degrees around the patient’s head to acquire the two dimensional (2D) images, called “basis images;”7the MDCT x-ray source must make many rotations around the patient to acquire the volume of images.1,5,8 A CBCT study has higher spatial resolution, shorter scan time, lower cost, and lower radiation dose when compared with MDCT study, except poor soft tissue contrast.
All imaging modalities should follow the ALARA rule — As Low As Reasonable Achievable.4,9 This principle is the basis for radiologic safety. Not all patients require a CBCT scan or 3D imaging.10 Radiation exposure is only warranted if the proposed study will provide beneficial clinical information. A clear objective needs to be established prior to scanning a patient and should not be used on a routine basis or for screening purposes on all individuals.9‑11