Principles of CBCT
CBCT units can be categorized according to patient positioning, field of view, clinical functionality, and detector type. Clinicians should consider all these characteristics prior to purchasing a unit. The first CBCT unit was similar in design to MDCT, with the patient in a supine (reclined) position during the scan. Most CBCT units have the patient seated or standing; this type of unit requires less space in the dental office and may be more accessible for wheelchair users.1,2
Field of view (FOV) is another way to categorize these units. FOV refers to the anatomical area that will be included in the data volume, or the area of the patient that will be irradiated.7,9 Depending upon the type of machine/detector, and the geometry of the x-ray beam, the FOV could be classified as small or limited, medium and large (Figure 1).1,4,5
Figure 1. Examples of small, medium, and large field of view (FOV).
The units that scan small or limited areas, cover approximately 5 teeth (5 cm diameter) and surrounding anatomical structures, resulting in less volume for interpretation. Small FOV scans are normally used for endodontic purposes due to the capability of high spatial resolution and ability to visualize changes to the periodontal ligament spaces (PDL) or lamina dura, root fractures, periapical lesions, relationship of an impacted tooth with the surrounding anatomical structures, and root canal morphology. One capability of the small field of view units is the ability to create a larger volume of the complete arch rather than just a few teeth through the stitching process.5 In this process, multiple adjacent limited field of view scans are united during the reconstruction phase to recreate one dental arch. The main disadvantage of the stitching technique is that the patient is exposed to multiple CBCT scans.1
Medium field of view is normally referred to those scans that image one arch or both dental arches, approximately 6‑11 cm in height. When evaluation of the extent of a lesion or status of the temporomandibular joints is desired, a medium FOV is suggested. When both dental arches are scanned, a pseudopanoramic rendering can be derived from the data set. Medium field of view is also indicated in implant planning cases.
The large field of view is recommended for specific cases with skeletal anomaly/asymmetry and where orthodontic/orthognathic surgery is planned. The scanned area may range from 11 to 24 cm in height and covers most of the craniofacial skeleton. For comprehensive surgical planning, oral surgeons and orthodontists require a scan that extends from the inferior border of the mandible to the supraorbital ridges or vertex of the skull. The main disadvantage of the large FOV is the larger radiation exposure. With the large field of view the osseous structures of the temporomandibular joints and facial asymmetry can be evaluated. There are some manufacturers that offer the clinician the ability of acquiring different types of FOV according to their own needs. Each patient will present a unique situation, but it is important to always request the smallest possible FOV to avoid unnecessary radiation exposure to the patient.9
There are also “hybrid systems” that have the capability of acquiring a panoramic projection image, posteroanterior or lateral cephalometric image, and limited FOV 3D images (CBCT).
Another way to categorize the CBCT units is according to the type of detector: CCD/image intensifier (II) or flat panel detector.5 The size of the detector dictates the FOV capabilities of each unit. This is directly related to the price of the unit; the larger the detector the more expensive the unit will be. The CCD/II are bulkier and require greater floor space in the dental office, compared to the flat panel detector that is smaller, and requires less space.1