Digital volume tomography (DVT; synonyms: digital volume tomography; Cone Beam Computed Tomography, Cone-Beam-CT, CBCT) in dentistry is a radiological imaging procedure that depicts the anatomy of the teeth, jaws, and facial skull in three dimensions and can thus make a significant contribution to preoperative and post-traumatic diagnostics. The procedure allows excellent visualization of bony structures due to the high contrast with air and soft tissues. DVT entered dentistry in 1998 and has certain advantages over other radiographic techniques. For example, it can depict the bony structures of the facial skull even more comprehensively than the usual pantomograms (panoramic tomograms, orthopantomograms, radiographic overviews of the jaws). Unlike the computed tomography (CT) procedure, which is used in specialized radiology practices, a digital volume tomogram (DVT) can be created by the dentist with the appropriate expertise in his own practice rooms, which results in a significant time saving for the patient and therapy planning. Another advantage is based on the fact that DVT provides less interference shadows caused by metal than CT, which is of great importance in the area of teeth restored with metal restorations.
Indications
The diagnostic issues in dentistry that justify the preparation of a DVT are wide-ranging. In principle, the procedure is always indicated when a three-dimensional representation of a structure is of importance for further therapy, i.e., when a comprehensive initial diagnosis appears useful, for example:
- Localization of displaced teeth that can only be detected two-dimensionally with an X-ray image.
- Implant insertion planning (placement of implants) (3D implant planning).
- Maxillary sinus diagnostics for clarification of dentogenic (triggered by a tooth) causes.
- Periodontology (periodontal diseases)
- Suspicion of space-occupying processes such as cysts or tumors.
- TMJ diagnostics
- Special endodontic (concerning the inside of the tooth) issues, e.g. clarification of root fractures, resorptions (dissolutions) or accessory (additional) root canals.
- Special orthodontic issues
- Preoperative imaging of structures at risk in the vicinity of the surgical site.
- Post-traumatic assessment (after accident) of injuries such as midface fractures.
Contraindications
Due to the radiobiological effect of X-rays in tissues, the following contraindications arise:
- Gravidity (pregnancy), unless there is a vital threat.
- Lack of justifying indication
Before the examination
Since DVT is a radiological procedure, appropriate precautions must be taken to protect the patient and the treatment team from X-ray radiation:
- Questioning women of childbearing age about possible pregnancy.
- Inquiring about recently taken radiographs
- Protecting body parts not to be imaged with a lead apron or shield
- Correct adjustment technique on the patient and all technical parameters to avoid repeat exposures.
The procedure
Digital volume tomography, like computed tomography (CT), is a slice imaging technique that makes it possible to create a three-dimensional reconstruction on the computer.The procedure is used in the context of dental diagnostics on a sitting patient. For the exposure, the patient’s head is positioned in the so-called isocenter. An X-ray tube and a flat image detector positioned opposite it rotate synchronously 360° around the patient’s head. A 3D object is virtually reconstructed by computer from the 360 (up to 400) individual images produced during a single rotation. Unlike conventional CT, which uses a fan-shaped beam and captures thin individual layers of the body, the beam of the DVT is cone-shaped, which explains the English synonym of cone-beam CT (CBCT). The beam cone captures the volume of the hard tissue structures to be examined in three dimensions.This results in a so-called Field of View (FOV; maximum section that the device can display), which is usually cylindrical in shape and measures 4 cm x 4 cm to 19 cm x 24 cm.During the examination, there is only a single circulation of the beams, which cover the entire area to be examined in a cone shape. The radiation is reflected by the tissue, a detector (CCD detector) measures the reflected radiation and converts it into images. The latest generation of DVT devices also has a Hounsfield calibration. Here, the values of different X-ray densities are converted into standardized Hounsfield units (hounsfield units=HU). Note: The Hounsfield scale describes the attenuation of X-rays in tissue and is displayed in grayscale images. The values can thus be assigned to tissue types and pathological deviations can be detectedImage reconstruction by the computer allows viewing of any slice from almost any direction as well as the three-dimensional object. Radiation exposure (simplified rule of four).
It is about four times that of an orthopantomogram (radiographic overview of the jaws), which is very common in dentistry, but only about a quarter of a computed tomogram required for dental diagnostics. Compared with natural radiation exposure, the dose of a DVT is slightly less than eight daily doses.
After the examination
DVT is followed by quality-assured digital image processing and documentation of the imaging parameters and, ultimately, their diagnostic evaluation.
Possible complications
Possible complications arise from procedural errors such as incorrect positioning of the patient, incorrect setting of exposure parameters, or computer malfunctions, among others. They would lead to repeat exposures and thus to increased radiation exposure for the patient.
