Digital volume tomography (DVT; synonyms: digital volume tomography; cone beam computed tomography, cone beam CT, CBCT) is a radiologic imaging procedure that provides three-dimensional images of the smallest bone structures of the skull, extremities, and joints. The procedure allows excellent visualization of bony structures due to the high contrast with air and soft tissues. DVT entered the field of dentistry in 1998 and has certain advantages over other X-ray 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 that DVT provides fewer interference shadows from metal than CT, which is of great importance in the area of teeth restored with metal. In the meantime, digital volume tomography has also found its way into ENT diagnostics and orthopedic diagnostics. In the ENT guidelines, this examination procedure is increasingly being accorded equivalent status to CT in terms of contrast formation for many indications.In orthopedic diagnostics, DVT is used for imaging bone tissue and joints. In essence, DVT is a variant of computed tomography (CT) with higher resolution of bony tissues on DVT.
The procedure
Digital volume tomography, like computed tomography (CT), is a slice imaging technique that allows a three-dimensional reconstruction to be created on a computer.The procedure is used in dental diagnostics and in ENT on sitting patients. 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. In orthopedic diagnostics, only the body region to be examined is advanced into the device. In the case of cross-sectional images of the foot and ankle, the diagnosis is made with the patient in a standing position, as in the case of X-rays. This results in the entire body weight being applied to the joints to be examined. As a result, detailed 3-D images of a stress situation of the joints are possible.DVT technology, in conjunction with a contrast medium display, also enables the display of the joint interior (three-dimensional arthrography).Furthermore, the procedure also enables functional diagnostics i.e. functional X-ray examination and podometry (foot pressure measurement).Radiation exposure
For information on radiation exposure, see the topics “Digital volume tomography (DVT) in dentistry,” “Digital volume tomography (DVT) in ENT,” and “Digital volume tomography (DVT) in orthopedics.”
