Digital volume tomography (DVT; synonyms: digital volume tomography; cone beam computed tomography, cone beam CT, CBCT) in orthopedics is a radiological imaging procedure that depicts bones and joints in three dimensions and can thus make a significant contribution to preoperative and posttraumatic 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). Following the introduction of DVT into dentistry, the procedure has also found its way into ENT diagnostics and orthopedic diagnostics. In orthopedic diagnostics, DVT is used to image bone tissue and joints. Essentially, DVT is a variant of computed tomography (CT) with higher resolution of bony tissues in DVT.
Indications
The diagnostic issues that justify obtaining a DVT are broad. 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:
- Imaging of bone tissue: extremities and joints (e.g., wrist, foot, and ankle).
- Post-traumatic assessment (after accident) of injuries to hands, feet, elbows, knees, or ankles.
Contraindications
Due to the radiobiological effects 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 allows a three-dimensional reconstruction to be made on the computer.The procedure is used in orthopedic diagnostics on standing and sitting patients. 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 performed on the standing patient, as in the case of X-rays. This results in the entire body weight bearing down on the joints to be examined. As a result, detailed 3-D images of a stress situation of the joints are possible. For imaging, the region of the patient to be examined 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. In the process, an image resolution up to 4 times higher than that of conventional CT devices is achieved. This allows ultra-high resolution images of hard tissue structures with a voxel size (equivalent to one pixel in a 2D image) of up to 75 μm. This enables even the finest hairline fractures in the extremities or early arthritis (joint inflammation) to be detected. 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.DVT technology, in conjunction with a contrast medium display, also allows visualization of the interior of the joint (three-dimensional arthrography).Furthermore, the procedure also allows functional diagnostics i.e. functional X-ray examination and podometry (foot pressure measurement). Radiation exposure
In orthopedic diagnostics, DVT diagnostics results in radiation exposure in the order of 50% or less of a normal CT examination.
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.
