Cone Beam CT is a medical imaging modality, which has been applied in different fields of medicine (e.g. cardiac imaging, radiotherapy). Recently, this technique has been applied to dental imaging.
The principle behind this technique, as its name implies, is a cone-shaped X-ray bundle, with the X-ray source and detector (Image Intensifier or Flat Panel Detector) rotating around a point (or field) of interest of the patient (Figure 1). The conical shape of the beam distinguishes this technique from helical CT, which used a fan-shaped beam. As a result of the acquisition of two-dimensional projections throughout this rotation, only one rotation or less is needed to acquire a full (three-dimensional) dataset. The images received by the detector are then compiled by the computer into volumetric data (primary reconstruction). This can then be visualized as two-dimensional multi-planar reformatted slices or in three dimensions by using surface reconstruction or volume rendering.
Figure 1. CBCT (right) compared to 'fan-beam' CT (left).
The cone-shaped beam used in CBCT is produced in a vacuum tube called an X-ray tube. Figure 2 shows a diagram of a hot cathode (Coolidge) tube. There are different parameters characterising the X-ray beam. The beam quality is defined by the X-ray spectrum, the shape of which is defined by the voltage peak (kVp) over the tube, the anode material and the filtration of the produced X-ray beam. The quantity of X-rays is linearly related to the anode current (mA) and exposure time (s). Furthermore, the collimation defines the width and height of the primary beam and therefore the size of the reconstructed field of view (FOV).
Figure 2. Coolidge tube, consisting of cathode (C) and anode (A).
By rotating the beam around a fixed point (isocentre) in the object of interest and acquiring projections from many different angles, a (typically cylindrical) three-dimensional volume can be reconstructed. Typically, a few hundreds of projections are collected. Although a 360° rotation is used in general, some devices have implemented a 180° or slightly greater rotation arc, which suffices for image reconstruction and leads to significant radiation reduction. The two-dimensional attenuation profiles obtained from all angles are then reconstructed into a three-dimensional matrix, containing volume elements (voxels) each having a certain grey value which represents the average density within this volume element. The grey value for each voxel is determined by the reconstruction algorithm, by combinding the information from all obtained projections.
There are generally two types of algorithms used in clinical practice. The most commonly used algorithm is the modified Feldkamp algorithm, which uses filtered backprojection. Recently, iterative reconstruction methods have gained attention as an alternative reconstruction method. Using iterative reconstruction, the acquisition process is modeled and consecutive cycles of reconstruction and reprojection are performed. After an initial reconstruction, projections of the reconstructed volume are simulated and compared with the actual raw data (i.e. the acquired projections). The reconstruction is then repeated and altered based on the difference between actual and simulated projections. This process is repeated for a number of iterations. A great benefit for this technique is that every part of the acquisition process can be modelled, which indicates its great potential in artefact reduction. However, iterative reconstruction requires a great amount of computing power, and it is generally expected to be implemented into practice more and more with the continuing increase of processing speed in workstations.
In conclusion, after a gradual introduction of this modality into dental radiology, the use of CBCT has steadily increased, and the market has been growing with a wide range of CBCT devices, as shown in the Comparison of CBCT Machines. These devices, although based on the same principe, exhibit great differences regarding exposure parameters and other quality factors, requiring an objective analysis of the performance of these devices, and an optimized implementation into dental practice.
