A voxel is the smallest 3D element of the volume,2 and is typically represented as a cube or a box, with height, width and depth. Just as 2D images are made of several pixels (represented as squares, with height and width) and the smaller the pixel the better the quality of the picture, the same concept applies to a 3D data volume. Each three-dimensional voxel represents specific x-ray absorption. The voxel size on CBCT images is isotropic, which means that all the sides are the same dimension with uniform resolution in all directions. In contrast, an MDCT voxel is in general nonisotropic meaning that one side of the voxel is different in dimension (Figure 7). This is considered an advantage of the CBCT because if a certain structure needs to be measured, the measurement will be exact in all the three orthogonal planes. There are different voxel sizes depending on the capabilities of each unit. The small field of view units may use a small voxel size of 0.076 mm, which enables visualization of very small changes to structures. Other voxel sizes available for CBCT units are variable, such as 0.2 mm, 0.3 mm, and 0.4 mm. It is important to note that the larger the voxel size, the less resolution the image will have and less capability to differentiate between small structures (Figure 8). The voxel size is dependent of the imaging objective and the size of the unit detector.
Figure 7. Illustration of CBCT and MDCT voxel size.
Figure 8. Voxel size affects details of the objects imaged.
Voxel size needs to be smaller than the desired anatomical structure for adequate representation. For example, the first sign of periapical inflammatory lesion is discontinuity of the lamina dura; thus, if visualization changes to the periapical area (lamina dura and PDL space) is desired, a CBCT less than 0.2 mm needs to be acquired. Structures smaller than the voxel size will not be visualized in the scan (example, small cracks in the enamel). As previously mentioned, soft tissue structures including mucosa, gingiva, cartilage, nerves and blood vessels cannot be evaluated in a CBCT study.