·Understanding the 2D and 3D image -
A 2-dimensional (2D) image is composed of numerous ‘pixels’. Pixels have 2 dimensions - height and width. Pixels determine the resolution (spatial) of an image. Images with higher resolution have more pixels per inch. Thus more number of pixels per inch produce an image with higher sharpness.
A 3-dimensional (3D) image is composed of numerous ‘voxels’. Voxels have 3 dimensions – height, width, and depth. Thus, Voxels are to 3D images, what pixels are to 2D images. They form the basis of CT and CBCT images. Resolution of the primary image in a CT/CBCT is determined by the voxel size. A voxel is thus a three dimensional element of anatomy represented by the two dimensional pixel. The volume of a voxel = pixel height X pixel width X slice thickness. Hence it is expressed in units of mm³.
Understanding the voxel in CBCT -
With a CBCT, the acquired data are reconstructed to achieve similar dimensions in all 3 planes. Hence, the volume data are considered to be isotropic. Isotropic data are composed of cube-shaped voxels of equal length on each side. This is applicable to CBCT for the maxillofacial region which has limited field of view (FOV). Voxels in CBCT are isotropic having equal height, width and depth. Hence CBCT offers the advantage of providing exact measurements of structures in all 3 orthogonal planes. For comparison purpose, voxels in conventional CT are anisotropic. Thus, in conventional CT one side of the voxel is different in dimension.
CBCT has excellent high-contrast resolution as a result of the small size and geometry of its isotropic voxels. This high contrast resolution also called as spatial resolution is the ability to distinguish sharp edges between small objects that have prominent difference in density. CBCT offers high contrast images i.r.t mineralized tissues in the maxillofacial region which are bones and teeth. The voxel resolution of CBCT ranges from 0.076 mm to 0.4 mm. The other voxel sizes for CBCT units are in the ranges of 0.18 mm, 0.2 mm, 0.3 mm and 0.4mm. (1)
There are several CBCT systems in the present market. The difference between these systems lies primarily in their spatial resolution and technical features. With a smaller voxel size, a high spatial resolution image can be obtained. The factors that determine the use of appropriate voxel size are imaging objective and size of the unit detector. Hence it is very important to know the purpose for the diagnosis, so that an appropriate FOV can be selected accordingly. An example is a survey for endodontic treatment purposes, which requires images with high resolution to clearly visualize the root canal morphology and details of the related structures. This requires a small FOV (5x5 cm; 5x4 cm or 4x4 cm) with a smaller voxel size (0.09 mm, 0.08 mm, 0.06 mm) and therefore is able to generate high resolution images. However, when the diagnostic aim is to visualize the extent of a cyst or tumor with the jaws, the FOV has to be larger (10x10 cm; 12x9 cm; 17x11 cm). This larger FOV generates images with a comparatively larger voxel size (0.18 mm; 0.20 mm ; 0.25 mm).
Thus selection of an appropriate FOV must be done while keeping in mind the ALADA concept * (as low as diagnostically acceptable), so as to minimize the radiation dose to the patient.(2)
The above arranged cross sectional images (from left to right) have a voxel size of 0.30mm (1); 0.18mm (2) and 0.09mm (3). Resolution of the images increases or becomes better as the voxel size reduces.
The above arranged sagittal images (from left to right) have a voxel size of 0.30mm (1); 0.18mm (2) and 0.09mm (3)
References
Diane J. Flint, Regina Casian Ruiz Velasco. Cone-Beam Computed Tomography (CBCT) Applications in Dentistry.
Radiation Safety in Dentistry. FDI Policy statement. Read it at – FDI World Dental Federation.
