Following is an excerpt from a well written paper by Michael Sonick DMD and others. The paper has some straight forward discussions about the history of 3D imaging as well as its usefulness and safety.
Two-dimensional digital radiographs are a compilation of data collected in the form of pixels, or picture elements. These are captured with CCD sensors, phosphor plate sensors or from scans of conventional film-based radiographs. Once in the computer and viewed on a monitor a number of digital tools are used to evaluate the image diagnostically. 3-D cone beam images are derived from voxels instead of pixels. Voxels are three dimensional cubes of information. When many voxels are compiled into an image, digital tools allow viewing any portion of that image, from any axis. These images become virtual representations of the objects studied, and the diagnostic utility of these images is a quantum leap above its 2-D counterparts.
Follow this link to the complete paper.