According to Webster’s dictionary, the word “tomography” is derived from the Greek word “tomos” to describe “a technique of x-ray photography by which a single plane is photographed, with the outline of structures in other planes eliminated. This concise definition illustrates the fundamental limitations of the conventional radiograph: superposition and conspicuity due to overlapping structures. In conventional radiography, as illustrated in Fig. 1.1(a), the three-dimensional (3D) volume of a human body is compressed along the direction of the x ray to a two-dimensional (2D) image. All underlying bony structures and tissues are superimposed, which results in significantly reduced visibility of the object of interest.