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.”1 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. Figure 1.1(b) shows an example of a chest x-ray radiograph. The superposition of the ribs, lungs, and heart is quite evident. Consequently, despite the image’s superb spatial resolution (the ability to resolve closely placed high-contrast objects), it suffers from poor low-contrast resolution (the ability to differentiate a low-contrast object from its background). A recognition of this limitation led to the development of conventional tomography.
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