According to Webster's dictionary, the word âtomographyâ is derived from the Greek word âtomosâ to describe âa technique of x-ray photography by which a single plane is photographed, with the outline of structures in other planes eliminated.â This concise definition illustrates the fundamental limitations of the conventional radiograph: superposition and conspicuity due to overlapping structures. In conventional radiography, the three-dimensional (3D) volume of a human body is compressed along the direction of the x ray to a two-dimensional (2D) image, as shown in Fig. 1.1(a). 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 study. 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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