In the first figure of the introduction, we described the X-ray CT scan as accomplishing the following imaginary sequence of events: The head is frozen and a fine saw makes two transverse cuts one centimeter apart, thereby creating a cross-sectional slab of head one centimeter thick. This slice is removed intact, placed on high-contrast X-ray film, and exposed to a broad beam of X-rays at right angles to the slice of tissue. When the radiograph of the slice has been processed and placed on a viewbox, the image would be essentially the same as that produced by the CT scanner. The CT scanner produces these results without physically altering the head. Except for slight tissue ionization, this process is entirely harmless (Figure 1).
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