Biomedical Engineering Reference
In-Depth Information
FIGURE 15.12
Four generations of scanning gantry designs. With modern slip ring technology, third- or
fourth-generation geometry allows spiral volumetric scanning using slice widths from 1 to 10 mm and pixel
matrixes to 10,242. Typically, a 50-cm volume can be imaged with a single breath hold.
diagnostically confusing, the image quality did suffer. Finally, and perhaps the biggest draw-
back, was that since the patient had to remain absolutely immobile during the scan, its use was
restricted solely to brain scans.
In an attempt to solve these problems, a second generation of gantries evolved. The scan-
ning procedures in this approach included a translation step as in the first generation, but it
incremented the scanning gantry 180
around the patient in 10
steps. In addition, in an
effort to obtain more profiles with each traverse, a larger fan beam was used. By using a
10
fan beam, it was possible to take ten profiles—at 1
intervals—with each traverse. By
indexing through 10
before taking the next set of profiles, it was possible to obtain a full
set of 180 profiles by making only 18 traverses. At the rate of approximately one second
for each traverse, scanning systems of this type could operate in the range of 18-20 seconds.
Even in this extremely reduced scan time, the scanning and detection system was able to
obtain over 300,000 precise absorption measurements during the complete (i.e., over 180
)
scan. These values were then used to construct a picture of the slice under investigation.
With this new generation of CT scanners, it became possible to obtain cross sections of
