Biomedical Engineering Reference
In-Depth Information
FIGURE 15.13
An ultrafast CT scanner. In this device, magnetic coils focus and steer the electron beam
through the evacuated drift tube, where they strike the target rings. The presence of four target rings and multiple
x-ray collimators allows four unique x-ray beams to pass through the patient. These beams are then interrupted by
two contiguous detector arrays and form eight unique image slices.
Courtesy of Imatrol.
volumetrically examine the heart and evaluate the presence and severity of coronary calcific
atherosclerosis. Although the ultrafast scanner can almost stop biologic motion, its contrast
and spatial resolution are presently less than third- or fourth-generation mechanical scanners,
so its general applications are somewhat less.
X-Ray Tubes and Detectors
It must be emphasized that the whole purpose of the scanning procedure is to take
thousands of accurate absorption measurements through the body. Taken at all angles
through the cross section of interest, these measurements provide an enormous amount
of information about the composition of the section of the body being scanned. Since these
readings are taken by counting photons, the more photons counted, the better will be the
quality of the information. With these factors in mind, it is essential that the radiation dose
be sufficient to obtain good-quality pictures. Since the total radiation dose is a function of
the maximum power of the tube and its operating time, faster scanners have to use a signif-
icantly higher-powered tube. These aspects of dose efficiency are very important in tube
and detector design.
