Biomedical Engineering Reference
In-Depth Information
any part of the body. However, it still was not fast enough to eliminate the streak artifacts,
nor did it seem possible to further reduce scan time with this approach. In essence, the first
and second generations had reached their limits. Scanning times could not be reduced further
and still achieve an acceptable image quality. Nevertheless, this geometry had the potential to
achieve the highest spatial and contrast resolutions of all the gantry designs.
A major redesign ensued, and many of the scanning system components were improved.
In the process, a third generation of gantries evolved that consisted of a pure rotational
system. In these systems, the source-detector unit had a pulsing, highly collimated wide-
angle (typically, 20 -50 ) fan beam, and a multiple detector array was rotated 360 around
the patient. This single 360 smoothly rotating movement produced scan times as low as
500 ms, increased appreciably the reliability of the data because they were taken twice
and increased the quality of the reconstructed image. A by-product of this design was that
the pulsing x-ray source could be synchronized with physiologic parameters, enabling
rhythmic structures such as the heart to be more accurately imaged.
The main advantages of this type of system were its simplicity and its speed, but it had
two major disadvantages. First, its fixed geometric system, with a fan beam usually estab-
lished for the largest patient, was inefficient for smaller objects and in particular for head
scanning. Second, it was particularly prone to circular artifacts. These “ring” artifacts were
most obvious in early pictures from this type of machine. The magnitude of the difficulty in
achieving stability can be appreciated when it is realized that an error of only one part in
10,000 can lead to an artifact that is diagnostically confusing.
Consequently, a fourth-generation gantry system consisting of a continuous ring (360 )of
fixed detectors (usually numbering between 600 and 4,800) was developed. In this system,
the x-ray source rotated as before, and the transmitted x-rays were detected by the stationary
detector ring. The fan beam is increased slightly so the detectors on the leading and trailing
edge of the fan can be continuously monitored and the data adjusted in case of shifts in detec-
tor performance. As a result, the image obtained using this approach is more reliable, and the
ring artifact is usually eliminated. Fourth-generation scan times of approximately 500 ms or
less with reconstruction times of 30 seconds or less are now commonplace.
The development of high-voltage slip rings has enabled third- and fourth-generation
scanners to continuously rotate about the patient. By moving the patient couch at a uniform
speed in or out of the scan plan, the x-ray beam will describe a helix or spiral path through
the patient. Slice widths of 1-10 mm and pixel matrixes up to 1,024 2 may be employed, and
typically a 50-cm-long volume can be imaged in a single breath hold. Because of improve-
ments in x-ray tube technology, spatial and contrast resolution are maintained. New display
techniques allow for real-time 3D manipulations and virtual reality fly-through of tubular
structures such as the aorta or large intestine.
Another gantry design, which might be called a fifth generation, utilizes a fixed detector
array arranged in a 210 arc positioned above the horizontal plane of the patient and four
target rings of 210 arc positioned below the horizontal plane and opposite from the detector
array. There is no mechanical motion in the gantry except the movement of the couch into
and out of the imaging plane (Figure 15.13).
X-rays are produced when a magnetic field causes a high-voltage focused election beam
to sweep along the path of the anode target. Up to four 1.5-mm simultaneous slices are
created in 50 or 100 ms and displayed in up to a 512 2 pixel matrix. This ultrafast CT can
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