Biomedical Engineering Reference
In-Depth Information
way. In a phased array, the center of the active aperture is always the same, and the
scanning of acoustic lines is accomplished through electronic angular steering. Each
line is steered by a small
incremental angle from the previous one, as shown in
Figure 16.14b.When
lines have been received, these lines form the basis for an image
frame. Examples of the image formats formed by these two array types can be seen in the
B-mode images of Figures 16.15 and 16.16. A variant of the linear array is the curved lin-
ear array, which operates like a linear array but on a curved convex surface rather than
on a flat surface. An example of an image from a curved linear array can be seen in
Figure 16.17.
N
EXAMPLE PROBLEM 16.7
For the example shown in Figure 16.14a, determine the frame rate for a scan depth of
s d ¼
150 mm, 100 lines per frame, and
c 0 ¼
1.5 mm/
m s
.
Solution
The round-trip time for one line is 2*
s d /
c 0 ¼
200
m
s. The time for a full frame is
lines/frame
N
or, in this case, 100 lines/frame
200
m
s/line
¼
20 ms/frame or 50 frames per second.
Transducer arrays come in a variety of sizes, shapes, and center frequencies to suit differ-
ent clinical applications, as shown in Figure 16.18. Access to the body is made externally
through many possible “acoustic windows,” where a transducer makes contact by coupling
FIGURE 16.15 B-mode image of lymph nodes in the breast at 12 MHz, an example of a linear array format.
Courtesy of Philips Medical Systems.
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