Page 283 - Biomedical Engineering and Design Handbook Volume 2, Applications

INSTRUMENTATION DESIGN FOR ULTRASONIC IMAGING 261

20
Summing Delay Wavefronts
Depth along transducer, mm 0 Σ
lines
state
before correction
10

–10

elements
Wavefront Array
–20
after correction

–40 –30 –20 –10 0 10 20 30 40
Depth, mm
FIGURE 9.8 Beam steering in a phased array system during reception. A linearly increasing time delay dif-
ferential is introduced for each of the delay lines to correct for the linear time difference in the arrival times.

wavefront is linear, the arrival times of the remaining echoes have a linear relationship; hence the
linear decrement on the delays from one element to the next.
In addition to beam steering, one can combine focusing and beam-steering delays in the same
process. This is illustrated in Fig. 9.9. Echoes from a near field point source (at 40 mm) are shown
arriving at the array elements. The arrival times of the echoes have a nonlinear component so the

20
Summing Delay Wavefronts
Distance along transducer, mm 0 Σ before correction
lines
stage
10

–10

Wavefronts Array Point
–20 elements
after beam source
steering and focusing
–40 –30 –20 –10 0 10 20 30 40
Depth, mm
FIGURE 9.9 Schematic of beam steering and focusing during reception in a phased array type system. In
addition to the focusing correction (also shown in Figure 3), phased array systems add a linearly increasing
time shift to the receive delay lines to achieve constructive interference in the desired direction.