 |
| Fig. 2 |
The ultrasound beam can also be steered electronically, without
moving the transducer. Electronically steered, or "phased array"
systems (Figs.
2 to 4) typically comprise 96 to 128 small elements (only a
few are shown), which are pulsed in a very rapid, precisely controlled
sequence. The top element is pulsed first (Fig.
2); because it is very small, the ultrasound wave it generates
is circular. Very soon afterwards, the second element is pulsed,
and so on. The individual wavelets combine to make one compound
wave that, because of the pulsing sequence, travels at an angle
to the axis of the transducer array. Returning echoes do not reach
all the transducer elements simultaneously; electronic circuits
delay the signals from those arriving first, allowing the remainder
to catch up (Fig.
4).
 |
| Fig. 3 |
Continuously changing the pulsing sequence scans the ultrasound
beam in a manner similar to a mechanical scanner. Despite the necessary
complexity of the electronic circuitry the phased array technique
offers methods for reading the effective beam width not possible
with mechanical systems. This is a very important factor in improving
image quality. Focusing can be achieved by fitting a plastic lens
over the face of the transducer. Phased array systems can provide
additional focusing electronically; a lens works by delaying portions
of the wavefront and a phased array can achieve the same effect
electronically by further modifying the pulsing sequence (Fig.
3).
 |
| Fig. 4 |
A phased array system can also employ a technique called
"dynamic focusing" (Fig.
4). If a pulse is transmitted across two interfaces, A and B,
the echo from A returns first. Its curved wavefront reaches the
center transducer elements before those at the edges. The electrical
signals from the central elements are delayed to allow those from
the edges to catch up. All the signals are then added together (A1).
A few microseconds later, echoes from B arrive. This wavefront is
less curved, so the delay pattern is altered. In this way the receiver
changes its focal distance as echoes from more distant structures
arrive, just as a pair of binoculars can be adjusted to keep an
airplane in focus as it flies past. This technique rejects off-axis
echoes that reduce effective beam width.
2004