THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

16.6 Transducer Arrays 46916.6 Transducer ArraysA single-element ultrasound transducer tends to radiate a rather narrow beam orreceive signals over a narrow spatial range. In order to cover a wider area througha process called scanning, and, in many instances, to emit more powerful signalsthan is possible with a single element, an especially arranged group of transducersor arrays are used to extend the versatility of transducers.Array transducers are also used to focus an acoustic beam. Variable delays areapplied across the transducer aperture. The delays are electronically controlled ina sequential or phased array and can be changed instantaneously to focus the beamon different areas.With linear-array transducers, which are far more versatile than piston transducers,the electronic scanning entails no moving parts, and the focal point canbe changed readily to any position in the scanning plane. A broad variety of scanformats can be generated, and received echoes can be processed for other applicationssuch as dynamic receive focusing, correction for phase aberrations, andsynthetic aperture imaging. The principal disadvantages of linear arrays obviouslylie in the greater complexity and increased costs of the transducers and scanners.In order to ensure high quality imaging, many (as high as 128 and on theincrease) identical array elements are required. Each array element tends to beless than 1 mm on one side and is connected to its own transmitter and receivingelectronics.Phased Arrays: Focusing and SteeringWe examine how a phase-array transducer can focus and steer an acoustic beamalong a specified direction. An ultrasound image is created by repeating the scanningprocess more than a hundred times to probe a two-dimensional (2D) or athree-dimensional (3D) locale in the medium. In Figure 16.7(a), a simple sixelementarray is shown focusing the transmitted beam. Each array element maybe considered a point source that radiates a spherical wavefront into the medium.Because the topmost element is the farthest away from the focus in this example, itis activated first. The other elements are triggered progressively at the appropriatetime so that the acoustic signals from all the elements reach the focal point simultaneously.According to Huygens’ principle, the resultant acoustic signal constitutesthe sum of the signals that came from the source. The contributions from eachelement add in-phase at the focal point to yield a peak in the acoustic signal. Elsewhere,some of the contributions add out-of-phase, lessening the signal relative tothe peak.On receiving an ultrasound echo, the phased array works in reverse. An echois shown in Figure 16.7(b) originating from receive focus 1. The echo is incidenton each array element at a different time interval. The received signals undergoelectronic delay so that they add in phase for an echo originating at a focal point.Echoes originating elsewhere have some of their signals adding out of phase,thereby reducing the receive signal relative to the peak at focus.