1. Introduction and Scope

Medical ultrasound has been one of the most widely adopted and rapidly developing diagnosis and therapy tools today because of its non-destructive and non-ion radiative nature. Ultrasound transducers, as key components in medical ultrasound systems, have been developed for a variety of ultrasound 2D and 3D imaging with sub-millimeter to mm spatial resolution and Doppler blood flow information. Medical ultrasound imaging stems from the sonar technology, which was first developed in World War I for detecting submarines using acoustic waves with frequencies ranging between >20 kHz and 1 MHz. For a sonar transducer, a piece of piezoelectric material was used to produce and detect the acoustic pulses. The concept of sonar was later applied to image the human tissues by increasing the ultrasound frequencies above 1 MHz. Medical imaging with ultrasound was first demonstrated to be a useful clinical tool in the early 1950s by Wild and Reid, who reported the first two-dimensional imaging of soft tissues. With the first medical ultrasound system, a transducer was scanned mechanically across the body to form a two-dimensional (2-D) image slowly. Later in 1970, the development of linear arrays and electronic beamforming made real-time 2D imaging possible. The development of digital electronic beamforming in the 1980’s resulted in improved image quality and flexibility in scanning. Ultrasound is now applied in a broad range of topics of biology and medicine, and accounts for about one-third of all diagnostic imaging procedures.