CHAPTER 6 M EDICAL U LTRASOUND S ENSORS Thomas L. Szabo Department of Biomedical Engineering Boston University, Boston, MA, USA 6.1. INTRODUCTION The transducer is the one indispensable part of a medical ultrasound system. Although much of this chapter describes the properties of piezoelectric transducers in general, emphasis will be placed on arrays for diagnostic imaging. Like many other sensors, medical ultrasound transducers perform their ordinary transduction functions of converting electrical signals to pressure (or stress) waves and vice versa, but what distinguishes them is their ability to perform imaging as well. In this chapter the physi- cal principles of transducers, preferred piezoelectric materials for transduction, design considerations, single-element and array configurations, the characteristics needed for imaging and focusing, and new developments in transducer technology are explored. A transducer can be considered to be a black box, as shown at the top of Figure 6.1. A short elec- trical pulse, v i (t), enters the left side and emerges out the other end as a pressure (or stress) wave, p i (t). This wave reflects from an object in its path and returns to the transducer, p r (t), where it is converted back to an electrical signal, v 0 (t), as illustrated at the bottom of Figure 6.1. These conversions are possible because of piezoelectric material inside the transducer. Piezoelec- tricity was discovered by the Curie brothers in the 1880s. They found that an electric charge appeared on electrodes placed on a compressed quartz crystal: the direct piezoelectric effect. They also verified the reverse piezoelectric effect, that a displacement is caused by a voltage applied to a quartz crystal. These reciprocal piezoelectric effects enable the reception of pressure pulse echoes and the transmission of pressure waves into media such as the body. 285