discovered in single-crystal quartz by Jacques and Pierre Curie in 1880 [3]. This discovery formed the basis for further research studies con­ducted by the physicist Paul Langevin, doctoral student of Pierre Curie– the inventor of the ultra­sonic echography and later a professor at the Collège de France in Paris. Piezoelectric materi­als encompass solid substances, for instance, cer­tain crystals such as quartz, ceramic, or ceramic perovskites, e.g., lead zirconate titanate, lead magnesium, or niobate–lead titanate. When elec­tric currents with different potentials are applied to these piezoelectric materials, these currents induce rapid and rhythmical changes in the thick­ness of the material (Fig. 2.1). In consequence the mechanical energy is transformed into emit­ted sound waves. For each US pulse, a series of echoes are returned when the pulse is reected from objects at greater or lesser distance. Vice versa the crystals can also absorb reected sound waves and convert them into electric signals. Thus, US transducers always have dual functions (see Fig.2.1). The US echoes are received by the transducer and converted into electronic signals that can be displayed on a monitor. The trans­ducer probes may contain a single element or an ensemble of arranged individual single transduc­ers. These are named as multiple-element probes (MEP) or transducer arrays. A schematic drawing of a US probe is given in Fig.2.2. While single­element probes were used in the early days of US applications, they have gradually been replaced by MEPs from the early 1980s onward [4]. The most outstanding advantage of MEP is the possi­bility of beam steering. In MEPs each crystal has its own circuit; thus the US beam can be “steered” by changing the timing in which each element gets pulsed. Beam steering allows the multiple angle and/or multiple point inspection from a single probe and a single probe position (Fig.2.3).

In principle, 3D US examination consists of four steps (data acquisition, visualization, image processing, and storing of data), and there are four ways of 3D data acquisition: