Transducers are defined as devices converting one form of energy into another. In the case of ultrasound, electrical energy is converted into mechanical (acoustic) energy. The most familiar transducer is a telephone receiver, with an earpiece that converts electrical impulses into sound waves and a mouthpiece that converts sound energy into electricity. Imaging transducers combine both functions by emitting and receiving ultrasound pulses and converting them into electrical impulses for further processing. Non-imaging continuous-wave (CW) Doppler transducers, just like the telephone receiver, have two elements; one is constantly emitting sound and the other is receiving sound. Figure 3.1 shows the anatomy of the imaging transducer.
An imaging transducer both emits and receives signals. The PZT (piezoelectric) crystal converts electrical impulses from the wire into ultrasound and vice versa. A matching layer reduces internal reflections within the probe by gradually decreasing acoustic impedance. Backing material reduces the length of the pulse by preventing after-ringing (dampening effect) Acoustic lenses improve focus. The case prevents electrical shock exposure for the patient and the operator.
At the core of the ultrasound transducer (probe) is a sheet of piezoelectric material known as an active element, or simply the “crystal.” It is usually made of lead zirconate titanate, or PZT. This material will create electricity when mechanically deformed (direct piezoelectric effect) and it itself deforms when electrical voltage is applied to its surface (reverse piezoelectric effect). The ability of some natural and man-made materials to create electricity when physically deformed was discovered by the brothers Pierre and Jacques Curie in 1880 and first used to produce ultrasound in sonar to track German U-boats during World War I in France in 1917. The piezoelectric effect of PZT irreversibly disappears as temperatures rise above 360° C (Curie point), making it impossible to sterilize ultrasound transducers with heat. The PZT crystal is one-half–wavelength thick (for the speed of sound in the active element itself). Connected to the PZT crystal is a wire that transmits electrical impulses from a pulse generator to the crystal during a pulse-generation phase, and away from it to the processor, during the “listening” phase, when an electrical impulse is generated in the PZT crystal by the returning echo. The listening phase is 10 times longer than the pulse duration, so the duty factor in imaging ultrasound transducers is 0.1–1% (see Chapter 2). The transducer can be also set to emit sounds of a so-called fundamental frequency, but receive echoes with frequencies that are multiples of the fundamental one. This tissue harmonic imaging is usually performed with returning frequencies that are twice (first harmonic) or even four times (second harmonic) higher than the fundamental one. Because the harmonic frequencies are generated in the tissues themselves, the image is resistant to certain artifacts and tends to be of a better quality. Behind the active element (PZT) is a backing or damping material. Just ...