The ultrasound transducer & piezoelectric crystals
The ultrasound transducer generates ultrasound (ultrasonic) waves. The transducer is held with one hand and its position and angle are adjusted to send ultrasound waves through structures to be visualized.
Ultrasound waves are emitted rapidly from the transducer. These sound waves travel through tissues and fluids. Some of the sound waves are reflected back to the transducer. By analyzing the reflected sound waves, the ultrasound machine creates an image of the tissues. Thus, the principle of ultrasound imaging is simple: sound waves are sent into the tissue and the reflected waves are used to create an image of the tissue (Figure 1).
The ultrasound waves are generated by ceramic crystals exhibiting piezoelectric properties (i.e piezoelectric crystals). Thousands of piezoelectric crystals are attached to the front of the transducer (Figure 2). The crystals are connected to the ultrasound machine via electrodes.
Piezoelectric crystals have unique electromechanical properties. When an electric current is applied to a piezoelectric crystal, it starts to vibrate and these vibrations generate sound waves with frequencies between 1.5 and 8 MHz (i.e ultrasound). Thus, piezoelectric crystals can convert electric currents into ultrasound waves. The crystals can also do the opposite; when the crystals are hit by reflected ultrasound waves, they begin to vibrate and these mechanical vibrations are converted into electric current that is sent back to the ultrasound machine, where the electrical signal is interpreted and translated into an image (Figure 3).
As can be seen from Figure 2, the ultrasound transducer contains several components. The transducer contains acoustic insulation that ensures that no other sound waves affect the transducer. The crystals are supported by a backing layer that suppresses the vibrations of the crystals, allowing sound waves to be sent out in shorter pulses and this improves resolution (discussed below). In front of the crystals are materials (matching layer) that reduce the difference in impedance between the crystals and the tissue to be studied. Without this layer, the impedance difference becomes large, which causes too much of the sound waves to be reflected (leaving fewer sound waves to penetrate the tissues). At the front of the transducer is an acoustic lens. This is the hard rubber that focuses the ultrasound waves, which results in less scatter of the waves and thus increase the resolution of the image.
From the transducer, ultrasound waves are sent out in pulses. Each pulse consists of a few sound waves emitted in 1 to 2 milliseconds. These sound waves travel through the skin, chest, pericardium, myocardium, etc. In the transition between each medium (tissue, blood, etc.), a significant portion of all sound waves will be reflected back to the transducer. When the reflected sound hits the piezoelectric crystals, they begin to vibrate and generate electric currents, which are transmitted to the ultrasound machine for analysis.
The reflected sound waves will have the same speed as the emitted sound waves, but the amplitude, frequency and angle of incidence may differ from the emitted sound waves. The ultrasound machine utilizes variations in the amplitude, frequency and timing of the reflected sound waves to create an image of the medium (tissue).