Ultrasound machines have greatly evolved during the last few decades. Bulky machines with limited capability have been transformed to much smaller units with enhanced capability of performing all modalities of echocardiography that we take for granted at present (Figure 2-1). Portable and even handheld devices have capabilities unimaginable 20 to 30 years ago. These units are technologically advanced, enabling performance of high-quality, complete studies. Regardless of their sizes, all have basically the same components and operate in the same manner.1,2
Three sizes of ultrasound systems, from a full-size system on the left to a handheld on the right and a portable system in the middle.
Ultrasound machines have two major components: the transducer (or probe) and the main unit.
The transducer is the part of the system that generates and sends out ultrasound pulses, and then receives echoes coming back from the target (Figure 2-2). These sound waves are shaped via the transducer as beams that are focused and electronically scanned through tissues. This process is done by converting electrical energy into ultrasonic (mechanical) energy, and vice versa. The element that facilitates this conversion of energy is PZT, a ceramic plate made of lead, zirconium, and titanium. PZT has piezoelectric properties, which means that applied voltage deforms the material, causing it to vibrate and generate ultrasonic waves, and reflected ultrasound received by the PZT element(s) produces voltage changes that are processed and converted to echocardiographic images (Figure 2-2). Since transducer elements are solid and have high acoustic impedance, if no compensation is made for a much lower impedance of skin and tissues, most ultrasound energy will be reflected and little will be transmitted to the target organs, resulting in virtually no usable information (see Chapter 1). To prevent this, and to facilitate ultrasound transmission, the face of the transducer is covered by a matching layer with an impedance value intermediate to the transducer and tissue. A backing layer is also applied to absorb the extra sounds, thus keeping the pulses short. This will ease the transfer of pulses between the transducer and the patient, and vice versa. The transducer element is then placed in a casing and covered by a lens to enable more effective transmission and reception from a particular region. Figure 2-3 demonstrates the basic structure of a single-element transducer.
This figure demonstrates how a transducer works both as a transmitter and receiver at the same time. A. Electricity applied to the transducer produces ultrasound pulses. B. Echoes received by the transducer produces voltages that can be processed further.
Basic structure of a single element transducer.