A History of Ultrasound and the Transducer

¶ … History of Ultrasound Physics and the Properties of the Transducer

Today, innovations in medical imaging allow clinicians to view the soft tissues, organs and blood flow inside the human body in ways that provide the opportunity for early diagnosis and treatment of a wide range of disorders as well as to monitor neonatal health (Goldberg & Verny, 2007). The introduction of ultrasound technologies, though, has only taken place since the second half of the 20th century, although there were a number of earlier attempts in the late 19th century that met with limited success. To determine the facts about this important innovation, this paper reviews the relevant literature concerning the history of ultrasound physics and the properties of the transducers that are used for ultrasound imaging, followed by a summary of the research and important findings concerning these issues in the conclusion.

Review and Discussion

According to Levine (2010), ultrasound is defined as "[S]ound with frequencies above 20 kHz (twenty thousand cycles per second)" (p. 179). Ultrasound physics relate to a physical or electromagnetic wave that is capable of interacting with some type of object; the extent of the interaction, or the crests and waves that are created, is a function of the size of the object versus the radiation wavelength that is used (Levine, 2010). An important point made by Levine is that, "Only objects similar in size or larger than that wavelength interact significantly with the [ultrasound] wave" (p. 179).

Ultrasound for medical diagnostic purposes was first attempted in 1937 but technical problems limited their use until after World War II when the use of the same transducer to send and receive ultrasound was used, a configuration that remains the standard for ultrasound imaging today (Levine, 2010). For medical diagnostic applications, ultrasounds are capable of traveling more rapidly compared to the time required to travel through the air because soft tissues are comprised mainly of water (1,540 meters/second in soft tissue versus 330 meters/second in air) (Levine, 2010). In this regard, Levine advised that, "Ultrasound frequencies on the order of megahertz are used for diagnostic soft tissue imaging. At 2 -- 20 MHz, the most useful frequencies for diagnostic imaging, the wavelengths in water are between 0.77 mm and 0.08 mm, which theoretically can distinguish structures of similar sizes" (2010, p. 179).

As depicted in Figure 1 below, the transducer that is used in ultrasound imaging involves a number of key components.

Figure 1. Key Component's of an Ultrasound Transducer

Source: http://www.vaultrasound.com/wp-content/uploads/2012/03/Transducer-basics-diagram-II.jpg

The transducer sends high-frequency sound wave streams into the body and collects the echoes that are returned from internal structures and the sound waves are transformed into electrical impulses that are used to generate computer displays (Goldberg & Very, 2007). The components contained in transducers use the piezoelectric effect which was first described by Jacques and Pierre Curie in 1877 (Levine, 2010). The piezoelectric effects generate sounds that can then be used to safely image human tissues by transmitting them into the body and then receiving their echo return that is translated into an image by computer applications (Transducers, 2016). According to the Virginia Ultrasound Authority (2016), "Linear transducers, like those commonly used for regional anesthesia and central venous access, contain as many as 120 individual ceramic elements aligned side-by-side along the face of the transducer" (Transducers, 2016, para. 4). The size of the piezoelectric crystals that are used in ultrasound transducers determine its corresponding frequency, including the depth of the near (Fresnel), focus and far (Fraunhofer) zones as shown in Figure 2 below.

Figure 2. Near, Focal and Far Zones

Source: http://keyweb14.com/vault/wp-content/uploads/2012/03/Near-and-Far-Zones.png

Finally, there are three main types of ultrasound used today as follows: