Brief history of acoustics

The goal of this chapter is to acquaint the reader with some of the significant milestones and pioneers in the history of acoustics and medical ultrasound (Table 1-1). The taproots of diagnostic ultrasound lie in one of the oldest sciences—the science of sound, or acoustics—which dates back to the ancient Greeks. Earlier cultures, such as the Egyptians, Persians, and the Chinese, developed musical instruments and became interested in how sound was created. However, a systematic study of sound really began with the Greek mathematician Pythagoras, thought to have been born around 550 BC. Pythagoras observed the relationship between sound pitch and frequency and is thought also to have invented the sonometer, an instrument used to study musical sounds. The Greek scholar Archytas, a Pythagorean and contemporary of Plato, correctly showed that pitch is related to the movement of vibrating air. However, he incorrectly asserted that the speed at which the vibrations travel to the ear is a factor in determining pitch. It was not until 350 BC that Aristotle developed the theory of sound propagation, the idea that sound is carried to the ears by the movement of air. Boethius compared sound to water (Box 1-1).

Although scholars in the Middle East and India were developing new ideas about sound by studying music and working out systems of music theory, the study of sound remained relatively dormant during the Middle Ages in the West. Interest in sound was revived in Europe during the upsurge of scientific investigation in the Renaissance. In 1500 Leonardo da Vinci (1452-1519) became interested in the physical properties of sound. He is thought to have originated the idea that sound travels in waves. Da Vinci also discovered that the angle of reflection is equal to the angle of incidence. Galileo Galilei (1564-1642) is credited with starting modern studies of acoustics by elevating the study of vibrations to scientific standards. In 1638 he demonstrated that the frequency of sound waves determined the pitch. In the early eighteenth century Sir Isaac Newton (1643-1727) studied the speed of sound in air and provided the first analytic determination of the speed of sound. Newton announced the mathematic derivation of the theory of velocity, based on his conclusions that light is made up of many tiny particles.

Unconvinced by the particle theory of light advanced by Newton, Christiaan Huygens (1629-1695) argued that light consists of waves. In 1687 he began a landmark treatise on the physics of light entitled Traité de la Lumière, published in 1690. Huygens’s principle asserted that each point in a light wave could be explained by miniature “wavelets” that combined to form a wavefront. From his theories Huygens was able to derive the laws of reflection and refraction. Huygens’s work was later elaborated on by French physicist Augustin Fresnel (1788-1827), who theorized that light “wavelets” possess the same frequency as their original wave (Huygens-Fresnel principle). This theory became important for scientists studying acoustics when they began to compare the similarities and differences between light waves and sound waves.

Over the next two and a half centuries, various experiments paved the way to our current understanding of the fundamentals of acoustics. The end of the nineteenth century marked the beginning of the modern study of acoustics, with the publication of The Theory of Sound by the British scientist Lord Rayleigh (1842-1919). In his remarkable book, Rayleigh gathered, clarified, and expanded the current knowledge of acoustics. The first volume, on the mechanics of a vibrating medium that produces sound, was published in 1877, and the second volume on acoustic wave propagation was published the following year.

The nature of ultrasound

Ultrasound describes sound frequencies beyond (ultra-) the range of normal human hearing (i.e., 20 hertz [Hz] and 20 kilohertz [kHz]). Thus ultrasound refers to sound frequencies greater than 20 kHz. One of the first experiments having to do with ultrasound took place in 1793 when Lazzaro Spallanzani (1729-1799), an Italian priest-scientist, was studying the activities of bats. Observing that bats could function effectively in the dark, even if blinded—but not if deafened—Spallanzani theorized that the bats were listening to something he could not hear. Exactly what it was eluded him. Spallanzani’s questions were answered 145 years later, in 1938, when George Washington Pierce invented a “supersonic receiver,” an instrument that was able to pick up the high-frequency squeaks emitted by bats and convert them into sound audible to humans.

During the scientifically rich nineteenth century, theories, investigations, and discoveries about sound abounded. Important among them was the theory of the effect of motion on the pitch of sound. In 1842 Austrian scientist Christian Johann Doppler (1803-1853) formulated the principle that when a source of wave motion moves, the apparent frequency of the emitted wave changes. This is known today as the Doppler effect.

In 1880 Paul-Jacques Curie (1856-1941) and his brother Pierre Curie (1859-1906) discovered piezoelectricity. These researchers established the presence of the piezoelectric effect by observing that certain crystals would expand and contract slightly when placed in an alternating electrical field. Reverse piezoelectricity permitted the same crystal to create an electric potential, or voltage, making the crystals useful as receivers and sources of sound waves, with ranges from audible to ultrasonic frequencies. Their accomplishments ultimately led to the development of the modern ultrasound transducer.

Not until the twentieth century did scientists learn how to produce ultrasound waves and put them to work. Until then, ultrasonic waves were little more than a scientific curiosity. During World War I, French physicists Paul Langevin (1872-1946) and Constantin Chilowsky (1880-1958) studied controlled sound frequency and intensity and discovered a way to use the property of echoing sound waves to detect underwater objects. Their device, the hydrophone, was used extensively in the surveillance of German submarines. In 1916, during World War I, the first submarine detected by the hydrophone was sunk. Even more important was the fact that their studies laid the groundwork for the development of sonar in World War II. Langevin also discovered harmful effects of ultrasound on marine life after observing that when small fish swam through ultrasound beams, they were killed instantly. He realized the potential power of the energy with which he was dealing when one of his assistants, holding his hand briefly in the path of the sound wave, experienced agonizing pain—as if his very bones were being heated.

Science and industry are responsible for the great strides made in the understanding and refinement of ultrasonic energy. Industrialists such as Floyd Firestone, with his ultrasonic invention called the reflectoscope, harnessed its awesome power and found many uses for it, including the detection of flaws in metal and the cleansing of metals.

Medical applications of ultrasound

In 1927 Robert Williams Wood and Alfred Lee Loomis first discussed the destructive nature of ultrasound on biologic organisms and living tissues. The effect of high doses of ultrasonic energy on the body is as injurious as atomic radiation. The effects produced by high-energy ultrasonic waves normally are irreversible and arise from cavitation, intense mechanical stresses, or intense localized heating. The minimum dose for damaging ultrasound is not defined easily nor is it possible to correlate a definite type of tissue damage with a universally standardized dosage. In lower doses, however, the effects can be therapeutic. Focused high-energy ultrasound waves are used for therapeutic ultrasound to remove unwanted tissue.

Low-intensity ultrasonic waves, on the other hand, can be used to visualize the interior of the body in a noninvasive and painless way. Since the late 1960s, scientists have been probing the soft tissues of the human body, seeing with sound. One of the first physicians to use ultrasound for medical diagnostic purposes was Austrian Karl Dussik (1908-1968). During the 1940s, he used two transducers positioned on opposite sides of the head to measure ultrasound transmission profiles. He also discovered that tumors and other intracranial lesions could be detected by this technique. He referred to these images of the brain as hyperphonograms. Dussik was using A-mode (amplitude modulation), which produces a one-dimensional image.

The development of metal flaw detectors and naval sonar also made possible the work of three independent American investigators: George Ludwig, John Wild, and Douglass Howry. Between 1947 and 1949, Ludwig, a Naval medical research surgeon, joined colleagues at the Massachusetts Institute of Technology. Using A-mode studies exclusively, Ludwig was able to detect successfully gallstones embedded in animal tissues (Figure 1-1). Wild, an English surgeon who immigrated to America, was the first to consider using ultrasound to detect tissue thickness. In his work at the University of Minnesota, he realized that ultrasonically, cancerous tissues differed greatly from normal tissues. Along with engineer John Reid, he constructed an early prototype breast scanner that employed the use of an externally placed water path. Their B-mode (brightness modulation) techniques used 2D presentations of echo-producing interfaces (Figure 1-2). In 1955 Wild and Reid presented the first paper on ultrasound used for medical purposes.¹ Wild was also a pioneer in the development of early internal scanners and devised a rectal transducer to obtain images of the large bowel.

In Denver, radiologist Douglass Howry worked independently of the other groups. Howry had pursued an interest in diagnostic ultrasound since 1948. In 1951, using war surplus electronic components, he constructed a “water-path” scanner. A laundry tub and later a cattle tank were the first prototypes for holding the water in which the subject or the body part to be imaged was submersed in water (Figure 1-3). Unfortunately, the one-dimensional images that resulted were disappointingly incomplete. Howry joined his friend and mentor, Joseph Holmes, who was at the University of Colorado in Denver. Their collaboration resulted in the development of a compound scanner. Howry and Holmes discovered that by simultaneously moving the transducer in two different motion patterns, a more complete anatomic picture could be formed. The cattle tank eventually was replaced by an upturned B-29 gun turret, in which the subject was immersed and weighted down to avoid introducing motion artifacts as the mechanically circling transducer cut a path through the water (Figure 1-4). The impracticality of using this method on sick patients spurred the two scientists to simplify the procedure and to develop a water-bath or “pan” scanner in 1957 that permitted the patient to sit next to a small pan of water through which the transducer moved.

Their experience eventually led Howry and Holmes to the development of a compound contact scanner, which provided direct scanning of the body using a light film of oil or lubricating gel to replace the cumbersome water path (Figure 1-5). The early bi-stable images registered on a phosphorus-coated oscilloscope screen as dots of light. All echoes greater than predetermined specific amplitudes were displayed as constant-intensity dots of light.

A major limitation of bi-stable imaging was that the storage phosphor was either “off” or “on,” and the resulting image existed in either black or white. Thus a white dot from an echo, at or above threshold strength, would appear on the screen. If the echo reflection was below the established threshold strength, no echo dot would be visible on the screen.

After Howry left Denver to work at the Massachusetts Institute of Technology, Holmes worked closely with William Wright, an electronic engineer from Loveland, Colorado. The fruits of their labor were realized in 1962 with the introduction of the first commercially available portable ultrasonic system, a compound contact scanner known as the Physionics Engineering Porta-Arm (Figure 1-6). This system gained worldwide acceptance and use. The work of the Denver group is considered one of the most seminal pioneering contributions in B-mode imaging and contact scanning. The Porta-Arm scanner was the direct precursor to the imaging systems in use today.

In the mid-1950s, obstetrician Ian Donald, aware of Howry’s work and that of Japanese researchers, began his study of diagnostic ultrasonography in Scotland. His interest stemmed from his experiences with the Royal Air Force during World War II, during which time he witnessed the ultrasonic testing of aircraft to detect metal stress and fatigue. Donald vowed to pursue his notion that ultrasound may be used in a similar fashion on patients. By means of comparative examinations of an excised tumor and a beefsteak (from the local butcher shop), Donald proved that tumors possessed echo patterns different from those of normal tissue. This work was conducted in an atomic boiler plant outside Glasgow with use of a borrowed flaw detector. Later, again using borrowed equipment and A-mode technique, he began detecting ovarian cysts, ascites, and polyhydramnios in his patients. Donald is credited with perfecting the first A-mode measurement of the fetal biparietal diameter, making it possible to ultrasonically estimate fetal age, weight, and growth rate (Figure 1-7).

In 1957 Donald collaborated with industrial engineer Tom Brown to develop a contact compound scanner, which they mounted on a bedside table and suspended over the patient. Manipulating the transducer by hand underneath the table, Donald, using brightness modulation (B-mode) technique, produced the first crude fetal scans. By 1960 Donald and Brown had developed a mechanical sector scanner and later a hand-held scanner, the Diasonograph, which was suitable for commercial distribution.

Although Donald was involved in the research and development of ultrasound equipment, his primary interest was in applying diagnostic ultrasonography to his specialty of obstetrics and gynecology. He is credited with contributing to the diagnosis of multiple pregnancies, hydramnios, and hydatidiform mole, in addition to the introduction of the fluid-filled bladder technique used in early pregnancy and gynecologic studies. His crowning achievement occurred in 1954 when he was the first to demonstrate a fetal gestational sac, which earned him the title “Father of Obstetric Ultrasound.”

A few months earlier, on October 29, 1953, equipped with the Siemens Ultrasound Reflectoscope, Inge Edler (1911-2001) and his physicist friend Hellmuth Hertz recorded the first moving pictures of the heart. Employing A- and B-mode techniques, they added a continuously moving display of the returning cardiac echoes (Figure 1-8). At first they were stymied by being unable to identify the various motion patterns, but then Edler realized that he was seeing the characteristic pattern of the anterior leaflet of the mitral valve. Their description of M-mode (motion mode) echocardiography marked the beginning of a new diagnostic noninvasive technique. Later, Sven Effert, placing a transducer directly on the heart, was able to verify all of Edler’s previous identifications and motion patterns. With this validation, the diagnostic potential of echocardiography became apparent. To further the effort in cardiology, while completing his Ph.D. in electrical engineering (1957-1965), John M. Reid worked on echocardiography with cardiologist Claude Joyner. Together they produced and used the first echocardiography system in the United States.

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