Sound and Ultrasound

When Russ Hobbie and I started to prepare the 4th edition of Intermediate Physics for Medicine and Biology, we discussed if we should add any new chapters. After reviewing comments from users of the 3rd edition, we concluded that there was one essential topic not covered in previous editions: medical ultrasound. So, Chapter 13 of the 4th edition is our attempt to describe the physics of sound and ultrasound, as applied to biology and medicine.

Sound (or acoustics) plays two important roles in our study of physics in medicine and biology. First, animals hear sound and thereby sense what is happening in their environment. Second, physicians use high-frequency sound waves (ultrasound) to image structures inside the body. This chapter provides a brief introduction to the physics of sound and the medical uses of ultrasonic imaging. A classic textbook by Morse and Ingard (1968) provides a more thorough coverage of theoretical acoustics, and books such as Hendee and Ritenour (2002) describe the medical uses of ultrasound in more detail.

In Sec. 13.1 we derive the fundamental equation governing the propagation of sound: the wave equation. Section 13.2 discusses some properties of the wave equation, including the relationship between frequency, wavelength, and the speed of sound. The acoustic impedance and its relevance to the reflection of sound waves are introduced in Sec. 13.3. Section 13.4 describes the intensity of a sound wave and develops the decibel intensity scale. The ear and hearing are described in Sec. 13.5. Section 13.6 discusses attenuation of sound waves. Physicians use ultrasound imaging for medical diagnosis, as described in Section 13.7. Ultrasonic imaging can provide information about the flow of blood in the body by using the Doppler effect, as shown in Sec. 13.8.

At 16 pages, this chapter on sound is the shortest in our book. Throughout the 4th edition, we moved many interesting applications into the homework problems, and nowhere is this more evident than in Chapter 13. Many aspects of ultrasonic imaging are only addressed in the problems, so you really need to work them in order to get a full understanding of ultrasound techniques.

Air and Water,
by Mark Denny.

Chapter 13 has the fewest references (only six) of any chapter in our book. To provide some additional guidance, let me comment on some these references. Theoretical Acoustics by Morse and Ingard is excellent but quite mathematical. Don’t look there for a gentle introduction. I always recommend Denny’s Air and Water for insight into biological physics, in this case his Chapter 10, Sound in Air and Water: Listening to the Environment. Hendee and Ritenour's Medical Imaging Physics (4th Edition) has several excellent chapters on medical ultrasound.

Where can you get additional information? First, there is always Wikipedia. Another online source can be found at How Stuff Works. The American Institute of Ultrasound in Medicine has a useful website, and publishes the Journal of Ultrasound in Medicine. Hendee and Ritenour repeatedly cite Zagzebski’s Essentials of Ultrasound Physics. Although I haven’t read it, my understanding is that it is aimed at students who are preparing to take the ARDMS exam. The American Registry for Diagnostic Medical Sonography (ARDMS) administers accreditation examinations in the area of diagnostic medical sonography, and publishes a list of topics covered on the Ultrasound Physics and Instrumentation exam. Another book that I haven’t read but have heard good things about is Diagnostic Ultrasound, by Rumack, Wilson, and Charboneau. At $325, the third edition of this two-volume tome is a bit pricey, but perhaps your library will have a copy. Finally, Russ and I always like to point out good American Journal of Physics articles, such as Mark Denny’s “The physics of bat echolocation” (Volume 72, Pages 1465–1477, 2004). Those brave souls who are teaching medical or biological physics with a lab incorporated into the class may find the paper “Undergraduate experiment to measure the speed of sound in liquid by diffraction of light” (Luna et al., American Journal of Physics, Volume 70, Pages 874–875, 2002) useful.

Theory of Sound,
by Lord Rayleigh.

Before I end, let me acknowledge one physicist who contributed much to our understanding of sound, Lord Rayleigh. I am a big fan of Victorian physicists (such as Faraday, Maxwell, and Kelvin), among whom Rayleigh is one of my favorites. This Nobel Prize winner, who published the two-volume Theory of Sound, was one of the first to discover how humans localize sound. He is one more of a long list of physicists who have made fundamental contributions to biology and medicine.

Note added October 26: A friend of mine, Neb Duric of the Karmanos Cancer Institute, has developed a way to detect breast cancer using ultrasound. For more, see the video on the Medical Physics in the news website.