214 The Absorption of Electromagnetic and Ultrasonic Energy /I I : 5 



some of the longitudinal (that is, compressional) wave is converted into 

 a transverse (or shear) wave. This wave is attenuated very rapidly in 

 viscous media such as protoplasm. In addition, ultrasonic waves are 

 scattered and absorbed at all cell interfaces. This is greatly accentuated 

 when there is a large change in pc, such as at a bone-soft tissue interface, 

 but is an important factor- even if the change in pc is small. 



The wavelength of ultrasonic energy is much smaller than that of 

 electromagnetic energy of the same frequency. This means the passage 

 of ultrasonic energy through tissues can be confined to a much smaller 

 volume than can electromagnetic energy. For example, the ultrasonic 

 wavelength is about 1 .5 mm at 1 mc, whereas the electromagnetic wave- 

 length is about 300 meters. An ultrasonic beam 0.25 mm in diameter 

 is feasible at 1 mc ; an electromagnetic beam would be at least 50 meters 

 in diameter at the same frequency. 



A result of this short ultrasonic wavelength is that ultrasonic absorption 

 or reflection can be used, just as X rays, to determine the structures 

 within living organisms. Various systems have been devised for this. 

 All of them are superior to X rays, in that ultrasonograms (as they are 

 called) have no known harmful effects. In contrast, more information 

 is usually obtained from an X-ray photograph than from most of the 

 ultrasonograms developed to date. 



If the intensity and duration of the ultrasonic energy is raised suffi- 

 ciently, destructive effects can be produced. These are discussed in 

 greater detail in Chapter 12. In the present chapter, only non- 

 destructive absorption will be discussed further. 



5. Nondestructive Effects of Ultrasound 



The absorption of ultrasonic energy at frequencies above about 250 kc 

 has been studied for many different types of tissues obtained from various 

 mammals. The real part of the acoustic impedance (that is, pc) is 

 essentially the same for all tissues. The exception is bone, which has a 

 much higher pc than any other tissue. In contrast, the absorption of 

 ultrasonic energy varies markedly from one soft tissue to another. 

 Perhaps the easiest data to interpret are those obtained for blood. It is 

 possible to measure separately the contributions to the absorption due 

 to the proteins dissolved in the plasma, the hemoglobin within the red 

 blood cells, and the cellular structure itself. These measurements show 

 that the absorption per wavelength increases monotonically as the 

 frequency is raised from 0.5 to 20 mc. This gradual increase, however, 

 is slower than would be expected for a simple viscoelastic medium. 

 The studies of blood showed that the ultrasonic absorption due to 



