11:2/ The Absorption of Electromagnetic and Ultrasonic Energy 205 



Other types of radiation can damage cells without producing ioniza- 

 tion. For example, under certain conditions single cells suspended in a 

 liquid are fractured when irradiated with acoustic energy. This is 

 always accompanied by a process called cavitation, in which small bubbles 

 or holes form in the liquid. These and other destructive actions of 

 ultrasonics are discussed in Chapter 11. If the acoustic power is not 

 too high, or if cavitation is suppressed, exposed biological cells absorb 

 some of the ultrasonic energy. This nondestructive absorption of 

 ultrasound is also discussed in the present chapter. 



When any type of energy is absorbed, it is eventually converted to 

 heat. The phenomena considered in this chapter are grouped together 

 because the conversion of incident energy to heat is the direct, immediate 

 effect. The different tissues of the human body, and different single 

 cells, all have differing absorptions, both for electromagnetic energy 

 and for ultrasonic energy. Thus, it is possible to selectively heat certain 

 tissues and certain portions of the human body. This heating action is 

 known as diathermy. Local heating of tissues promotes recovery from 

 many disorders. Diathermy is the direct medical application of the 

 phenomena discussed in this chapter. 



The absorption of electromagnetic and ultrasonic energy has, however, 

 a more fundamental significance. It is an important tool for building 

 a complete picture of the physical nature of biological cells and tissues. 

 As such, it supplements knowledge gained by looking through a micro- 

 scope and also supplements studies of molecular biology. 



2. Electrical Impedances 



The absorption of electromagnetic energy in tissues can be described 

 only in the language of electricity and magnetism. Several important 

 definitions are summarized in Table I of Appendix C. Any reader not 

 well versed in electrical terminology and definitions is asked to study 

 that appendix before proceeding with the current chapter. Magnetic 

 terms were completely omitted from Table I, but magnetic fields exist 

 whenever a current flows. Thus, if one passes an alternating current 



through tissues (or other conductors), a magnetic field H will be 

 generated. Likewise, if a tissue (or other conductor) is subjected to a 

 changing magnetic induction B, an emf will be induced in it. The 



proportionality constant between B and H is called the magnetic 

 permeability, /x. These added terms, along with a few others, are 

 summarized in the following table. 



