11:6/ The Absorption of Electromagnetic and Ultrasonic Energy 217 



further support the existence of mechanical relaxations although it 

 would not give any clue as to the origin of them. However, the data in 

 Figure 5, for example, indicate that if relaxation processes occur, there 

 must be several for each protein molecule. 



Much greater absorptions of ultrasonic energy occur at boundaries 

 between media of very different density and hardness. For example, 

 when an ultrasonic wave passes through soft tissue and is incident on 

 bone, it encounters a region of much greater pc. At this interface, a 

 large part of the ultrasonic energy is converted into heat. This gross 

 phenomenon, although easy to observe experimentally, is difficult to 

 analyze in detail. Ultrasonic heating at bone surfaces forms the physical 

 basis for many of the applications of ultrasonic diathermy. 



6. Diathermy 



Both ultrasonic and electromagnetic radiation are used clinically to 

 heat selective portions of patients. This process is known as diathermy. 

 It is more effective than the application of external heat because the heat 

 is generated throughout the tissues. A variety of disorders are alleviated 

 and recovery is speeded in others through the use of diathermy. The 

 physical action is one of heating, although stirring the contents and 

 surroundings of the cells may also be important in ultrasonic diathermy. 



At very low frequencies, the absorption of both electromagnetic and 

 ultrasonic radiations is so weak that neither is effective for diathermy. 

 Absorption increases as does the frequency up to about 1 mc; as the 

 frequency is raised above this the impedances (both electrical and ultra- 

 sonic) remain constant until much higher frequencies are reached. The 

 depth of penetration measured in wavelengths also remains constant. 

 Because the wavelength is inversely proportional to the frequency, the 

 shorter wavelengths have a lower penetration. The extreme high 

 frequency limit of electromagnetic diathermy is the use of an infrared or 

 heat lamp whose radiations barely penetrate the skin. 



In spite of their similarities, there are certain gross differences between 

 ultrasonic and electromagnetic diathermy. Electromagnetic energy is 

 only poorly absorbed in fatty tissues and hardly at all in bone. As a 

 result, most of the heating occurs in the muscles. In contrast, ultrasonic 

 energy is equally absorbed in fat and muscle and is markedly absorbed 

 at the bone-tissue interfaces. It is used widely in the treatment of 

 various bone diseases. 



Another difference concerns the wavelength. At any given frequency, 

 the wavelength of ultrasound is much shorter than that of an electro- 

 magnetic wave. Accordingly, ultrasonic diathermy treatment can be 



