218 The Absorption of Electromagnetic and Ultrasonic Energy /I I : 7 



administered to a joint such as the elbow, or even part of a joint. 

 Electromagnetic diathermy treatment would affect all the muscles of the 

 arm at conventional frequencies (20 mc). 



7. Summary 



Ultrasonic and electromagnetic energy are absorbed by all tissues and 

 cells. These absorptions have been studied over a wide range of fre- 

 quencies. Such investigations have contributed to an understanding of 

 the physical nature of biological cells and tissues. Certain gaps in 

 present knowledge are emphasized by these studies. 



The electromagnetic absorptions are explained by the model of a cell 

 consisting of an insulating dielectric wall surrounding an interior 

 protoplasm which is a fair conductor of electricity. The electrical 

 impedance from 10 kc to 10 mc, both of suspensions of single cells and 

 also of whole tissues, fits this model very well. Above 10 mc, certain 

 relaxation phenomena associated with the water itself- are important. 

 These phenomena are predicted by conventional electromagnetic theory. 

 Below 10 kc, other electrical relaxations occur in the tissues. The 

 origin or significance of the low frequency relaxations is not known. 



Absorption of ultrasonic energy in cells fails to fit any simple visco- 

 elastic model above 300 kc. This failure emphasizes the incompleteness 

 of current knowledge of protein structure. The motion of single cells in 

 blood contributes a small but measurable amount to the total ultrasonic 

 absorption. However, the major effect in blood-cell suspensions and 

 tissues is due to the proteins. Many different tissues with widely varying 

 protein types have similar ultrasonic absorptions. Thus, some common, 

 but not understood, molecular property of proteins is measured by these 

 experiments. 



Both electromagnetic and ultrasonic irradiation of tissues are used 

 clinically. The success of this application has been the inspiration for 

 many of the more basic studies of the cellular and molecular origin of 

 these absorptions. 



REFERENCES 



1. Cole, K. S., and H. J. Curtis, " Bioelectricity : Electric Physiology," 

 Otto Glasser, ed., Medical Physics (Chicago, Illinois: Year Book Publishers, 

 Inc., 1950) Vol. 2, pp. 82-90. 



2. Schwan, H. P., "Electrical Properties of Tissue and Cell Suspensions," 

 Advances in Biological and Medical Physics,]. H. Lawrence, and C. A. Tobias, 

 eds. (New York: Academic Press, Inc., 1957) Vol. 5, pp. 147-209. 



