212 



The Absorption of Electromagnetic and Ultrasonic Energy /I I : 3 



much higher than that of the softer tissues, particularly at low frequencies. 

 Fatty tissue is very different because fat is an excellent electrical 

 insulator. Tissues of this nature show a much higher resistivity and a 

 much lower dielectric constant than do those with more water. The 

 general shape of the resistivity and dielectric curves is similar to that for 



Figure 4. Ultrasonic absorption by blood. Curve 1 shows 

 absorption per wavelength for packed red blood cells. Curve 

 2 illustrates similar absorption per wavelength for whole blood. 

 Curve 3 is a plot of the absorption per wavelength for whole 

 blood computed from the absorption of plasma proteins and 

 hemoglobin. Curves 4 and 5 diagram the ultrasonic absorp- 

 tion of plasma in db per wavelength and db per cm, 

 respectively. After E. L. Carstensen and H. P. Schwan, J. 

 Acous. Soc. America 31 : 185 (1959). 



muscle. Brain tissue has more fat-like material (lipids) than does muscle. 

 At lower frequencies, its resistivity is close to that of fatty tissues. How- 

 ever, this resistivity falls rapidly as the frequency is raised from one to 

 10 megacycles (10 6 to 10 7 cps). Its value above this frequency range is 

 close to that of the nonfatty tissues. Its dielectric constant is within 

 the range of the watery tissues at all frequencies. 



In concluding this section, it should be noted that the electrical 

 impedance of biological cells supports the picture of a cell consisting of 



