12:4/ Destructive Effects ot High Intensity Ultrasound 



227 



cells. Increases in the viscosity and in the wetting ability of the sus- 

 pending medium have been found to raise the ultrasonic pressure 

 necessary for cavitation. Thus, red blood cells in isotonic saline are 

 ruptured at lower ultrasonic pressures than are necessary in whole blood. 

 The threshold for cavitation always depends on the part of the field 



where the greatest ultrasonic pressures 

 occur because stirring invariably 

 accompanies cavitation. For non- 

 focused sound fields, these maxima 

 occur at or near the transducer surface. 

 Experiments have shown that over a 

 wide frequency range, from perhaps 

 250 cps to 1 mc, the threshold is un- 

 changed. By contrast, in focused 

 sound fields where cavitation occurs 

 away from the surface of the trans- 

 ducer, the ultrasonic pressure neces- 

 sary to produce cavitation increases 

 rapidly as the frequency is raised. 



The threshold for cavitation in the 

 body of the liquid is controlled, at a 

 given frequency, by the existence of 

 submicroscopic pockets of gas or vapor 

 called nuclei. If the amplitude of the 

 pressure changes during the ultrasonic 

 cycle is sufficiently great, the nucleus 

 grows, while the pressure is decreasing, 

 to such a large volume that it col- 

 lapses violently when the pressure starts to increase. The product of the 

 pressure amplitude times the period of ultrasonic wave determines the 

 pressure amplitudes necessary for violent collapse of the cavities. At about 

 10 kc, one atmosphere of pressure amplitude is sufficient, whereas at 1 

 mc the threshold for cavitation in air-saturated water is about 30 atm. 

 Over very wide frequency ranges, the relative rates at which different 

 types of biological cells are destroyed remain unaltered. These relative 

 rates can be used to indicate the relative fragility of different cell types. 

 To make this quantitative, one must study the time rate of cell destruc- 

 tion in a constant sound field. Studies of these rates show that as long 

 as at least 1 per cent of the population remains undamaged, one may 

 write 



Figure 3. The distortions of a cell wall 

 which might be caused by an oscil- 

 lating bubble near the cell. After 

 Eugene Ackerman, " Pressure Thresh- 

 olds for Biologically Active Cavita- 

 tion," J. Appl. Physics 24: 1371 (1953). 



dt 



(1) 



