12:5/ Destructive Effects of High Intensity Ultrasound 



231 



pressure of about 13 atm is applied. However, as is illustrated in 

 Figure 5, neuronal destruction is still observed when the acoustic 

 pressure amplitude is 6 atm, whereas the static or average pressure is 

 13 atm. While making cavitation less likely as a cause for neuronal 

 destruction, these observations do not rule it out completely because 

 cavitation is observed in particulate suspensions at positive pressures. 



Another criterion for cavitation is the existence of a sharp pressure 

 threshold below which no effects would be observed. A sharp threshold 



0.4 



0.3- 



CD 



0.2- 



0.1 



1.0 2.0 3.0 4.0 5.0 6.0 



Voltage Driving Crystal (kv) 



FigureS. Minimum time for paralysis. The lines on the graph 

 represent measurements, at two hydrostatic pressures, of the 

 relationship of the driving voltage on the crystal to the recip- 

 rocal of the "minimum time for paralysis" for frogs cooled 

 to 1 C. After W. J. Fry, " Action of Ultrasound on Nerve 

 Tissue— a Review," J. Acous. Soc. Am. 25: 1 (1953). 



of this nature does exist for the ultrasonic destruction of neurons. More- 

 over, this threshold depends on the applied external pressure, although 

 the rate of change is relatively small. 



Thus, the application of extremely high intensity bursts of ultrasonic 

 energy in destroying neurons is not fully understood. It appears 

 possible that this might be a different sort of cavitation effect. Cellular 

 injury due to effects other than cavitation was demonstrated by 

 Goldman and Lepeschkin using algae and rotifers. This injury was 

 similar to the neuron damage in that the cells were not disrupted. 

 However, the injuries to algae and rotifers could be observed immedi- 

 ately after exposure to ultrasound, whereas the damage to neurons 

 could not be detected histologically for the first ten minutes after ex- 

 posure. Whatever its origin, the ultrasonic destruction of neurons 

 is of clinical importance to the surgeon. 



