230 Destructive Effects of High Intensity Ultrasound / 1 2. : 5 



5. Neurosurgery with Ultrasound 



Ultrasonic agitation can be used for surgery within the central nervous 

 system. This application is very different from the effects of cavitation 

 on single cells in suspension ; further, it appears not to involve the general 

 tissue heating discussed in Chapter 1 1 . Essentially, neurosurgery with 

 ultrasound depends on the fact that neurons can be destroyed by short 

 ultrasonic pulses at high acoustic pressure levels. The pulses can be 

 made sufficiently short so that negligible heating occurs. 



The neuron damage can be restricted to a small volume by the use of 

 sharply focused ultrasonic beams. The wavelength of sound at 1 mc in 

 water or tissue is 



c 1.5 x 10 5 cm/sec 

 A = - = tt^t, ■ — = 0.15 cm 



V 



10 6 /sec 



Thus, it should be possible by the use of suitable focusing devices to 

 restrict the damage to a volume of about 1 mm 3 , that is, about 10" 3 ml. 

 Very complex apparatus has been used to approach this extreme of 

 focusing. 



Neurosurgery with ultrasound has several advantages over more 

 conventional surgery. First, and perhaps most important, the pulse 

 duration and intensity can be adjusted so that the blood vessels and 

 supporting (glia) cells are undamaged. Second, it is possible to destroy 

 neurons within the brain without damaging the surface of the brain. 

 As in other types of brain surgery, a part of the skull must be removed 

 before the application of the ultrasound. 



The action of destroying the neurons but leaving the surrounding cells 

 undamaged is not clearly understood. However, a number of possible 

 explanations can easily be eliminated. For example, the action on the 

 neurons is not due to heating. Experiments at different temperatures 

 showed the same results. Moreover, intermittent exposures produced 

 the same destruction as continuous exposures of the same total exposure 

 time. Likewise, the neurosurgical effects do not depend critically upon 

 the frequency of the applied signal. Thus, they are not a resonant 

 type of phenomenon. Static pressures of a magnitude comparable to 

 those occurring during the positive pressure of the acoustic cycle do not 

 produce any damage. 



The absence of other effects suggests cavitation as a possible cause of 

 neuron destruction. Traditionally, the most reliable test for cavitation 

 has been to apply an excess static pressure. If the effects observed are 

 due to cavitation, these should be decreased when an excess pressure is 

 applied. This is, indeed, the case for the damage to neurons; at any 

 ultrasonic pressure levels, the destruction is much less when a static excess 



