Mechanical Bioeffects of Pulsed High Intensity Focused Ultrasound on a Simple Neural Model Open Access
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In the study of traumatic brain injury, an understanding of how pressure pulses affect nerves through mechanisms that are neither thermal nor cavitational in nature is important. This study used a simple neural model - the giant axon of the earthworm - in combination with a train of 825 kHz ultrasound pulses of adjustable magnitude and duration, to study the effects of low-energy ultrasound pulse trains on nerve function. The amplitude and conduction velocity of action potential triggered in the worm were measured as a function of the strength of the ultrasound pulse train. Various measures of the strength of the pulse train, including acoustic impulse for an individual exposure, and cumulative impulse of all exposures delivered up to the given time, were examined for their usefulness as a correlating parameter of the data. It was found that using cumulative impulse as the independent variable and conduction velocity of the dependent variable provided the best correlation between experiments conducted on different worms at different pulse strengths and exposure times. The experiments showed a reduction in the action-potential amplitude and nerve conduction velocity as the impulse was increased, beginning at much lower values of the impulse (about 2000 Pa-sec) than was measured in previous studies in frog sciatic nerves that had thermal effects. When compared with studies involving neurological damage in rats produced by shock-tube or detonation overpressures, the impulses needed to produce bioeffects were higher in the present experiments. However, in terms of the radiation-force impulse associated with the oscillatory pulse train, the impulse was lower for the present study, more in line with the level of damage likely to occur in the two types of experiments. This highlights the possible role of radiation-force impulse in affecting the nervous system.