Heating vs. cavitation in the induction of mouse hindlimb paralysis by ultrasound

Grip strength tests were performed on hairless mice before and after variou s ultrasound exposures in a temperature-controlled water bath at 37 degrees C. Lithotripter exposure of 800 shock waves produced no effect on hindlimb function, In contrast, 1.09-MHz exposures at 1 MPa with 10:100 ms burst mo de did produce a statistically significant reduction in grip strength of ab out 60%. The exposure duration was important for the 1.0-MPa burst mode exp osure, with grip-strength reductions appearing after 150 s or longer exposu res. Continuous exposure at 3.3 W cm(-2) (0.32 MPa peak) for 200 s produced the same effect as burst mode exposure at 3.3 W cm(-2) (1-MPa peak) for 20 0 s, which implicates the temporal average intensity as an important factor . The temperature elevations for 1-MPa burst mode was estimated from thermo couple measurements in the spine to be 12 degrees C after 200-s exposure. A lthough tests of exposures in cool (32 degrees C) and warm (42 degrees C) b aths produced inconclusive results in regard to the thermal mechanism, the effects observed appear to result from ultrasonic heating (rather than from cavitation). Thus, any potentially harmful consequences associated with th e effects examined might be related more, for example, to ultrasonic hypert hermia therapy than to shock-wave lithotripsy. (C) 1999 World Federation fo r Ultrasound in Medicine & Biology.