The interaction of ultrasonic heating and cavitation in vascular bioeffects on mouse intestine

Anesthetized hairless mice were exposed at the midsection to 400-kHz focused ultrasound that was continuous or pulsed with 100 microseconds pulses, in a temperature-controlled water bath. After exposure, the intestines were evaluated for petechiae, presumably induced by heating, and hemorrhages, presumably induced by cavitation. Petechiae (up to about 100) occurred above 0.28 MPa (2.6 W cm-2) for 1000 s continuous exposure at 37 degrees C, and the threshold increased to 6.5 MPa (1.4 W cm-2 temporal average) for 1000 s pulsed exposure (0.001 duty factor). Hemorrhages (up to about 10) were seen above 0.65 MPa for continuous exposure (10 s, 100 s or 1000 s), and the threshold increased only to 1.6 MPa for 1000 s pulsed exposure (0.001 duty factor). Fractionating a brief continuous exposure into a low duty-factor pulsed exposure greatly decreased the petechiae, but actually increased the hemorrhages. For example, 1 s continuous exposure at 3.2 MPa (340 W cm-2) averaged 0.33 hemorrhages per mouse, and 1 s on-time pulsed exposure (1000 s duration, 0.001 duty factor) at 3.2 MPa (0.34 W cm-2 temporal average) averaged 4.3 hemorrhages per mouse. More petechiae were induced at 42 degrees C bath temperature relative to 32 degrees C or 37 degrees C, and the hemorrhage effect was somewhat enhanced by elevated temperature. Generally, heating and cavitation appeared to have largely independent roles in vascular bioeffects on mouse intestine.