Heating as a mechanism for ultrasonically-induced petechial hemorrhages in mouse intestine

Anesthetized hairless mice were exposed to 1.035 MHz unfocused ultrasound in a temperature-controlled water bath. Visible petechial hemorrhages and hyperemia were produced in the intestines. Histologically, the petechiae appeared to be localized mostly to the lamina propria of the mucosa. For 2 min exposure, the number of petechiae and length of hyperemic tissue both increased with increasing spatial peak pressure amplitude above a threshold of about 0.4-0.57 MPa. The magnitude of these effects increased, and the threshold decreased, with increasing exposure duration from 1 min to 4 min. The effects also increased for increasing bath temperature from 32 degrees C to 42 degrees C. The effects decreased markedly for 1 ms burst-mode exposure with 2 ms and 4 ms repetition periods with constant pressure amplitude, but were essentially constant for constant temporal-average intensity. Both effects were also produced by heating from radio-frequency diathermy at 10.35 MHz. The effects were associated with abdominal temperatures above about 42-43 degrees C, which were estimated with a thermocouple at the end of the exposure. The hyperemia is a clearly thermal effect, while the petechiae have been previously associated with the cavitation mechanism. However, the ultrasonically-induced petechiae observed in this study do not represent an unequivocal marker for cavitation, but rather appear to be attributable to heating. These findings help to reduce the expectation of ultrasonic cavitation during medical therapy treatments.