Superthreshold behavior and threshold estimation of ultrasound-induced lung hemorrhage in adult mice and rats

Threshold estimates and superthreshold behaviors for ultrasound-induced lun g Hemorrhage were investigated as a function of species (adult mice and rat s) and ultrasound frequency (2.8 and 5.6 MHz). A total of 151 6-to-7-week-o ld female ICR mice and 160 10-to-11-week-old female Sprague-Dawley rats wer e randomly divided into two ultrasonic frequency groups, and further random ly divided into seven or eight ultrasonic peak rarefactional pressure group s, Each group consisted of about 10 animals. Animals were exposed to pulsed ultrasound at either 2.8-MHz center frequency (1-kHz PRF, 1.42-mus pulse d uration) or 5.8-MHz center frequency (1-kHz PRF, 1.17-mus pulse duration) f or a duration of 10 seconds, The in situ (at the pleural surface) peak rare factional pressure levels ranged between 2.5 and 10.5 MPa for mice and betw een 2.3 and 11.3 MPa for rats. The mechanical index (MI) ranged between 1.4 and 6.3 at 2.8 MHz for mice and between 1.1 and 3.1 at 5.6 MHz for rats. T he lesion surface area and depth were measured for each animal as well as t he percentage of animals with lesions per group. The characteristics of the lesions produced in mice and rats were similar to those described in previ ous studies by our research group and others, suggesting a common pathogene sis in the initiation and propagation of the lesions at the gross and micro scopic levels. The percentage of animals with lesions showed no statistical differences between species or between ultrasound frequencies. These findi ngs suggest that mice and rats are similar in sensitivity to ultrasound-ind uced lung damage and that the occurrence of lung damage is independent of f requency. Lesion depth and surface area also showed no statistically signif icant differences between ultrasound frequencies for mice slid rats. Howeve r, there was a significant difference between species for lesion area and a suggestive difference between species for lesion depth. The superthreshold behavior of lesion area and depth showed that rat lung had more damage tha n mouse lung, and the threshold estimates showed a weak, or lack of, freque ncy dependency, suggesting that the MI is not consistent with the observed findings.