Hemorrhage near fetal rat bone exposed to pulsed ultrasound

Ultrasound-induced hemorrhage near the fetal rat skull was investigated to determine if the damage could be correlated with temporal-average intensity. A 0.92-MHz f/1 spherically focused transducer (5.1-cm focal length) was used to expose the skull of 18- to 19-day gestation exteriorized Sprague-Dawley rat fetuses (n = 197). There were four ultrasound-exposed groups (n = 36 each), one sham exposed group (n = 36) and one cage control group (n = 17). Three of the ultrasound-exposed groups had the same peak compressional (10 MPa)/peak rarefactional (6.7 MPa) pressure but different spatial-peak temporal-average intensities (I_TA) of 1.9, 4.7 and 9.4 W/cm²; the pulse repetition frequency (PRF) was varied (100, 250 and 500 Hz, respectively). The fourth ultrasound-exposed group had a peak compressional (6.7 MPa)/peak rarefactional (5.0 MPa) pressure and corresponding I_TA of 4.6 W/cm²; PRF was 500 Hz. Hemorrhage occurrence increased slightly with increasing I_TA, as well as peak rarefactional pressure and PRF, but the hemorrhage area did not correlate with any of the exposure parameters. (E-mail: timothybigelow@mail.und.nodak.edu).