Ultrasonic heating of the brain of the fetal sheep in utero

The fetal sheep was used as a model to determine the extent of ultrasound-induced heating of brain tissue in procedures involving pulsed Doppler examination of fetal intracranial arteries. Temperature measurements were recorded in late-gestation fetuses insonated in utero. The centre frequency was 3.5 MHz and a pulse repetition rate of 6 to 10 kHz produced a power output of 0.6 or 2 W. The brain was insonated in the near field of a focussed beam where the -6-dB beam width was 1.7 cm for the 0.6-W transducers and 1.2 cm for the 2-W transducers. Mean (standard error) maximal temperature increases of 3.0 degrees C (0.3) and 12.5 degrees C (1.3), respectively, were recorded in dead fetuses. The mean values obtained in normally perfused living fetuses were lower by 43% and 30%, respectively, showing that vascular perfusion substantially limited ultrasonic heating in sheep fetal brain tissue. There were no changes in blood flow to the heated brain tissue as measured using radiolabelled microspheres.