ULTRASONIC HEATING OF THE BRAIN OF THE FETAL SHEEP IN-UTERO

The fetal sheep was used as a model to determine the extent of ultraso und-induced heating of brain tissue in procedures involving pulsed Dop pler examination of fetal intracranial arteries, Temperature measureme nts were recorded in late-gestation fetuses insonated in utero, The ce ntre 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 fo r the 0.6-W transducers and 1.2 cm for the 2-W transducers, Mean (stan dard error) maximal temperature increases of 3.O degrees C (0.3) and 1 2.5 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 substantia lly 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.