Pulsed ultrasound and electrocortical activity in fetal sheep

The aim of this study was to determine if low intensity ultrasound affects electrocortical activity in chronically instrumented fetal sheep. A 3.5-MHz transducer was attached to each fetal skull (n = 9) and activated periodically to emit pulses of 5.8 microseconds duration at a repetition rate of 2 kHz generating a power output of 60 mW. The power supply was activated in 30-s bursts every 180 s but random electronic switching energized the transducer with 50% of bursts. On average, 175 bursts of ultrasound were delivered to the fetus over 19.5 h. Blinded analysis of continuous recordings of electrocortical activity showed it to be unaffected by the ultrasound exposures. Therefore, it is unlikely that ultrasonic examination of human fetuses will affect normal cyclical electrocortical activity.