Pulsed ultrasound and the fetal sheep brain

Abstract

Recent work indicates that some diagnostic equipment operating in pulsed Doppler mode has the potential to cause biologically significant heating of the fetal brain. Such heating is attributable to substantial absorption of ultrasound by the skull. This study examined. ultrasonic heating and related effects in the fetal sheep brain. The principal hypothesis was that vascular perfusion has an important cooling effect on ultrasound-induced heating of the superficial cortex sited adjacent to the skull within the beam path. The threshold average intensity required to heat the brain to 1.5'C (regarded as a safe maximum for fetuses) within a l2O s exposure period was established. The exposure period was selected because it was relevant to pulsed Doppler studies of the fetal internal carotid artery where scan durations of up to 80 s have been observed. The study was designed to overcome the limitations of previous reports by measuring ultrasound-induced temperature changes in the soft tissue of the brain in sheep fetuses that were unanaesthetised and in a physiologically stable state.