Heating of guinea-pig fetal brain during exposure to pulsed ultrasound.

Ultrasound-induced temperature elevations in fresh and formalin-fixed fetal guinea-pig brains were measured during in vitro insonation, with a stationary beam in a tank containing water at 38 degrees C. The pulsing regimen used 6.25 microseconds pulses, repeated at a frequency of 4 kHz emitted from a focussed transducer operating with a centre frequency of 3.2 MHz. The greatest temperature rise in brain tissue occurred close to bone and correlated with both gestational age and progression in bone development. After a 2 min insonation with a spatial peak temporal average intensity (ISPTA) of 2.9 W/cm2, a mean temperature elevation of 5.2 degrees C was recorded in fetuses of 60 days gestation (dg). The same exposure produced an increase of 2.6 degrees C in the centre of whole brains of 60 dg fetuses when the bony cranium was removed. As most of the heating occurs within 40 s, these findings have implications for the safety of pulsed Doppler examinations where dwell-time may be an important factor.