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 pul
sing regimen used 6.25 mus pulses, repeated at a frequency of 4 kHz em
itted 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 bo
ne development. After a 2 min insonation with a spatial peak temporal
average intensity (I(SPTA)) of 2.9 W/cm 2, a mean temperature elevatio
n 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 implicati
ons for the safety of pulsed Doppler examinations where dwell-time may
be an important factor.