In vivo heating of the guinea-pig fetal brain by pulsed ultrasound and estimates of thermal index.

Temperature was measured in the brain in live near-term fetal guinea pigs (62-66 d gestational age), during in utero exposure to a fixed beam of pulsed ultrasound at intensity ISPTA 2.82 W/cm2. Mean temperature increases of 4.3 degrees C close to parietal bone and 1.1 degrees C in the mid-brain were recorded after 2-min exposures. These values were lower (12%) than those obtained for ultrasound-induced heating near the bone in dead fetuses insonated in utero. A significant cooling effect of vascular perfusion was observed only when guinea pig fetuses reached late gestation, near term, when the cerebral vessels were well developed. The estimated value for the thermal index (TIB), as used in AIUM/NEMA output display standard, underestimated the measured temperature increase at the bone-brain interface. The ratio of measured temperature to the TIB is 1.3. A modification of the cranial thermal index provided a more reasonable, conservative, estimate of the temperature increase at a biologically significant point of interest at the brain-bone interface.