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 pulse d ultrasound at intensity I-SPTA 2.82 W/cm(2). Mean temperature increases o f 4.3 degrees C close to parietal bone and 1.1 degrees C in the mid-brain w ere recorded after 2-min exposures. These values were lower (12%) than thos e obtained for ultrasound-induced heating near the bone in dead fetuses ins onated in utero. A significant cooling effect of vascular perfusion was obs erved only when guinea pig fetuses reached late gestation, near term, when the cerebral vessels were well developed. The estimated value for the therm al index (TIB), as used in AIUM/NEMA output display standard, underestimate d 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 th ermal index provided a more reasonable, conservative, estimate of the tempe rature increase at a biologically significant point of interest at the brai n-bone interface. (C) 1998 World Federation for Ultrasound in Medicine & Bi ology.