Measurement of Temperature Rise in Prenatal Rats During Exposure of the Exteriorized Uterus to Ultrasound

We have found that ultrasound produces intensitydepen- dent increases in embryotoxicity after exposure of the rat conceptus within the exteriorized uterus. To estimate the range where thermal mechanismspertain,implanted thermocouples and adigital thermometer were used to measure fetal and maternal temperatures under the same exposure conditions. Results with protracted exposures to focused beams were analyzed by calculating polynomial equations from the data for individual animals and developing families of curves describing tem- perature-time relationships for progressively greater intensities at two frequencies used in our bioeffects studies. Ancillary experiments examined heat dissipation by circulating blood. In experiments with plane beams, intensities were increased at S-minute intervals and separate polynomials were fit to each segment. The composite step-function curves indicated that patterns of temperature rise in the fetus were similar at 0.8 and 2.4 MHz, but the rises were greater at the higher fre- quency. Temperature rises in maternal tissues were frequency depen- dent and were great enough to account for effects observed in dams ex- posed at high intensities. The data with both focused and plane beams are generally consistent with the shapes of our embryotoxicity response curves and implicate thermal mechanisms in many of the effects.