In utero measurement of ultrasonically induced fetal mouse temperature increases

The temperature increase in the murine fetus, exposed in vivo to 1 MHz ultrasound in the spatial-peak temporal-average intensity range 0.5 to 10 W/cm2 for durations of 30 to 400 s, was determined with implanted thermocouples at 9, 12 and 15 days of gestation by a method that preserved the normal uterine environment. The results show that the temperature increase is related approximately linearly to the energy flux It, where I is the acoustic intensity and t is the exposure duration. The temperature increase was observed to be nearly the same for the 9- and 12-day fetuses but less for day-15 specimens, possibly due to the smaller size in early gestation and increased vascularization in later gestation. The results are compared with calculations based on a model of a pair of nested parallelpipeds for which the properties of the outer body remain unchanged, the dimensions and absorption coefficient of the inner body increase with gestational age and the perfusion constant varies with gestational age.