Measurement of ultrasound energy density distributions in vivo.

Thermocouple junctions coated with absorbing material have been used for measurement of ultrasound energy density distribution in vivo. The response of this type of probe has been measured in tumours implanted in rats, in ox liver, and in a water bath, using 1 and 3 MHz focussed ultrasound, and these measurements are reported in this paper. The initial rapid rate of temperature rise during the first 60 ms after the ultrasound is switched on provides a reliable index of ultrasound energy density. The coating material increases the magnitude of the temperature rise and reduces variations caused by differences in tissue properties and changes in blood flow. The response is a linear function of ultrasound energy density over the range of interest for therapeutic applications. Thus probes calibrated in known ultrasound fields in water tanks can be used to estimate energy densities in tissue directly.