Sound velocity versus temperature measurements in tissues

Abstract

We have developed a sensitive interferometric technique for measuring the variation of the velocity of ultrasound in tissue as a function of temperature. The tissue samples are placed between two transducers mounted in an invar holder, and this assembly is situated in a temperature-controlled water bath. The transmitting transducer is excited continuously by a crystal-controlled oscillator at a frequency of 10 MHz. The phase of the received signal is compared with the phase of the transmitted signal by a vector voltmeter. Variations in velocity are detected as changes in the relative phase between the transmitted and received signals. The overall accuracy of the system allows resolution to within 3° of phase difference. This corresponds to an accuracy of 10 ppm for a transducer spacing of 10 cm. The experimental data will be used in a larger project aimed at measuring internal body temperature nonintrusively with computer-aided tomography. Temperature resolution on the order of 0.1°C across 1 cm of tissue are expected in the final instrument.