Noninvasive Tissue Temperature Estimation Using Nonlinear Ultrasound Harmonics

Non-invasive tissue temperature estimation is important in thermal therapies for having an efficient treatment. A noninvasive ultrasonic technique for monitoring tissue temperature changes is proposed based on the changes in the harmonics of ultrasound backscatter as a function of temperature. The backscattered pressure amplitudes of the fundamental frequency (p(1)), the second (p(2)) and the third (p(3)) harmonics generated by nonlinear ultrasound propagation and the ratios of the second and the third harmonics over the fundamental frequency (p(2)/p(1) and p(3)/p(1)) were investigated as a function of temperature. The acoustic harmonics were generated and detected with a commercial high frequency ultrasound imaging system in pulse-echo mode. The experiments were performed on tissue-mimicking gel phantoms and ex vivo bovine muscle tissues. The temperature was increased from 26 degrees C to 46 degrees C in increments of 2 degrees C. The average values of p(1), p(2), p(3), p(2)/p(1), p(3)/p(1) increased by 14%, 50%, 117%, 37% and 92% for the gel phantoms, and for the tissue samples increased by 29%, 50%, 170%, 10% and 109%, respectively. The results indicate that the harmonic amplitudes and their ratios are highly sensitive to propagation medium's temperature and could potentially be used for noninvasive ultrasound thermometry.