Temperature dependence of acoustic harmonics generated by nonlinear ultrasound beam propagation in ex vivo tissue and tissue-mimicking phantoms

Abstract

Purpose: Hyperthermia is a cancer treatment technique that could be delivered as a stand-alone modality or in conjunction with chemotherapy or radiation therapy. Noninvasive and real-time temperature monitoring of the heated tissue improves the efficacy and safety of the treatment. A temperature-sensitive acoustic parameter is required for ultrasound-based thermometry. In this paper the amplitude and the energy of the acoustic harmonics of the ultrasound backscattered signal are proposed as suitable parameters for noninvasive ultrasound thermometry.

Materials and methods: A commercial high frequency ultrasound imaging system was used to generate and detect acoustic harmonics in tissue-mimicking gel phantoms and ex vivo bovine muscle tissues. The pressure amplitude and the energy content of the backscattered fundamental frequency (p₁ and E₁), the second (p₂ and E₂) and the third (p₃ and E₃) harmonics were detected in pulse-echo mode. Temperature was increased from 26° to 46 °C uniformly through both samples. The amplitude and the energy content of the harmonics and their ratio were measured and analysed as a function of temperature.

Results: The average p₁, p₂ and p₃ increased by 69%, 100% and 283%, respectively as the temperature was elevated from 26° to 46 °C in tissue samples. In the same experiment the average E₁, E₂ and E₃ increased by 163%, 281% and 2257%, respectively. A similar trend was observed in tissue-mimicking gel phantoms.

Conclusions: The findings suggest that the harmonics generated due to nonlinear ultrasound beam propagation are highly sensitive to temperature and could potentially be used for noninvasive ultrasound tissue thermometry.

• Acoustic harmonics
• high frequency ultrasound
• noninvasive thermometry

Acknowledgements

The authors wish to acknowledge the technical assistance of Arthur Worthington, Graham Pearson and Luke Yaraskavitch, all from the Department of Physics, Ryerson University, Toronto, Canada.

Declaration of interest

This work was partially supported by the Ontario Research Fund – Research Excellence (ORF-RE) grant and the Natural Sciences and Engineering Research Council of Canada (NSERC Discovery grants) that were awarded to Jahan Tavakkoli and Michael Kolios. Funding to purchase the equipment was provided by the Canada Foundation for Innovation, the Ontario Ministry of Research and Innovation, and the Ryerson University and the Canada Research Chairs Program. The authors alone are responsible for the content and writing of the paper.