Archives of Acoustics, Online first
10.24425/aoa.2024.148814

Ultrasound Imaging of Nonlinear Media Response Using a Pressure-Dependent Nonlinearity Index

Andrzej NOWICKI
ORCID ID 0000-0002-9260-8237
Department of Ultrasound Institute of Fundamental Technological Research
Poland

Jurij TASINKIEWICZ
Department of Ultrasound Institute of Fundamental Technological Research
Poland

Piotr KARWAT
ORCID ID 0000-0002-2104-1130
Department of Ultrasound Institute of Fundamental Technological Research
Poland

Ihor TROTS
ORCID ID 0000-0002-7055-3116
Department of Ultrasound Institute of Fundamental Technological Research
Poland

Norbert ŻOŁEK
ORCID ID 0000-0002-2416-7783
Department of Ultrasound Institute of Fundamental Technological Research
Poland

Ryszard TYMKIEWICZ
ORCID ID 0000-0002-9311-0043
Department of Ultrasound Institute of Fundamental Technological Research
Poland

It has been shown that within the range of acoustic pressures used in ultrasound imaging, waveforms are distorted during propagation in tissue due to the physically nonlinear behavior of the tissue. This distortion leads to changes in the spectrum of the received ultrasound echoes, causing the transfer of signal energy from the fundamental frequency to higher harmonics. Interestingly, adipose tissue exhibits up to 50 % stronger nonlinear behavior compared to other soft tissues. The tissue nonlinearity parameter B/A is typically measured ex vivo using an ultrasound method in transmission mode, which requires extensive receiving systems. Currently, there is no improved ultrasound method for measuring the B/A nonlinearity parameter in vivo, which could be used in assessing the degree of fatty liver disease.
We propose a new, simple approach to estimating nonlinear tissue properties. The proposed method involves transmitting ultrasound waves at significantly different acoustic pressures, recording echoes only in the fundamental frequency band at various depths, and introducing a nonlinearity index (NLI) based on specific echo amplitude ratios.
The NLI at a given depth is calculated using the ratio of two dimensionless parameters. The first parameter is a predetermined constant obtained by dividing the total echo values from transmitting a signal at higher sound pressure by those from a signal at lower sound pressure, summed over a small tissue sample volume located near the transducer. The second parameter is calculated at a fixed distance from the transducer, determined by dividing the total echo values from transmitting a signal at higher sound pressure by those from a signal at lower pressure, summed over a small tissue volume of the tissue at that distance from the transducer. The reliability of the proposed measurements for assessing tissue nonlinearity has been substantiated through experimental confirmation of the existing correlations between the values of NLI and B/A in water, sunflower oil, and animal liver tissue samples with oil-enriched regions. The NLI was more than 15 % higher in sunflower oil than in water. The NLI in bovine liver sample below the area with injected oil (mimicking “steatosis”) was
more than 35 % higher than in regions without oil. This method represents a promising modality for the nonlinear characterization of tissue regions in vivo, particularly for diagnosing fatty liver disease.
Keywords: ultrasound imaging; abdominal ultrasound; nonlinear propagation; tissue harmonic imaging; nonlinearity index
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Copyright © 2024 The Author(s). This work is licensed under the Creative Commons Attribution 4.0 International CC BY 4.0.


DOI: 10.24425/aoa.2024.148814