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

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Nowicki A. , Tasinkiewicz J. , Karwat P. , Trots I. , Żołek N. , Tymkiewicz R.

tom Vol. 49, nr 4

557--563

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.

Zjawisko generowania fal harmonicznych oraz jego wykorzystanie praktyczne ze szczególnym uwzględnieniem współczesnej ultrasonografii

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Jeka S. , Sokólska E. , Dura M. , Żuchowski P.

tom Vol. 17, nr 2

140-143

Od wielu lat z postępem w dziedzinie fizyki związany jest postęp w naukach przyrodniczych. Jednym ze zjawisk, które odgrywa coraz większą rolę w naukach przyrodniczych, jest zjawisko generacji drugiej harmonicznej w ośrodkach anizotropowych. Możliwość uzyskania fali o podwojonej częstotliwości w stosunku do fali podstawowej pozwoliło na prowadzenie badań, wcześniej trudnych bądź niemożliwych do przeprowadzenia. Dzięki wyższej jakości obrazu badania ultrasonograficzne stały się bardziej czułe i są powszechnie wykorzystywane, m.in. w kardiologii, radiologii, chirurgii, reumatologii czy diagnostyce onkologicznej. Ultrasonografia staje się niezwykle cenioną metodą obrazowania obok rezonansu magnetycznego czy tomografii komputerowej. Ze względu na jej dostępność wiązane są z nią duże nadzieje na przyszłość.

The progress in physics results in life sciences advancement. One of the phenomena, which plays more and more important role in life sciences, is the second harmonic generation in anisotropic materials. The possibility of achieving the wave, characterized by double frequency – with relation to the initial wave, allowed to conduct examinations, which were previously very difficult or impossible to perform. Thanks to higher quality of image, ultrasound examination has become more sensitive and is widely used, even in cardiology, radiology, surgery, rheumatology and oncology diagnostics. Ultrasonography with the use of tissue harmonic imaging is very valued method, so as magnetic resonance imaging or CT scan and due to its availability, high expectations are associated with it for the future.