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Ultrasound contrast response to variation of incident pulse length and polarity
Linköping University, Department of Biomedical Engineering, Physiological Measurements. Linköping University, The Institute of Technology.
Institute of Biomedical Engineering, Kaunas University of Technology, Kaunas LT-3006, Lithuania.
Department of Electrical Measurements, Lund University, SE-22100 Lund, Sweden.
Linköping University, Department of Biomedical Engineering, Physiological Measurements. Linköping University, The Institute of Technology.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Microbubbles are used as ultrasound contrast agents (UCA) in diagnostic ultrasound as they considerably enhance the backscattered signal and generate specific signal characteristics that can be used to isolate echoes that originate from the blood volume. Emerging new advanced contrast specific insonation techniques have shown to better discriminate the backscattered UCA-signal but has not gained clinical practice due to their complexity and the need for additional soft- and hardware, or due to the debated safety aspects regarding microbubble cavitation at mechanical index (MI >>0.4). In this study we investigate a simplified approach to improve the nonlinear signal contribution from UCA at low MI < 0.4 by utilizing the asymmetry between positive and negative peak pressures for pulse lengths ≤3.5 cycles. In vitro registrations of the transmitted pulse peak pressure asymmetry from a single element transducer were obtained with a needle hydrophone after a transducer excitation pulse with increasing length from 0.5 to 5 cycles. A computational model (Bubblesim) was used to investigate the response from a single microbubble after interaction with transmitted pulse with variations of length, shape and polarity. Our results show that small changes (quarters of a pulse cycle) will change the transmitted pulse shape and distribution of peak pressures and that this effect can be used to change the scattering behavior of UCA in simulations and in vitro. This effect will increase with decreasing pulse lengths <5 cycles. The best case scenario for differentiation of harmonic UCA response with polarity change at MI <0.4 and real time imaging can for transducer frequencies of 2-2.5 MHz be found for pulse lengths of 2.25 and 2.75 cycles in the acoustic pressure interval of 300-500 kPa.

Keyword [en]
Ultrasound contrast agents, nonlinear imaging, harmonic imaging, contrast echocardiography
National Category
Natural Sciences
URN: urn:nbn:se:liu:diva-65417OAI: diva2:395506
Available from: 2011-02-07 Created: 2011-02-07 Last updated: 2011-02-07
In thesis
1. On Nonlinear Acoustics in Contrast Echocardiography
Open this publication in new window or tab >>On Nonlinear Acoustics in Contrast Echocardiography
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Ultrasound is one of the most commonly used noninvasive medical imaging techniques. Ultrasound contrast agents (UCA), consisting of encapsulated gas-filled microbubbles, have shown to increase the diagnostic precision in selected low echogenic patients. UCA also holds promise for bedside evaluation of myocardial perfusion quantification, but is not yet reproducible and specific enough for clinical use. In addition risks have been addressed when used, as first recommended, together with high mechanical index (MI) for reperfusion assessment by contrast destruction. We clinically observed increased myocardial velocities after UCA-administration when applied simultaneously with color tissue Doppler imaging (CTDI) arising the question if this increase was due to physiological factors or physical changes in the backscattered signals when UCA were present.

The aims of the thesis was to explain this velocity shift and simultaneously to contribute to a future safe and contrast specific application by further characterizing the non-linear acoustic properties of UCA when located in an acoustic field. Of specific interest was to evaluate in which way nonlinear wave propagation affects the response from UCA and if a change in pulse shape, length or polarity can be utilized to increase the nonlinear signal contribution.

Twelve patients with ischemic heart disease were examined with CTDI before and after UCA-administration in order to verify the change in peak systolic velocity. An experimental in vitro model including flow and tissue phantoms for UCA was established for CTDI. Raw data from single-element transducers and clinical ultrasound systems were collected for three different UCA and analyzed to determine if the observed velocity shift could be reproduced in vitro and to find a possible cause. Our results show in vivo and in vitro that UCA will affect the autocorrelation phase shift estimator used for CTDI in terms of contribution from rupturing UCA microbubbles, which explains the velocity shift. CTDI during contrast infusion should therefore be avoided unless it can be performed at low MI where the majority of the UCA are intact.

The computational model for spatial superposition of attenuated waves was modified to include an operator for pulse distortion from nonlinear wave propagation. The Matlab™ toolbox Bubblesim based on a modified Rayleigh-Plesset-equation and with insonation parameters such as frequency, pressure amplitude, pulse length and polarity was used to study the response from single microbubbles either for simulated pulses or for pulses generated by clinical ultrasound systems and single element transducers. The combination of the two models also provided a computational platform to asses pulse distortion from nonlinear wave propagation, the response of the UCA bubble and the linear backscatter of the low amplitude bubble echo. When evaluating the harmonic response in simulations and in vitro, the interaction of the excitation pulses with the contrast bubbles was identified as the main cause of nonlinear scattering, and a 2-3 dB increase of the second harmonic amplitude depends on nonlinear distortions of the incident pulse. By applying small changes of short (<3.5 cycles) and fragmented transmitted wideband pulses of 2-2.5 MHz, it is shown that inverted pulse polarity considerably modulates power without affecting a low and safe MI (<0.4), and the results lodged promise to further to enhance a contrast response.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2010. 114 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1338
National Category
Engineering and Technology
urn:nbn:se:liu:diva-65418 (URN)978-91-7393-315-5 (ISBN)
Public defence
2010-11-12, Eken, Campus US, Linköpings universitet, Linököping, 09:15 (Swedish)
Available from: 2011-02-07 Created: 2011-02-07 Last updated: 2011-02-07Bibliographically approved

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