Multi-bubble scattering acoustic fields in viscoelastic tissues under dual-frequency ultrasound

Linear pressure waves in mono- and poly-disperse bubbly liquids: Attenuation and propagation speed in slow and fast and evanescent modes

Using volumetric averaged equations from a two-fluid model, this study theoretically investigates linear pressure wave propagation in a quiescent liquid with many spherical gas bubbles. The speed and attenuation of sound are evaluated using the derived linear dispersion. Mono- and poly-disperse bubbly liquids are treated. To precisely describe the attenuation effect, some forms of bubble dynamics equations and temperature gradient models are employed. Focusing on the dissipative effect, we analyze the stop band that occurs in the linear dispersion relation. In the two-fluid model, even if the dissipation effect is considered, the inconvenience that the wavenumber diverges to infinity in the resonance frequency cannot be resolved. Additionally, the validity of terminating that wavenumber value in the middle of the frequency is demonstrated. To determine a linear dispersion relation that can exactly predict thermal conduction and acoustic radiation, wave propagation velocities and attenuation coefficients are compared with some experimental data and existing models. The results show that thermal conduction and acoustic radiation should be set appropriately to accurately predict the propagation velocity and attenuation except in the high frequency range, the phase velocity in the resonance frequency range, or the attenuation in the high frequency range.

Multimodality quantitative ultrasound envelope statistics imaging based support vector machines for characterizing tissue scatterer distribution patterns: Methods and application in detecting microwave-induced thermal lesions

Ultrasound envelope statistics imaging, including ultrasound Nakagami imaging, homodyned-K imaging, and information entropy imaging, is an important group of quantitative ultrasound techniques for characterizing tissue scatterer distribution patterns, such as scatterer concentrations and arrangements. In this study, we proposed a machine learning approach to integrate the strength of multimodality quantitative ultrasound envelope statistics imaging techniques and applied it to detecting microwave ablation induced thermal lesions in porcine liver ex vivo. The quantitative ultrasound parameters included were homodyned-K α which is a scatterer clustering parameter related to the effective scatterer number per resolution cell, Nakagami m which is a shape parameter of the envelope probability density function, and Shannon entropy which is a measure of signal uncertainty or complexity. Specifically, the homodyned-K log10(α), Nakagami-m, and horizontally normalized Shannon entropy parameters were combined as input features to train a support vector machine (SVM) model to classify thermal lesions with higher scatterer concentrations from normal tissues with lower scatterer concentrations. Through heterogeneous phantom simulations based on Field II, the proposed SVM model showed a classification accuracy above 0.90; the area accuracy and Dice score of higher-scatterer-concentration zone identification exceeded 83% and 0.86, respectively, with the Hausdorff distance <26. Microwave ablation experiments of porcine liver ex vivo at 60-80 W, 1-3 min showed that the SVM model achieved a classification accuracy of 0.85; compared with single log10(α),m, or hNSE parametric imaging, the SVM model achieved the highest area accuracy (89.1%) and Dice score (0.77) as well as the smallest Hausdorff distance (46.38) of coagulation zone identification. We concluded that the proposed multimodality quantitative ultrasound envelope statistics imaging based SVM approach can enhance the capability to characterize tissue scatterer distribution patterns and has the potential to detect the thermal lesions induced by microwave ablation.

Influence of the liquid ionic strength on the resonance frequency and shell parameters of lipid-coated microbubbles

The correct measurement of the resonance frequency and shell properties of coated microbubbles (MBs) is essential in understanding and optimizing their response to ultrasound (US) exposure parameters. In diagnostic and therapeutic ultrasound, MBs are typically surrounded by blood; however, the influence of the medium charges on the MB resonance frequency has not been systematically studied using controlled measurements. This study aims to measure the medium charge interactions on MB behavior by measuring the frequency-dependent attenuation of the same size MBs in mediums with different charge densities. In-house lipid-coated MBs with C3F8 gas core were formulated. The MBs were isolated to a mean size of 2.35 μm using differential centrifugation. MBs were diluted to ≈8×105 MBs/mL in distilled water (DW), Phosphate-Buffered Saline solution (PBS1x) and PBS10x. The frequency-dependent attenuation of the MBs solutions was measured using an aligned pair of PVDF transducers with a center frequency of 10MHz and 100% bandwidth in the linear oscillation regime (7 kPa pressure amplitude). The MB shell properties were estimated by fitting the linear equation to experiments. Using a pendant drop tension meter, the surface tension at the equilibrium of ≈6 mm diameter size drops of the same MB shell was measured inside DW, PBS1x and PBS10x. The surface tension at the C3F8/solution interface was estimated by fitting the Young-Laplace equation from the recorded images. The frequency of the peak attenuation at different salinity levels was 13, 7.5 and 6.25 MHz in DW, PBS1x and PBS-10x, respectively. The attenuation peak increased by ≈140% with increasing ion density. MBs' estimated shell elasticity decreased by 64% between DW and PBS-1x and 36% between PBS-1x and PBS-10x. The drop surface tension reduced by 10.5% between DW and PBS-1x and by 5% between PBS-1x and PBS-10x, respectively. Reduction in the shell stiffness is consistent with the drop surface tension measurements. The shell viscosity was reduced by ≈40% between DW and PBS-1x and 42% between PBS-1x and PBS-10x. The reduction in the fitted stiffness and viscosity is possibly due to the formation of a densely charged layer around the shell, further reducing the effective surface tension on the MBs. The changes in the resonance frequency and estimated shell parameters were significant and may potentially help to better understand and explain bubble behavior in applications.