Vol 65, No 2 (2019)

The paper presents the results of a numerical experiment comparing the rates of volumetric thermal ablation of bovine liver tissue ex vivo, generated by a multielement ultrasound array Sonalleve V1 3.0T (Philips Healthcare) using various exposure protocols. Pulsed sonications with the same time-average, but different peak power and duty cycle were modeled. The treatment trajectory consisted of a discrete set of single foci located at the center and along two concentric circles. Beam focusing in tissue was modeled using the Westervelt equation, the temperature field was calculated using the bioheat equation, and the threshold of tissue damage was determined according to the thermal dose formulation. It is shown that pulsed shock-wave exposures can provide up to three times faster volumetric ablation of tissue as compared to continuous quasi-harmonic wave treatments.

The relationship between the experimentally obtained attenuation coefficient and dispersion of the phase velocity of longitudinal ultrasonic waves in polymer composite materials has been analyzed. The laser optoacoustic method has been used to measure the ultrasonic attenuation and velocity in a wide frequency range. Verification of the Kramers–Kronig relations for ultrasound-scattering and absorbing carbon fiber reinforced plastic (CFRP) composites has been performed using the analysis of the relationship between the frequency dependences of the attenuation coefficient and phase velocity of longitudinal acoustic waves in samples. We have shown that the Kramers–Kronig relations between the ultrasonic attenuation and phase velocity are valid within the 1–10 MHz frequency range without regard to a particular mechanism of the decay in the energy of an initial acoustic wave during its propagation in a composite.

A novel solid state structure consisting of piezoelectric plate sandwiched between two piezoelectric films is suggested as propagation medium for acoustic waves. Considering, as an example, quartz plate coated with AlN film and with AlN film together with ZnO film, the main characteristics of the Lamb and SH acoustic modes are numerically calculated and compared with each other. It is shown that i) the range of acoustic parameters achievable in structures is wider than that is for an uncoated plate, ii) generation of waves in the plate with one film is accomplished by 12 transducer configurations, while there are 32 configurations to excite the same waves in two film structure, iii) dispersion of the wave velocities and coupling constants depend on the mode order, mode type, film thickness, plate thickness, and transducer configuration. This property makes selection of appropriate modes more flexible. Results of calculations are partially verified experimentally.

The paper describes a theoretical study of horizontal anisotropy of dynamic ocean noise. It is shown that the anisotropic distribution of the ocean noise field is governed by the effect of its scattering by wind waves. The dependence of the degree of noise anisotropy on the mode number and wind speed is discussed.

As fluids and gases move through porous media, they generate acoustic noise. It is shown that the shape of the noise spectra is determined by the properties of the fluid and the porous reservoir, as well as by the flow regime. Our study has experimentally confirmed the conclusions that the location of sound intensity maxima are independent of the rate of fluid filtration through a porous medium. The criterion for the occurrence of filtration noise as a fluid moves through a porous medium is determined. The dependence of the sound intensity maximum on the fluid filtration rate is studied. The relationship between the porosity and permeability of porous media samples and the character of the filtration noise spectra is considered. The obtained experimental data can be used to create a sound generation model for gas filtration through a porous medium.

Field investigations of the influence of a drifting ice cover on the probability of bit errors in a hydroacoustic communication channel constructed by the orthogonal frequency-division multiplexing (OFDM) technology were carried out. Quantitative estimates of the effect of drifting ice on a multifrequency phase-modulated OFDM signal reflected from the lower ice surface have been determined. The negative influence is seen in the broadening of the carrier frequency spectrum and in the reduction of the mutual correlation of phase-frequency shifts between carrier frequencies at the reception point. As a result of the spectrum broadening and jump-like changes in the phase of information signals, there is a sharp increase in the probability of bit errors during the reception. Methods are proposed for choosing optimum broadband OFDM-signal parameters allowing the negative influence of the ice cover to be neutralized and the signal transmission through a hydroacoustic communication channel with given characteristics to be implemented.

Ultrasound (US) and microbubble (MB) interaction is an important factor in the research of bioacoustics, as well as targeted drug and gene delivery. In this study, we demonstrate the feasibility of pulse−echo M-mode imaging system to be used for the visualization and quantification of US–MB interaction in both spatial and temporal dimensions. The system incorporates an exposure chamber with the cell–MB suspension, a 2.7 MHz focused US transducer, a US pulser–receiver and the customized LabView software. The results of cell and MB interaction obtained after M-mode image analysis have showed the US–MB interaction to be non-uniform in space and non-stationary in time. In order to quantify the spatio-temporal US–MB interaction, we have introduced the time function of spatial homogeneity dynamics. We have observed that the effective duration of interaction can be characterized at the predefined threshold of spatial homogeneity. For example, at the US excitation of 360 kPa peak negative pressure (15 bursts transmitted at 80 Hz pulse repetition frequency), the US–MB interaction persists for more than 5 seconds in the range at 4 mm depth of the exposure chamber with more than 50% of homogeneity. The system proposed in this assay is feasible for the characterization of US–MB interaction and can be exploited to optimize the MB concentration and/or the US excitation parameters.