Guo Xiasheng

Two-Dimensional Lorentz Force Image Reconstruction for Magnetoacoustic Tomography with Magnetic Induction

Magnetoacoustic tomography with magnetic induction has shown potential applications in imaging the electrical impedance for biological tissues. We present a novel methodology for the inverse problem solution of the 2-D Lorentz force distribution reconstruction based on the acoustic straight line propagation theory. The magnetic induction and acoustic generation as well as acoustic detection are theoretically provided as explicit formulae and also validated by the numerical simulations for a multilayered cylindrical phantom model. The reconstructed 2-D Lorentz force distribution reveals not only the conductivity configuration in terms of shape and size but also the amplitude value of the Lorentz force in the examined layer. This study provides a basis for further study of conductivity distribution reconstruction of MAT-MI in medical imaging.

Microstreaming velocity field and shear stress created by an oscillating encapsulated microbubble near a cell membrane

Sonoporation mediated by microbubbles is being extensively studied as a promising technology to facilitate gene/drug delivery to cells. However, the theoretical study regarding the mechanisms involved in sonoporation is still in its infancy. Microstreaming generated by pulsating microbubble near the cell membrane is regarded as one of the most important mechanisms in the sonoporation process. Here, based on an encapsulated microbubble dynamic model with considering nonlinear rheological effects of both shell elasticity and viscosity, the microstreaming velocity field and shear stress generated by an oscillating microbubble near the cell membrane are theoretically simulated. Some factors that might affect the behaviors of microstreaming are thoroughly investigated, including the distance between the bubble center and cell membrane (d), shell elasticity (χ), and shell viscosity (κ). The results show that (i) the presence of cell membrane can result in asymmetric microstreaming velocity field, while the constrained effect of the membrane wall decays with increasing the bubble-cell distance; (ii) the bubble resonance frequency increases with the increase in d and χ, and the decrease in κ, although it is more dominated by the variation of shell elasticity; and (iii) the maximal microstreaming shear stress on the cell membrane increases rapidly with reducing the d, χ, and κ. The results suggest that microbubbles with softer and less viscous shell materials might be preferred to achieve more efficient sonoporation outcomes, and it is better to have bubbles located in the immediate vicinity of the cell membrane.

Effect of Tissue Inhomogeneity on Nonlinear Propagation of Focused Ultrasound

We study the influence of tissue inhomogeneity on the focused ultrasound based on the phase screen model and the acoustic nonlinear equation. The inhomogeneous tissue is considered as a combination of a homogeneous medium and a phase aberration screen. Six polyethylene (PE) plates with various correlation lengths and standard deviations are made to mimic the inhomogeneity induced by the human body abdominal. Results indicate that the correlation length affects the side lobe structure of the beam pattern; while the standard deviation is associated with the focusing capability. This study provides a theoretical and experimental basis for the development of a precise treatment plan for high intensity focused ultrasound.

Lattice Boltzmann method and its application in the modelling of high intensity focused ultrasound (HIFU)

High-intensity focused ultrasound (HIFU) is a breakthrough of noninvasive targeted therapeutic technique for tumor treatments. The operational procedure of HIFU is to concentrate the ultrasound energy into the focal region by using the ultrasound transducer, and the focused ultrasound energy is sufficient to rapidly rise the temperature of tumor located at the focal region up to above 65°C and locally destroy the tumor for coagulation necrosis. The ultrasound transducer is the key component in HIFU treatment to generate the high-intensity focused ultrasound energy, the dimension of focal region generated by the transducer is closely relevant to the safety of HIFU treatment. Therefore, it is necessary to simulate the acoustic field numerically for estimating the performance, optimizing the parameters and reducing the design cost of the focused ultrasound transducer. Besides, the common spherical transducer is the most widely used transducer in HIFU, but the size of its focal region still could not satisfy the requirements of some sophisticated applications. So, it is necessary to adopt some new kinds of focused ultrasound transducers with better focusing performance. Aiming at these issues, we presented a numerical simulation method called the lattice Boltzmann method (LBM) in this paper. It is a novel fluid dynamic simulation approach based on mesoscopic kinetic theory, which takes prominent advantages of distinct physical meaning, easy implementation and excellent parallel performance. The LBM has shown great potential in numerical simulations of complex flows that would be difficult for traditional methods. Firstly, we reviewed the developments and applications of the LBM. Then, we revealed the inherent relationship between the LBM and the Boltzmann equation, and presented two basic LBM models called the single-relaxation-time (SRT) model and multiple- relaxation-time (MRT) model, recovered the corresponding macroscopic Navier-Stokes equations respectively via the Chapman−Enskog expansion, presented two common boundary conditions called the non-equilibrium extrapolation scheme and the BFL scheme. Besides we introduced the conversion method between the physical units and lattice units based on dimensional analysis. After that, we built an axisymmetric multiple-relaxation-time (AMRT) LBM model with the BFL scheme, and simulated the acoustic fields generated by concave ultrasound transducers of different field angles respectively by the AMRT model, Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation and spheroidal beam equation (SBE). Results indicated that the AMRT model could be used to describe the acoustic field generated by the concave ultrasound transducer, and the transducer with bigger field angle had a better focusing performance. Lastly, we presented a novel spherical cavity transducer with two open ends for providing subwavelength focal region and sufficient pressure gain. We investigated the standing wave acoustic field generated by the spherical cavity ultrasound transducer via the AMRT model and experimental measurements. Results indicated that the AMRT model could be used to describe the standing wave filed generated by the spherical cavity ultrasound transducer, and this device exhibited much better focusing performance than the traditional concave ultrasound transducer, and could meet the requirement of some sophisticated HIFU treatments. The main aim of this work is to solve some practical problems for the numerical modeling of acoustic field in the HIFU treatments and provide new sights into the acoustic simulations.

Effects of acoustic vibration on the reorientationsof C. elegans

As a typical model organism, the nematode C. elegans detect and respond to diverse environmental stimuli, such as light, temperature, odors and chemicals. The underline neuronal circuitry has been defined by decades of work by C. elegans researchers. However, there’s no work done on its perception to acoustic vibration, which is a critical environmental cue in the natural habitat. For the first time, we quantified worm’s response to acoustic vibration with precisely controlled amplitude and frequency. In an isotropic environment, worm’s navigation can be described as a random walk. Periods of forward movements are interrupted by random reorientations. C. elegans respond to environmental cues by biasing its reorientations. In this work, we delivered acoustic wave with defined amplitude and frequency to freely moving C. elegans . A custom-built real-time computer vision system was used to record the movements of individual young adult worms navigating the agar surface. Data collected was analyzed using customized particle-tracking and shape analysis algorithms. To automatically flag reorientations, we considered both the posture of the animal and the movement of its center of mass. Rapid reorientations ( Ω turns or reversal-turns) were flagged when the heading change of the center of mass trajectory was > 60° over 1 second. It was found that worms reorientate more when sound is on and reorientate less when sound is off. To quantify animal’s response to acoustic wave, we defined a non-dimensional acoustic sensitive index according to the formula. The index approaches 0 or 1 if there is no response or big response, respectively. We tested animal’s response to acoustic wave of varying displacement amplitude and fixed frequency. It was found that the acoustic sensitive index is proportional to the displacement amplitude. However, when we further tested animal’s response to acoustic wave of fixed displacement amplitude and varying frequency, the data shows that the acoustic sensitive index first increases then decreases with the increase of frequency. To characterize the mechanical vibration of the agar surface during the acoustic stimuli, we used a laser vibrometer (Polytec). It was showed that there’s obvious response when the sound cause vibration is as low as ~200 nm. The vibration level is much lower than other mechanical stimuli, such as gentle hair touch or plate tapping, which were used for studying the mechanical sensation of C. elegans in most labs. Our results suggest that worm can sense acoustic vibration. It responds to acoustic vibration by biasing the reorientation rate during navigation. The acoustic sensitive index depends on the frequency and displacement amplitude of the acoustic wave. The vibration level C. elegans can sense is much lower than people normally thought. Our work laid the ground work for studying C. elegans acoustic sensation. It would be interesting to further understand the neural circuits and cellular mechanism of this behavior.

Real-Time Monitoring and Quantitative Evaluation of Cavitation Bubbles Induced by High Intensity Focused Ultrasound Using B-Mode Imaging

A software-based method is proposed to eliminate the flooding interference strips in B-mode images, and to evaluate the cavitation bubbles generated during high intensity focused ultrasound (HIFU) exposures. In vitro tissue phantoms are exposed to 1.12 MHz HIFU pulses with a fixed 100 Hz pulse repetition frequency. HIFU-induced cavitation bubbles are detected as hyperechoic regions in B-mode images. The temporal evolution of cavitation bubbles, generated by HIFU pulses with varying driving amplitude and pulse length, is analyzed by measuring the time-varying area of the hyperechoic region. The results show that: first, it is feasible to monitor HIFU-induced cavitation bubble activity in real-time using B-mode imaging; second, more cavitation bubbles can be generated with higher acoustic energy delivered; third, the hyperechoic region is observed to shrink gradually after ceasing the HIFU pulses, which indicates the dissolution of cavitation bubbles. This work will be helpful for developing an effective tool to realize real-time monitoring and quantitative evaluation of HIFU-induced cavitation bubble activity using a current commercialized B-mode machine.

Correlation between microbubble-induced acoustic cavitation and hemolysis in vitro

Microbubbles promise to enhance the efficiency of ultrasound-mediated drug delivery and gene therapy by taking advantage of artificial cavitation nuclei. The purpose of this study is to examine the ultrasound-induced hemolysis in the application of drug delivery in the presence of microbubbles. To achieve this goal, human red blood cells mixed with microbubbles were exposed to 1-MHz pulsed ultrasound. The hemolysis level was measured by a flow cytometry, and the cavitation dose was detected by a passive cavitation detecting system. The results demonstrate that larger cavitation dose would be generated with the increase of acoustic pressure, which might give rise to the enhancement of hemolysis. Besides the experimental observations, the acoustic pressure dependence of the radial oscillation of microbubble was theoretically estimated. The comparison between the experimental and calculation results indicates that the hemolysis should be highly correlated to the acoustic cavitation.