Reception pattern influence on magnetoacoustic tomography with magnetic induction

Based on the acoustic radiation theory of a dipole source,the influence of the transducer reception pattern is studied for magnetoacoustic tomography with magnetic induction(MAT-MI).Numerical studies are conducted to simulate acoustic pressures,waveforms,and reconstructed images with unidirectional,omnidirectional,and strong directional transducers.With the analyses of equivalent and projection sources,the influences of the model dimension and the layer effect are qualitatively analyzed to evaluate the performance of MAT-MI.Three-dimensional simulation studies show that the strong directional transducer with a large radius can reduce the influences of equivalent sources,projection sources,and the layer effect effectively,resulting in enhanced pressure and improved image contrast,which is beneficial for boundary pressure extraction in conductivity reconstruction.The reconstructed conductivity contrast images present the conductivity boundaries as stripes with different contrasts and polarities,representing the values and directions of the conductivity changes of the scanned layer.The favorable results provide solid evidence for transducer selection and suggest potential practical applications of MAT-MI in biomedical imaging.     

Acoustic dipole radiation model for magnetoacoustic tomography with magnetic induction

An acoustic dipole radiation model for magnetoacoustic tomography with magnetic induction (MAT-MI) is pro-posed,based on the analyses of one-dimensional tissue vibration,three-dimensional acoustic dipole radiation and acoustic waveform detection with a planar piston transducer.The collected waveforms provide information about the conductiv-ity boundaries in various vibration intensities and phases due to the acoustic dipole radiation pattern.Combined with the simplified back projection algorithm,the conductivity configuration of the measured layer in terms of shape and size can be reconstructed with obvious border stripes.The numerical simulation is performed for a two-layer cylindrical phantom model and it is also verified by the experimental results of MAT-MI for a tissue-like sample phantom.The proposed model suggests a potential application of conductivity differentiation and provides a universal basis for the further study of conductivity reconstruction for MAT-MI.     

电导率各向异性金属薄板感应式磁声图像的仿真

为了给金属薄板感应式磁声(MAT-MI)成像算法的研究提供数据源,提出一种电导率各向异性金属薄板表面MAT-MI图像的数值仿真方法。建立含缺陷的电导率各向异性金属薄板仿真模型,并将其置于静磁场中。将通入交变电流的折线线圈置于金属薄板上方,对金属薄板在静磁场和交变磁场共同作用下产生的感应涡流以及声源(即洛伦兹力)进行数值仿真,得到金属薄板表面波位移分布的灰度图像。仿真实验结果表明,根据表面波位移在缺陷处迅速衰减的特性,可从图像中准确地识别并定位金属薄板表面的缺陷。忽略金属材料的电导率各向异性会降低成像质量,进而导致对缺陷的误判。通过提高表面波位移信号的信噪比可改善成像质量。减小提高距离或增大激励电流频率,可提高系统对微小缺陷和不规则缺陷检测的分辨力。      

Carboxylated magnetic nanoparticles as MRI contrast agents: Relaxation measurements at different field strengths

At the moment the biomedical applications of magnetic fluids are the subject of intensive scientific interest. In the present work, magnetite nanoparticles (MNPs) were synthesized and stabilized in aqueous medium with different carboxylic compounds (citric acid (CA), polyacrylic acid (PAA), and sodium oleate (NaOA)), in order to prepare well stabilized magnetic fluids (MFs). The magnetic nanoparticles can be used in the magnetic resonance imaging (MRI) as contrast agents. Magnetic resonance relaxation measurements of the above MFs were performed at different field strengths (i.e., 0.47, 1.5 and 9.4 T) to reveal the field strength dependence of their magnetic responses, and to compare them with that of ferucarbotran, a well-known superparamagnetic contrast agent. The measurements showed characteristic differences between the tested magnetic fluids stabilized by carboxylic compounds and ferucarbotran. It is worthy of note that our magnetic fluids have the highest r2 relaxivities at the field strength of 1.5 T, where the most of the MRI works in worldwide.     

A theoretical and experimental study of the magnetoacoustic effect in alloys

An account of the magnetoacoustic effect in aluminum alloys having additions of iron, copper magnesium, zinc, and silicon is given. The problem of the torsional oscillations of a metallic, electrically-conducting isotropic specimen in a magnetic field is considered, and experimental data are given.     

The second harmonic generation in semiconductors in the DC magnetic field

The second harmonic radiation in millimeter wave region is found in semiconductors in the absence of the dc electric field. The observed resonant maximum of radiation intensity is shown to be related to the excitation of the free-electron plasma oscillations in the dc magnetic field.     

Boundary normal pressure-based electrical conductivity reconstruction for magneto–acoustic tomography with magnetic induction

As a kind of multi-physics imaging approach integrating the advantages of electrical impedance tomography and ultrasound imaging with the improved spatial resolution and image contrast, magneto–acoustic tomography with magnetic induction(MAT-MI) is demonstrated to have the capability of electrical impedance contrast imaging for biological tissues with conductivity differences. By being detected with a strong directional transducer, abrupt pressure change is proved to be generated by the gradient of the induced Lorentz force along the force direction at conductivity boundary. A simplified boundary normal pressure(BNP)-based conductivity reconstruction algorithm is proposed and the formula for conductivity distribution inside the object with the clear physical meaning of pressure derivative, is derived. Numerical simulations of acoustic pressure and conductivity reconstruction are conducted based on a 2-layer eccentric cylindrical phantom model using Hilbert transform. The reconstructed two-dimensional conductivity images accord well with the model, thus successfully making up the deficiency of only imaging conductivity boundary in traditional MAT-MI. The proposed method is also demonstrated to have a spatial resolution of one wavelength. This study provides a new method of reconstructing accurate electrical conductivity and suggests the potential applications of MAT-MI in imaging biological tissues with conductivity difference.     

Brillouin spectroscopy of nonlinear magnetoacoustic resonances in a layered YIG/GGG structure

Brillouin light scattering spectroscopy is used to visualize the spatial structure of magnetoacoustic resonances in an yttrium iron garnet (YIG) film on a gadolinium–gallium garnet (GGG) substrate under the strong influence of nonlinear processes of three magnon decay. It is shown that the decay processes result in the simultaneous excitation of magnetoacoustic resonances at two frequencies: those of the input signal and its half frequency. The distribution of coupled magnetic and elastic waves becomes much more complicated and the excitation threshold of magnetoacoustic resonances arises.     

Dependences of magnetic Barkhausen emission and magnetoacoustic emission on the microstructure of pearlitic steel

The relationships between microstructure, mechanical behaviour and magnetic properties of completely pearlitic steels have been investigated, with the objective of determining the applicability of magnetic measurements to non-destructive evaluation of the properties of high-strength pearlitic steels. High-carbon steels were heat treated to produce completely pearlitic structures with various interlamellar spacings, and their magnetic properties, including hysteresis loops, Barkhausen emission (BE) and magnetoacoustic emission (MAE) profiles, were measured. MAE profiles were found to have two peaks at the knee of hysteresis loop, while BE profiles show only a single peak at about the coercive field for all samples. The peak height ratio of the MAE profiles and the amplitude of the BE profiles increase monotonically with increasing pearlite spacing, whereas coercivity is inversely proportional to pearlite spacing. These results can be interpreted in the context of magnetic domain structures and magnetization reversal processes observed by Lorentz transmission electron microscopy, which revealed that, in specimens with smaller pearlite spacing, reverse domains nucleated and grew at higher reverse magnetic field, and domain wall jumps across cementite lamellae were smaller than in samples with coarser pearlites. The good correlation observed between the magnetic properties, mechanical strength and microstructures of these steels provides the basis for rapid and effective non-destructive assessment of their properties for industrial applications.     

Effect of an external magnetic field on the propagation velocities of magnetoacoustic waves in a magnetic fluid

Theoretical results on the propagation of magnetoacoustic waves in an ideal magnetic fluid with frozen magnetization are generalized. Expressions for the propagation velocity of fast and slow magnetoacoustic waves and Alfven waves are derived. Agreement between the theoretical and experimental results is demonstrated for the fast magnetoacoustic waves propagating in a water-based magnetic fluid.     

Magnetoacoustic position imaging for liquid metal in animal interstitial structure

Magnetoacoustic tomography with magnetic induction(MAT-MI), as a new kind of in-vivo imaging method, has potential application value in interstitial fluid research. In this paper, we propose the application of MAT-MI with liquid metal serving as a tracer of the interstitial structure to study its fluid behavior, and use it to implement the positional imaging of the spatial distribution of liquid metal. Owing to the particularity of liquid metal magnetoacoustic pressure(MAP) signals, we propose an envelope analysis method to extract the rising edge of the amplitude envelope of the detected waveform as effective position data. And for the first time, we propose the method of superpositing pixel matrix to achieve the position imaging of liquid metal. Finally, the positional imaging of the liquid metal sample embedded in the gel is achieved to have relatively accurate results. This study provides a method of effectively extracting data and implementing the position imaging for liquid metal in the interstitial structure in the frame of MAT-MI.     

A thermo-fluid analysis in magnetic hyperthermia

In the last years, hyperthermia induced by the heating of magnetic nanoparticles （MNPs） in an alternating magnetic field received considerable attention in cancer therapy. The thermal effects could be automatically controlled by using MNPs with selective magnetic absorption properties. In this paper, we analyze the temperature field determined by the heating of MNPs, injected in a malignant tissue, subjected to an alternating magnetic field. The main parameters which have a strong influence on temperature field are analyzed. The temperature evolution within healthy and tumor tissues are analyzed by finite element method （FEM） simulations in a thermo-fluid model. The cooling effect produced by blood flow in blood vessels from the tumor is considered. A thermal analysis is conducted under different distributions of MNP injection sites. The interdependence between the optimum dose of the nanoparticles and various types of tumors is investigated in order to understand their thermal effect on hyperthermia therapy. The control of the temperature field in the tumor and healthy tissues is an important step in the healing treatment.     

High frequency fast magnetoacoustic modes in the plasma core

Fast magnetoacoustic modes (FMM) [known also as compressional Alfvén eigenmodes (CAE) and magnetosonic modes] with frequencies exceeding or equal the ion gyrofrequency are considered. It is shown that edge-localized FMM, which presumably are responsible for the superthermal ion cyclotron emission (ICE) observed in many experiments on tokamaks and stellarators, represent a particular case of these modes. In general, FMMs with frequencies above/about the ion gyrofrequency have different radial locations and structures. They can extend over a large part of the plasma cross section and even can have maximum amplitudes at the magnetic axis. Modes with the same frequency and the same poloidal mode number are multiple, having different radial structures. These results are obtained in the approximation of cylindrical plasma with one-ion species.     

交变力磁力显微镜动态成像技术的研究

近年来磁力显微镜(magnetic force microscopy,MFM)对动态磁场信号的测量与分析由于其特殊的工业要求和重要用途而受到广泛关注,本文旨在利用交变磁力对磁性探针的周期性调制发展一种交变力磁力显微镜技术,为磁信息存储工业等重要领域关键技术的发展提供新型的有力的工具.与目前标准MFM采用的设计思路不同,本文的关键在于合理利用MFM频率调制机理,优化设计MFM磁性探针,并且引入动态信号处理模块,实现对交变磁场信号的MFM成像.为达到这些目的,需要从理论上研究MFM探针的频率调制机理,并由实验上设计出动态信号提取模块,二者相辅结合优化设计出具有动态信号测试和分析能力的交变力磁力显微镜技术,由此来测量和解释纳米尺度磁畴结构.     

Linear magnetoacoustic birefringence effects in FeBO3

A theory of magnetoacoustic birefringence in the weak, easy-plane ferromagnet FeBO₃ is formulated with allowance for the mechanical boundary conditions at the sample in the experiment. The amplitude of the acoustic wave transmitted through the crystal is calculated as a function of the magnetic field; the dependence satisfactorily describes the experimental results. Fiz. Tverd. Tela (St. Petersburg) 39, 901–904 (May 1997)     

Surface modification of magnetic nanoparticles in biomedicine

Progress in surface modification of magnetic nanoparticles(MNPs)is summarized with regard to organic molecules,macromolecules and inorganic materials.Many researchers are now devoted to synthesizing new types of multi-functional MNPs,which show great application potential in both diagnosis and treatment of disease.By employing an ever-greater variety of surface modification techniques,MNPs can satisfy more and more of the demands of medical practice in areas like magnetic resonance imaging(MRI),fluorescent marking,cell targeting,and drug delivery.     

Nonlinear oscillation characteristics of magnetic microbubbles under acoustic and magnetic fields

Microbubbles loaded with magnetic nanoparticles (MMBs) have attracted increasing interests in multimode imaging and drug/gene delivery and targeted therapy. However, the dynamic behaviors generated in diagnostic and therapeutic applications are not clear. In the present work, a novel theoretical model of a single MMB was developed, and the dynamic responses in an infinite viscous fluid were investigated under simultaneous exposure to magnetic and acoustic fields. The results showed that the amplitude reduces and the resonant frequency increases with the strength of the applied steady magnetic field and the susceptibility of the magnetic shell. However, the magnetic field has a limited influence on the oscillating. It is also noticed that the responses of MMB to a time-varying magnetic field is different from a steady magnetic field. The subharmonic components increase firstly and then decrease with the frequency of the magnetic field and the enhanced effect is related to the acoustic driving frequency. It is indicated that there may be a coupling interaction effect between the acoustic and magnetic fields.     

Manifestation of basal-plane anisotropy and mechanical boundary conditions in magnetic birefringence of sound in hematite

The angular dependence of the magnetic birefringence of sound in hematite is experimentally investigated as a function of the direction of a magnetic field applied in the basal plane of the hematite crystal. It is found that, at room temperature, the curve of magnetoacoustic oscillations in the magnetic field, i.e., the oscillatory dependence of the amplitude of an acoustic wave transmitted through the crystal on the magnetic field strength, is characterized by hexagonal and uniaxial anisotropy. It is shown that the hexagonal anisotropy is governed by the basal-plane anisotropy of higher orders. The appearance of the uniaxial magnetic anisotropy in the basal plane of the crystal is explained by the mechanical stresses arising in the sample when piezoelectric transducers are glued to the sample ends. This assumption is confirmed by the observed change in the direction of the uniaxial anisotropy axis under variations in the boundary conditions.     

Resonant third harmonic generation of a short pulse laser in plasma by applying a wiggler magnetic field

We examine the effect of wiggler magnetic field on pulse slippage of short pulse laser-induced third harmonic generation in plasma. The process of third harmonic generation of an intense short pulse laser in plasma is resonantly enhanced by the application of a magnetic wiggler. The laser exerts a ponderomotive force at second harmonic driving density oscillations. The second harmonic oscillations coupled with electron velocity at the laser frequency, produces a non-linear current, driving the third harmonic. Third harmonic pulse generates in the fundamental pulse domain. However, the group velocity of the third harmonic wave is greater than the fundamental wave. Hence, the third harmonic pulse saturates strongly and moves forward from the fundamental pulse at shorter distance than the second harmonic pulse.     

Features of excitation and detection of magnetoacoustic emission in ferromagnetic objects

We study how the conditions of excitation and detection influence the magnetoacoustic emission (MAE) parameters of ferromagnets. For a large group of metal ferromagnet samples differing in physical properties and dimensions, the dependence of the MAE amplitude on the frequency of the remagnetizing field has a similar nonmonotonic character. In all probed cases, the maximum of the amplitude corresponds to a field frequency of 3–5 Hz. The decrease in the MAE amplitude for a subsequent increase in the frequency of the alternating field is related to the action of eddy currents. It is shown that the size of the field corresponding to the MAE maximum, which for a given time dependence of the remagnetizing field can be determined by the time shift in an oscillogram, can be a new parameter of the structuroscopy of ferromagnets.