Abilities of nonlinear acoustic methods in locating gas bubbles in biological tissues

On the basis of a numerical solution to the equation of radial oscillations of a gas bubble in a liquid, the abilities of nonlinear acoustic methods in detecting gas bubbles in biological tissues with the use of pulsed acoustic location are analyzed. It is demonstrated that, with moderate amplitudes of the probing signal, it is possible to detect bubbles whose size is close to resonance. An improved version of the method of paired pulses is proposed, along with a method for detecting the bubbles and measuring their dimensions on the basis of a nonlinear excitation of their natural oscillations.     

生物组织光传输的系统模型及应用

以辐射传输方程为基础,讨论了生物组织光传输问题的线性空间平移不变性,同时建立了层状生物组织光传输问题的系统理论及相应的计算模型.与Monte Carlo方法的对比计算表明：该模型在保证结果准确性的前提下,可极大地提高生物组织光传输问题的计算效率.在此基础上,将该模型应用于层状生物组织内会聚光传输问题的理论计算,讨论了光源会聚角对生物组织内光场分布的影响.     

Influence of impurities on the de Haas-van Alphen effect

A general theory is given for the effects of impurities or other localized defects in metals on the amplitudes (“Dingle factor”) and the periods of the de Haas-van Alphen oscillations. Starting from the Green's function for the crystal with defects, after configuration averaging a simple expression for the spectral density and the level broadening is obtained, expressed in terms of the transition matrix for a single defect. The density of states leading to the de Haas-van Alphen oscillations is gained by summing the spectral density over the electron states in the presence of the magnetic field. Using the transition matrices obtained earlier, the changes of the oscillations may be calculated for general localized defects and Fermi surfaces. A previous paper by Brailsford on the same topic is critically discussed.     

The method of calculating forced oscillations in nonlinear discrete-time systems under periodic external actions

We consider a method for calculating forced oscillations in nonlinear discrete-time systems under periodic external actions. The method is based on representing the stationary oscillations in the form of an invariant set of nonlinear discrete point mappings and allows one to calculate the nonlinear-system response in the steady-state regime. The examples of using this method for calculating forced oscillations in the first- and second-order nonlinear recursive systems under the harmonic-signal action on such systems are presented.     

Nonlinear phenomena accompanying the development of oscillations excited in a layer of a linear dissipative medium by finite displacements of its boundary

A method of describing oscillations in resonators on the basis of evolution equations is proposed. The latter are obtained by simplifying the functional equations under the assumption that the distortions of travelling waves within the resonator length are small, that the Mach number for the moving boundary oscillations is small, and that the frequency is close to one of the natural frequencies of the resonator. The problems of nonstationary oscillations of a layer with a moving boundary are solved. The law that should govern the wall oscillations to provide the development of steady-state linear resonance oscillations is determined. The shape of the resonance curve formed in the presence of a boundary nonlinearity is calculated. The method of matching of asymptotics is applied to the singularly perturbed problem with small dissipation. It is shown that a boundary nonlinearity leads to a distortion of the temporal profile of the standing wave and to the generation of higher harmonics in the process of the development of steady-state oscillations. In contrast to the classical linear problems where the resonance occurs at the coincidence of the external force frequency with one of the natural frequencies, in the case under study the resonance behavior is observed in frequency bands, which are wider the higher the amplitude of the boundary oscillations is.     

Images of synchronized chaos: Experiments with circuits

Synchronization of oscillations underlies organized dynamical behavior of many physical, biological and other systems. Recent studies of the dynamics of coupled systems with complex behavior indicate that synchronization can occur not only in case of periodic oscillations, but also in regimes of chaotic oscillations. Using experimental observations of chaotic oscillations in coupled nonlinear circuits we discuss a few forms of cooperative behavior that are related to the regimes of synchronized chaos. This paper is prepared under the request of the editors of the special focus issue of Chaos and contains the materials for the lecture at the International School in Nonlinear Science, "Nonlinear Waves: Synchronization and Patterns," Nizhniy Novgorod, Russia, 1995. The main goal of the paper is to outline the collection of examples that illustrate the state of the art of chaos synchronization. (c) 1996 American Institute of Physics.     

Anomalous surface deceleration of a domain wall in an amorphous ferromagnet

On soft magnetic amorphous specimens, a rapid decrease in the surface amplitude of 180° domain wall oscillations relative to the bulk amplitude is observed with increasing frequency of the magnetizing field. The dynamics of the domain wall is studied by a magnetooptical method at the specimen surface and by the induction method in the bulk. The results of the experiment disagree with the theory, which takes into account the effect of eddy currents and predicts that, with increasing frequency, the surface amplitude of the domain wall oscillations should decrease slower than the bulk amplitude. The observed behavior of the domain wall is explained by its interaction with macroscopic defects at the specimen surface. This interaction gives rise to unsteady chaotic surface wall displacements, which lead to an increase by several orders of magnitude in the effective surface damping parameter in the Landau-Lifshits equation.     

Breathers in the one-dimensional roll structure of twisted nematics

The dynamics of nonsingular defects has been studied experimentally and theoretically in the periodic roll structures arising at electroconvection in nematic liquid crystals twisted by π/2. The presence of an axial component of the velocity of the hydrodynamic flow with the opposite direction in the neighboring rolls is characteristic of roll structures in twisted nematics. The quantitative estimates of the critical frequency of the oscillations of structural defects have been obtained on the basis of the nonlinear equation of motion for the roll displacement. It has been found that the periodic creation and annihilation of a pair of edge dislocations with the topological indices +1 and −1 occur in the process of oscillations of a defect with a nonsingular nucleus. It has been demonstrated that oscillating defects with zero topological indices correspond to the solution of the sine-Gordon equation in the form of standing breathers.     

The character of cooperative low-frequency oscillations in a model of a biopolymer

The oscillations in a model of a biopolymer are calculated under various excitation conditions. Relatively slow cooperative motions, which can be used to simulate conformational transitions in a biological macromolecule from one stable state to another, are analyzed.     

Acoustic nonlinearity parameter tomography for biological tissues via parametric array from a circular piston source--theoretical analysis and computer simulations

The acoustic nonlinearity parameter B/A describes the nonlinear features of a medium and may become a novel parameter for ultrasonic tissue characterization. This paper presents a theoretical analysis for acoustic nonlinear parameter tomography via a parametric array. As two primary waves of different frequencies are radiated simultaneously from a circular piston source, a secondary wave at the difference frequency is generated due to the nonlinear interaction of the primary waves. The axial and radial distributions of sound pressure amplitude for the generated difference frequency wave in the near field are calculated by a superposition of Gaussian beams. The calculated results indicated that the difference frequency component of the parametric array grows linearly with distance from the piston source. It therefore provides a better source to do the acoustic nonlinearity parameter tomography because the fundamental and second harmonic signals both have a near field that goes through many oscillations due to diffraction. By using a finite-amplitude insert substitution method and a filtered convolution algorithm, a computer simulation for B/A tomography from the calculated sound pressure of the difference frequency wave is studied. For biological tissues, the sound attenuation is considered and compensated in the image reconstruction. Nonlinear parameter computed tomography (CT) images for several biological sample models are obtained with quite good quality in this study.     

Self-sustained target waves in excitable media with only a long-range link

In this paper we investigate spatiotemporal pattern formation in excitable media with only a long-range link. Besides the trivial solutions of spiral patterns, we find the asymptotic self-sustained target waves in the autonomous tissues. The wave source supporting this kind of new pattern is the oscillatory one-dimensional Winfree-loop self-organized under the presence of a long-range link, which is explored by the dominant phase-advanced driving method. Based on this understanding we can effectively regulate the oscillations of excitable media by suitably arranging the long-range link, including construction of self-sustained target waves with controllable period and wave length, or manipulation of system states between different patterns.     

Viscoelasticity imaging of biological tissues with phase-resolved photoacoustic measurement

A method for noninvasive viscoelasticity imaging of biological tissues using phase-resolved photoacoustic measurement is presented. We deduced the process of photoacoustic effect on the basis of thermal viscoelasticity theory, and established the relationship between the photoacoustic phase delay and the viscosity-elasticity ratio for soft solids. Agar phantoms with different densities and different absorption coefficients were used to verify the dependence of photoacoustic phase-resolved viscoelasticity measurements. Moreover, viscoelasticity imaging of tissues was obtained with a photoacoustic point scanning system. The photoacoustic phase-resolved method provides a basis for viscoelasticity imaging, which can potentially be used for detection of viscoelastic properties and lesions of biological tissues.     

多谱段短波红外小鼠静脉荧光观测成像研究

短波红外(short-wave infrared,SWIR)一般指900～1700 nm的光波段,是肉眼不可见的光波段,这种波段目前主流的探测器以InGaAs为主,主要用于军事、生物以及材料光谱分析等领域.短波红外荧光成像以其对生物组织光学损伤小、成像深度大、成像信噪比高、空间和时间成像分辨率高等特点,使得基于InGa...     

Soliton excitations in a polariton condensate with defects

To study soliton excitations in a polariton condensate with defects, we use the Gross-Pitaevskii equation and its hydrodynamic form. An extra term is added to take into account the non-equilibrium nature of the polariton condensate and the presence of defects. The reductive perturbation method transforms these hydrodynamic equations into a modified Korteweg-de Vries equation in the long wavelength limit. We linearize this equation and study the soliton linear excitations.We give an analytic expression of traveling excitations using the variation of constants method. In the more general form,we show numerically that the excitations are oscillations, i.e., the amplitude and the width of the dark soliton oscillate simultaneously but in an opposite way.     

Defect modes in one-dimensional photonic lattices

Linear defect modes in one-dimensional photonic lattices are studied theoretically. For negative (repulsive) defects, various localized defect modes are found. The strongest confinement of the defect modes appears when the lattice intensity at the defect site is nonzero rather than zero. When launched at small angles into such a defect site of the lattice, a Gaussian beam can be trapped and undergo snake oscillations under appropriate conditions.     

The study of sensitivity and precession of a single-source diffused tomography for detecting of target depth in biological phantom

Optical imaging can be used to study the cancerous stages of breast cancer; and this imaging is noninvasive and safe for healthy tissues. The key problem of optical imaging is the image reconstruction which depends on photon migration in biological tissues; because the study of photon migration in the biological tissues is a complicated problem. The diffusion equation is sometimes used to simulate the photon migration in the biological tissues. Due to limitation of diffusion equation and its approximated nature, we want to explore the accuracy and precision of this method. So in this study, we report the design of a single-source diffuse imaging system with simulating code based on finite element method (FEM) to detect the location of tumor in breast phantoms. The comparison between the reconstructed results and actual values can be considered as a criterion of accuracy of this diffused system. In this experimental setup, to reduce the expense of computational procedure, single source is applied, and the accuracy and precision of this single-source are investigated.     

基于变分模态分解与多尺度排列熵的生物组织变性识别

根据高强度聚焦超声(HIFU)治疗中超声散射回波信号的特点,本文利用变分模态分解(VMD)与多尺度排列熵(MPE)对生物组织变性识别进行了研究.首先对生物组织中的超声散射回波信号进行变分模态分解,根据各阶模态的功率谱信息熵值分离出噪声分量和有用分量;对分离出的有用信号进行重构并提取其多尺度排列熵;然后通过Gustafson-Kessel (GK)模糊聚类确定聚类中心,采用欧氏贴近度与择近原则对生物组织进行变性识别.将所提方法应用于HIFU治疗中超声散射回波信号实验数据,用遗传算法对多尺度排列熵的参数优化后,对293例未变性组织和变性组织的超声散射回波信号数据进行了多尺度排列熵分析,发现变性组织的超声散射回波信号的多尺度排列熵值要高于未变性组织;多尺度排列熵可以较好地识别生物组织是否变性.相对于EMD-MPE-GK模糊聚类以及VMD-小波熵(WE)-GK模糊聚类变性识别方法,本文所提方法中变性与未变性组织特征交叠区域数据点更少,聚类效果和分类性能更好;本实验环境下生物组织变性识别结果表明,该方法的识别率更高,高达93.81%.     

Resonance parametric excitation of temperature oscillations in levitating aerosol droplets

The threshold conditions of excitation of temperature oscillations in high-Q water aerosol droplets have been studied under the conditions of slow evaporation. Selection rules have been obtained for interacting modes in a droplet. The threshold intensity of excitation of temperature oscillations has been analyzed in comparison with the threshold intensity of stimulated Brillouin and stimulated Raman scattering in a droplet. It is shown that, in the three-mode regime, the temperature oscillations can be excited at a rather low pumping level (about 10 W/cm²). A method is proposed for the remote measurement of the microphysical parameters of a droplet from the periodic temperature shift of eigenfrequencies of a droplet, the threshold intensity of excitation of temperature oscillations, and the thermal Raman frequency.     

Optical anisotropy of a biological tissue under conditions of immersion clearing and without them

The optical anisotropy of a biological tissue is studied under conditions of immersion clearing of the tissue and without them. It is shown that the rate of immersion clearing of a biological tissue depends on its type and is determined by the rate of diffusion of immersion liquids into the sample. The parameters that characterize the rate of diffusion of an immersion liquid into a biological tissue are determined. The types of anisotropy of different samples of biological tissues are found. Based on the Müller matrix formalism, a model for calculating the spectrum of collimated polarized transmission of a biological tissue is constructed. Comparison of calculated and experimental transmission spectra of biological tissues showed their good quantitative coincidence.     

Theil Entropy as a Non-Lineal Analysis for Spectral Inequality of Physiological Oscillations

Theil entropy is a statistical measure used in economics to quantify income inequalities. However, it can be applied to any data distribution including biological signals. In this work, we applied different spectral methods on heart rate variability signals and cellular calcium oscillations previously to Theil entropy analysis. The behavior of Theil entropy and its decomposable property was investigated using exponents in the range of [−1, 2], on the spectrum of synthetic and physiological signals. Our results suggest that the best spectral decomposition method to analyze the spectral inequality of physiological oscillations is the Lomb–Scargle method, followed by Theil entropy analysis. Moreover, our results showed that the exponents that provide more information to describe the spectral inequality in the tested signals were zero, one, and two. It was also observed that the intra-band component is the one that contributes the most to total inequality for the studied oscillations. More in detail, we found that in the state of mental stress, the inequality determined by the Theil entropy analysis of heart rate increases with respect to the resting state. Likewise, the same analytical approach shows that cellular calcium oscillations present on developing interneurons display greater inequality distribution when inhibition of a neurotransmitter system is in place. In conclusion, we propose that Theil entropy is useful for analyzing spectral inequality and to explore its origin in physiological signals.