k-space propagation models for acoustically heterogeneous media: application to biomedical photoacoustics

Biomedical applications of photoacoustics, in particular photoacoustic tomography, require efficient models of photoacoustic propagation that can incorporate realistic properties of soft tissue, such as acoustic inhomogeneities both for purposes of simulation and for use in model-based image reconstruction methods. k-space methods are well suited to modeling high-frequency acoustics applications as they require fewer mesh points per wavelength than conventional finite element and finite difference models, and larger time steps can be taken without a loss of stability or accuracy. They are also straightforward to encode numerically, making them appealing as a general tool. The rationale behind k-space methods and the k-space approach to the numerical modeling of photoacoustic waves in fluids are covered in this paper. Three existing k-space models are applied to photoacoustics and demonstrated with examples: an exact model for homogeneous media, a second-order model that can take into account heterogeneous media, and a first-order model that can incorporate absorbing boundary conditions.     

Limited projections laser speckle tomography of complex flows

Different approaches are discussed applied to reconstruction of local flow parameters using line-of-sight laser probing. The first one is based on single projection measurement and detailed numerical simulation of flow pattern. The second one is based on multi-projectional line-of-sight measurement and reconstructing local flow parameters by solving inverse integral transforms. Holographic interferometry (HI) and speckle photography (SP) are used for laser probing of complex 3D media under study. Computer-assisted tomography (CAT) approach based on Radon integral transform is described and examples of reconstructed flowfields are given including compressible flows with shock waves. Statistical information about flow studied is extracted as well, and maps of local statistical turbulence parameters are quantitatively determined by using obtained SP-data and a new Erbeck–Merzkirch integral transform.     

Transmission of polarized light through turbid media

The depolarization of light in multiple-scattering media with large (larger than the light wavelength) inhomogeneities is considered. The polarization state of the scattered light is described in the principal-mode approximation. Using the Fokker-Planck model, the polarization and intensity distribution of light are calculated in the vicinity of an inhomogeneity in the shape of an absorbing half-plane. The results of the calculations agree with the experimental data on transmission of light through turbid media.     

Investigation of multiple ultrasmall-angle neutron scattering with the aid of a double-crystal diffractometer

A method of multiple small-angle neutron scattering (MSANS), used to obtain information about inhomogeneities of substances, is considered. Experimental schemes and theory of a double-crystal diffractometer intended for measuring multiple small-angle neutron scattering are described. Methods used to approximate MSANS angular distributions at a low concentration of inhomogeneities and based on Moliére’s theory are reviewed. Their applicability is demonstrated for the examples ofMSANS spectra of samples from ferromagnetic iron-nickel alloys, aluminum powders, and high-T _c superconductor ceramic materials. Theoretical and experimental investigations of interference effects that manifest themselves in multiple small-angle neutron scattering on systems containing high concentrations of scatterers are reported.     

Enhanced backscattering of vortex waves from volume scattering media

The effect of phase vortices on the enhanced coherent backscattering from volume scattering media is studied theoretically and experimentally. The experimental results are well described by a theoretical model based on the diffusion approximation corrected for small path lengths contributions. Based on this approach, a self-referencing method for measuring the optical characteristics of a multiple scattering medium can be developed.     

Determining the structural parameters of fractal and nonfractal objects in multiple small-angle neutron scattering experiments

An experimental procedure employing setups with standard resolution characteristics for multiple small-angle neutron scattering in fractal and nonfractal media is described. Specific features of the proposed method, which are related to a limited resolution of the spectrometer, are considered in the case of large-scale inhomogeneities with the characteristic size exceeding the inverse spatial resolution. A new approach to the extraction of information about the fractal dimension of the system studied is demonstrated, which takes into account the dependence of the attenuation and broadening of the transmitted neutron beam on the sample thickness.     

Monte Carlo simulation in the theory of wave propagation in random media: II. Scintillation index, second and fourth moments

The purpose of this article (comprising parts I and II) is to develop and test the approach of combining a path-integral technique and a complex-valued Monte Carlo method to calculate the highest moments of the Green function of the stochastic wave equation for media with random small-scale inhomogeneities against the background of large-scale inhomogeneities. In part II calculations of the second and fourth moments of the Green function and the scintillation index have been performed for 1D and 2D cases in the framework of three models: a model of the stochastic wave equation and models of parabolic and Markov approximations. The finiteness of the correlation radius of inhomogeneities has been shown to be the reason for the significant difference between the Markov approximation and the other two. The results obtained prove that the applicability of the parabolic approximation (without the Markov approximation) is much wider than might be expected. A comparison has been made showing good agreement with reliable results for 1D media. The Monte Carlo results have exhibited the singularities existing at the localization centres and forming exponential decay of the second moment from distances of about one wavelength. The unexpected sharp oscillations interrupting the exponential decay of the Green function moments have been obtained at distances from the localization centre of several tens of times the average distance between scatterers. The effect of weak large-scale inhomogeneities on the behaviour of the second moment has also been investigated.     

A new method for reconstructing medium inhomogeneities using Rayleigh waves: Case studies

The problem of reconstructing local low-contrast inhomogeneities in the Earth’s surface layers by means of coherent Rayleigh surface waves is considered. It is shown that analysis of the frequency characteristics of shear projections in this wave on the surface allows construction of the function of inhomogeneity distribution in a specified depth range. The results from seismoacoustic reconstruction of inhomogeneities correlate with data obtained using standard geophysical methods and are confirmed by direct observations.     

Method of acoustic sweep monitoring of oceanic inhomogeneities (Review)

Theoretical grounds of the new method of monitoring of the temporal variability of oceanic inhomogeneities, which uses the data on frequency shifts of interference maxima of the sound field, were described. The method is free of limitations on both the resolution of signals coming in individual modes (rays) and the adiabatic approximation underlying the conventional methods of inhomogeneity reconstruction. The monitoring sensitivity was estimated, which allows us to estimate minimum detectable changes in the speed of sound by measurement data on frequency shifts of local maxima. Experimental data on shifts of the frequency spectrum of a broadband low-frequency signal on a stationary path in a shallow sea were presented. On their basis, the possibility of applying this method to diagnose tidal variations was shown. Within a numerical simulation, model reconstruction of the frequency spectrum of background internal waves was considered on the basis of the data on measurements of the spectrum of frequency shifts of the interference maximum. The results of the spectrum reconstruction with and without focusing of the conjugate wave field are presented. The problems of monitoring stability and efficiency with respect to the interference pattern formed by various groups of modes were discussed.     

相关哈特曼-夏克波前传感器波前重构新方法

对相关哈特曼-夏克波前传感器的波前重构方法进行了研究,提出了一种基于相邻子图像间相对平移量的波前重构新方法。与常用的基于单一参考子图像的波前重构方法相比,新方法的动态范围有了很大提高。在8×8去4角的子孔径划分下,测量离焦波面时,新方法的动态范围可提高约16倍,子孔径数目越多,动态范围提高的倍数也越高。精度分析表明,两种波前重构方法在8×8去4角的子孔径划分下精度相近,在子孔径数目更多时,相邻子图像处理法的波前重构精度低于单一参考子图像法。     

Methods of terahertz-subterahertz BWO spectroscopy of conducting materials

This paper considers present spectroscopic methods developed for measurement of dielectric response (conductivity and permittivity spectra) of condensed media in the terahertz-subterahertz spectral regions. The techniques based on the use of BWO spectrometers and designed for direct quantitative measurements (without invoking the Kramers-Kronig relations) in the terahertz-subterahertz frequency range (0.03–1.45 THz) of the conductivity σ(ν) and permittivity ?′(ν) spectra of conducting and absorbing materials are described. The techniques are based on measuring the amplitude and phase of the transmission coefficient of film samples on dielectric substrates and the reflection coefficient of a reference plane-parallel dielectric plate contacting the surface of the sample under study. The use of these techniques in measurement of the σ(ν)) and ?′(ν) spectra of conductors and superconductors is illustrated with examples.     

Assessment of nanostructured ceramic coating damage. Nanotribospectroscopy

The applicability of a tribospectral analysis to the detection of nanoscale inhomogeneities and discontinuities at the nanocoating-substrate interface has been substantiated theoretically. The approach put forward in this work is based on the measurement of the counterbody sliding resistance force coupled with an analysis of its frequency spectrum. A theoretical study on the potentialities and limitations of nanotribospectroscopy has been performed with the use of a computer simulation by the method of movable cellular automata. The results of the investigation have shown, among other things, the possibility to estimate a number of parameters of nanoscale discontinuities (characteristic spacing, linear size, etc.). To obtain reliable information on the defect structure and spacing, nanotribospectroscopy must be supplemented by other diagnostic methods. The design of a prototype nanotribospectrometer with spatial resolution up to 8 nm is described. Applications of the examined approach as a promising nondestructive-testing technique for assessment of nanostructured coating and surface layer damage are discussed.     

Acoustic diffraction tomography of the ocean

Tomographic monitoring of medium-scale inhomogeneities of the ocean using acoustic waves is considered. The basic integral equations are analyzed. A more detailed study is suggested for two patterns of diffraction tomography: pulsed translucent tomography of random inhomogeneities and hydroacoustic imaging. Some results of reconstruction of inhomogeneity images recovered based on laboratory-experiment data are presented.Institute of Applied Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 36, No. 8, pp. 738–751 August, 1993.     

Diffuse optical tomography of dynamic inhomogeneities in randomly inhomogeneous media

The transport of the time autocorrelation function in randomly inhomogeneous media with spatially separated static and dynamic regions is considered. Combining the photon mean trajectory method with the diffusing-wave spectroscopy technique, a method is proposed for fast reconstruction of images of dynamic inhomogeneities in a randomly inhomogeneous medium with a specified dynamics of scatterers, including media with Brownian diffusion and directional flows. The method of mean trajectories is complemented by Monte Carlo modeling, which makes it possible to apply this method to systems with a complex dynamics of scatters.     

On the theory of refractive radio-wave scattering

We consider in detail the frequency correlation of radio-wave fluctuations in one or several thick layers with strong large-scale inhomogeneities of turbulent origin. General expressions are obtained for the space-frequency fluctuation correlation of the radio-waves received. We analyze particular cases of radio wave scattering in turbulent media with inhomogeneities described by power-law spectra with indices p2 and p3. It is shown, in particular, that the coherence band of signals propagated in media with strong large-scale inhomogeneities is critically dependent on the spectral type of thOse inhomogeneities. The occurrence of an additional strongly scattering layer, which has radically different properties compared to the first layer, on the radio-wave path can increase or decrease considerably the frequency correlation of the radio waves received.Radiophysical Research Institute, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 10, pp. 1012–1022, October, 1995.     

Detection of inhomogeneities in biological tissues by the methods of confocal microscopy

The model of detection of local inhomogeneities of scattering and absorbing types in biological tissues by the methods of reflection and transmission confocal microscopy has been developed on the basis of the theory of vision in scattering media. General equations for calculation of the image contrast of an inhomogeneity against the background of a scattering medium are derived. The influence of the object characteristics and observation-system parameters on the maximum detection depth of inhomogeneities is analyzed. Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 5, pp. 625–639, May, 1998.