Tissue characterization based on scatterer number density estimation

The authors propose a robust model for characterizing the statistical nature of signals obtained from ultrasonic backscatter processes. The model can accommodate frequency-dependent attenuation, spatially varying media statistics, arbitrary beam geometries, and arbitrary pulse shapes. On the basis of this model, statistical schemes are proposed for estimating the scatterer number density (SND) of tissues. The algorithm for estimating the scatterer number incorporates measurements of both the statistical moments of the backscattered signals and the point spread function of the acoustic system. The number density algorithm has been applied to waveforms obtained from ultrasonic phantoms with known number densities and in vitro mammalian tissues. There is an excellent agreement among theoretical, histological, and experimental results. The application of this technique for noninvasive clinical tissue characterization is discussed.     

Enhancement of continuous estimation of distribution algorithms by density ensembles

In this article, two novel density ensembles methods – the resampling method and the subspaces method – are proposed for enhancing existing continuous Estimation of Distribution Algorithms (EDAs). In ‘resampling continuous EDAs’, a population of densities of the selected promising solutions is obtained by iteratively using the resampling operator and the density estimation operator, and new candidate solutions at the next generation are reproduced through sampling from all obtained densities of promising solutions. In ‘subspaces continuous EDAs’, a population of densities is obtained by randomly choosing a subset of all variables and estimating the density of all selected high-quality solutions in this subspace. The above steps iterate and many densities of high-quality solutions in different subspaces can be obtained. New candidate solutions at the next generation are reproduced through perturbing the old promising solutions by sampling from the densities in different subspaces. The results upon convergence with different numbers of variables and the effects of parameters on the performance of the density ensembles methods for continuous EDAs are studied based on the experimental results.     

Automatic spectral density estimation for random fields on a lattice via bootstrap

We consider the nonparametric estimation of spectral densities for second-order stationary random fields on a d-dimensional lattice. We discuss some drawbacks of standard methods and propose modified estimator classes with improved bias convergence rate, emphasizing the use of kernel methods and the choice of an optimal smoothing number. We prove the uniform consistency and study the uniform asymptotic distribution when the optimal smoothing number is estimated from the sampled data.     

Response spectral density determination for nonlinear systems endowed with fractional derivatives and subject to colored noise

A computationally efficient method for determining the response of non-linear stochastic dynamic systems endowed with fractional derivative elements subject to stochastic excitation is presented. The method relies on a spectral representation both for the system excitation and its response. Specifically, first the ordinary non-linear differential equation of motion is transferred into a set of non-linear algebra equations by employing the harmonic balance method. Next, the response Fourier coefficients are determined by solving these non-linear equations. Finally, repeated use of the proposed procedure yields the response power spectral density. Pertinent numerical examples, including a fractional Duffing and a bilinear oscillator, demonstrate the accuracy of the proposed method.     

Comparison of two non-convex mixed-integer nonlinear programming algorithms applied to autoregressive moving average model structure and parameter estimation

In this article, the stochastic modelling approach proposed by Box and Jenkins is treated as a mixed-integer nonlinear programming (MINLP) problem solved with a mesh adaptive direct search and a real-coded genetic class of algorithms. The aim is to estimate the real-valued parameters and non-negative integer, correlated structure of stationary autoregressive moving average (ARMA) processes. The maximum likelihood function of the stationary ARMA process is embedded in Akaike's information criterion and the Bayesian information criterion, whereas the estimation procedure is based on Kalman filter recursions. The constraints imposed on the objective function enforce stability and invertibility. The best ARMA model is regarded as the global minimum of the non-convex MINLP problem. The robustness and computational performance of the MINLP solvers are compared with brute-force enumeration. Numerical experiments are done for existing time series and one new data set.     

On the estimation of the material fatigue properties required to perform the multiaxial fatigue assessment

To accurately perform the fatigue assessment of engineering components subjected to in‐service multiaxial fatigue loading, the adopted design criterion must properly be calibrated, the used information usually being the fatigue strength under both pure uniaxial and pure torsional fatigue loading. Because of the complex fatigue response of metallic materials to multiaxial loading paths, the only reliable way to generate the necessary pieces of calibration information is by running appropriate experiments. Unfortunately, because of a lack of both time and resources, very often, structural engineers are requested to perform the multiaxial fatigue assessment by guessing the necessary fatigue properties. In this complex scenario, initially, the available empirical rules suitable for estimating fatigue strength under both pure axial and pure torsional fatigue loading are reviewed in detail. Subsequently, several experimental results taken from the literature and generated by testing metallic materials under a variety of proportional and non‐proportional multiaxial loading paths are used to investigate the way such empirical rules affect the accuracy in estimating fatigue strength, the damage extent being evaluated according to the modified Wöhler curve method. Such a systematic validation exercise allowed us to prove that under proportional loading (with both zero and non‐zero mean stresses), an adequate margin of safety can be reached even when the necessary calibration information is directly estimated from the material ultimate tensile strength. On the contrary, in the presence of non‐proportional loading, the use of the empirical rules reviewed in the present paper can result, under particular circumstances, in a non‐conservative fatigue design.     

Correlating defect density with carrier mobility in large-scaled graphene films: Raman spectral signatures for the estimation of defect density

We report a correlation between carrier mobility and defect density in large-scaled graphene films prepared by chemical vapor deposition (CVD). Raman spectroscopy is used for investigating the layer number and the crystal quality of graphene films, and the defect density is estimated by the intensity ratios of the D and G peaks. By carefully controlling the growth parameters, especially the H(2)/CH(4) ratios during growth, and employing H(2) during cooling, monolayer-dominant graphene films can be obtained with different D peak intensities in Raman spectra, which show good correspondence with their carrier mobility obtained by Hall measurements. Also, a progressive shift of neutrality points to a more negative gate voltage is observed with the increase in defect density. Both the connections of carrier mobility and the shift of neutrality points to a negative direction in relation to the defect density in graphene are observed for the first time in CVD-grown graphene films. With the best growth conditions, a cm-scaled graphene film with carrier mobility of ～ 1350 cm(2) V(-1) s(-1) (p-type in air) can be obtained.     

A filter for on-line estimation of spectral content

A robust filter algorithm to extract, a posteriori, the rational signal model from a noisy measurement, with little a priori information, is proposed. The spectrum and the statistics of the signal and of the corrupting noise are assumed unknown, except that the signal is assumed to have a rational spectrum. An algorithm based on system and signal theory is derived to select a set of frequencies where the signal-to-noise ratio (SNR) is high from a given measurement spectrum. The density of selected frequencies weights the importance of the measurement as a function of frequency, An estimate of the signal model is obtained from the best weighted least-squares fit to the measurement spectrum at the selected frequencies. The proposed filter has applications to control and signal processing, and a wide variety of applications are presented. Applications include: system identification of a dc motor and a two-link manipulator, extraction of a myo-electric signal from a noisy measurement, the assignment of a rational model to a vegetation tissue's impedance, and to the number density profile of atmospheric oxygen     

Viscoelastic parameter estimation based on spectral analysis

This paper introduces a new technique for the robust estimation of relaxation-time distribution in tissue. The main novelty is in the use of the phase of transfer functions calculated from a time series of strain measurements at multiple locations. Computer simulations with simulated measurement noise demonstrate the feasibility of the approach. An experimental apparatus and software were developed to confirm the simulations. The setup can be used both as a rheometer to characterize the overall mechanical properties of a material or as a vibro-elastography imaging device using an ultrasound system. The algorithms were tested on tissue mimicking phantoms specifically developed to exhibit contrast in elasticity and relaxation time. The phantoms were constructed using a combination of gelatin and a polyvinyl alcohol sponge to produce the desired viscoelastic properties. The tissue parameters were estimated and the elasticity and relaxation time of the materials have been used as complementary features to distinguish different materials. The estimation results are consistent with the rheometry, verifying that the relaxation time can be used as a complementary feature to elasticity to delineate the mechanical properties of the phantom.     

基于功率谱密度的盲信号分离

提出一种新的基于功率谱密度的盲信号分离方法。该方法在传感器数与源数关系不明确情况下,且源信号相互独立时,直接通过混合信号功率谱密度函数比值求解混合阵,并通过混合阵判定观测信号是完备混合、超定混合还是欠定混合,由此进一步分离信号。理论分析、仿真数据证明了算法的有效性。同时还仿真了噪声对分离性能的影响。     

A spectral conjugate gradient method for nonlinear inverse problems

In this study, a new spectral conjugate gradient method is presented to solve nonlinear inverse problems, which transferred into the unconstrained nonlinear optimization with a neighbour term. The global convergence and regularizing properties of the proposed method are analysed. In the end, some numerical results illustrate the efficiency and the robustness of this method.     

A family of solution algorithms for nonlinear structural analysis based on relaxation equations

A family of hierarchical algorithms for nonlinear structural equations are presented. The algorithms are based on the Davidenko-Branin type homotopy and shown to yield consistent hierarchical perturbation equations. The algorithms appear to be particularly suitable to problems involving bifurcation and limit point calculations. An important by-product of the algorithms is that they provide a systematic and economical means for computing the stepsize at each iteration stage when a Newton-like method is employed to solve the systems of equations. Some sample problems are provided to illustrate the characteristics of the algorithms.     

The method of separation for evolutionary spectral density estimation of multi-variate and multi-dimensional non-stationary stochastic processes

The method of separation can be used as a non-parametric estimation technique, especially suitable for evolutionary spectral density functions of uniformly modulated and strongly narrow-band stochastic processes. The paper at hand provides a consistent derivation of method of separation based spectrum estimation for the general multi-variate and multi-dimensional case. The validity of the method is demonstrated by benchmark tests with uniformly modulated spectra, for which convergence to the analytical solution is demonstrated. The key advantage of the method of separation is the minimization of spectral dispersion due to optimum time- or space–frequency localization. This is illustrated by the calibration of multi-dimensional and multi-variate geometric imperfection models from strongly narrow-band measurements in I-beams and cylindrical shells. Finally, the application of the method of separation based estimates for the stochastic buckling analysis of the example structures is briefly discussed.     

噪声谱估计算法对语音可懂度的影响

噪声谱估计是单通道语音增强算法的关键步骤,当前大部分语音增强算法旨在提高语音质量,提高语音可懂度的算法却很少。在传统的单通道语音增强算法中,语音质量的提高往往是以牺牲语音的可懂度为代价的。对目前主流的几种噪声谱估计算法对语音可懂度影响进行分析。在不同噪声背景、不同信噪比情况下进行噪声谱估计,并采用谱减法对含噪语音信号作去噪处理,对比分析不同噪声、不同信噪比下增强前后语音的短时客观可懂度(Short-Time Objective Intelligibility,STOI)值,最后根据信噪比,对比分析了不同噪声环境下,语音增强前后语音能量高于噪声能量的时频块所占比例。实验表明,相比其他噪声估计算法,最小统计(Minima Statistics,MS)算法由于保留了更多的以语音能量为主的时频块,使得去噪后的语音有较高的可懂度。     

A numerical study of various algorithms related to the preconditioned conjugate gradient method

We present the results of a numerical study of the preconditioned conjugate gradient algorithm, the minimal residual algorithm, the biconjugate gradient algorithm and the bi-minimal residual algorithm using both simple test matrices and more realistic test matrices derived from physical problems. The application of the methods to unsymmetric matrices is considered. We emphasize the importance of a good preconditioning, look at various methods including ICGG(n) and incomplete block factorization, and make some practical recommendations. Some of the folk-lore surrounding the semi-iterative methods is dispelled.     

关于影像密度测量时光谱条件的探讨

本文概括阐明了由ISO规定,通常用在不同影像技术工作中的主要几种光学密度测量的光谱条件。如:视觉密度、1类型和2类型印片密度、A状态和M状态密度。同时也介绍了光谱乘积的简明定义和教学表达式以及相关数据,且附有有关问题的简单讨论。