Electromagnetic imaging for an imperfectly conducting cylinder

A computational approach to the imaging or inverse scattering of an imperfectly conducting cylinder is presented. A conducting cylinder of unknown shape and conductivity scatters the incident wave in free space and the scattered field is recorded on a circle surrounding the scatterer. By properly processing the scattered data, the shape and conductivity of the scatterer can be reconstructed. The problem is formulated in the form of nonlinear integral equations, which can be solved numerically by the Newton-Kantorovitch algorithm. The pseudoinverse technique is used to overcome the ill-posedness, and the condition number of the matrix is also discussed. Numerical examples are given to illustrate the capability of the inversion algorithm using the simulated scattered fields in both near and far zones. Multiple incident directions permit good reconstruction of shape and, to a lesser extent, conductivity in the presence of noise in measured data     

Electrical conductivity imaging via contactless measurements: an experimental study

A data-acquisition system has been developed to image electrical conductivity of biological tissues via contactless measurements. This system uses magnetic excitation to induce currents inside the body and measures the resulting magnetic fields. The data-acquisition system is constructed using a PC-controlled lock-in amplifier instrument. A magnetically coupled differential coil is used to scan conducting phantoms by a computer controlled scanning system. A 10000-turn differential coil system with circular receiver coils of radii 15 mm is used as a magnetic sensor. The transmitter coil is a 100-turn circular coil of radius 15 mm and is driven by a sinusoidal current of 200 mA (peak). The linearity of the system is 7.2% full scale. The sensitivity of the system to conducting tubes when the sensor-body distance is 0.3 cm is 21.47 mV/(S/m). It is observed that it is possible to detect a conducting tube of average conductivity (0.2 S/m) when the body is 6 cm from the sensor. The system has a signal-to-noise ratio of 34 dB and thermal stability of 33.4 mV/degrees C. Conductivity images are reconstructed using the steepest-descent algorithm. Images obtained from isolated conducting tubes show that it is possible to distinguish two tubes separated 17 mm from each other. The images of different phantoms are found to be a good representation of the actual conductivity distribution. The field profiles obtained by scanning a biological tissue show the potential of this methodology for clinical applications.     

基于深度迭代网络的穿墙雷达成像方法

联合墙杂波抑制及图像重建迭代求解方法为当前较前沿的穿墙雷达成像(through-the-wall radar imaging, TWRI)算法,能够同时抑制墙体杂波和重构目标图像,但仍存在收敛速度慢、人工干预过多以及对初值的选取敏感等问题,难以快速精确地进行目标成像. 针对上述问题,本文提出一种联合低秩与稀疏分解驱动的可学习深度迭代网络的TWRI方法. 该方法利用穿墙雷达场景下墙体杂波的低秩特性以及待重建目标图像的稀疏特性,首先将穿墙雷达成像问题建模为联合低秩与稀疏分解的正则化优化问题,然后采用变分框架和轮换策略将优化问题转化成两个准线性优化子问题并推导其更新公式,最后将上述迭代更新公式映射到网络结构中,展开成深度迭代网络模型并采用端到端学习策略,形成融合物理模型的可学习深度迭代网络框架. 仿真结果表明该方法能够有效抑制墙体杂波,相对于其他方法显著提高了目标成像精度和速度.     

Electrical conductivity imaging via contactless measurements

A new imaging modality is introduced to image electrical conductivity of biological tissues via contactless measurements. This modality uses magnetic excitation to induce currents inside the body and measures the magnetic fields of the induced currents. In this study, the mathematical basis of the methodology is analyzed and numerical models are developed to simulate the imaging system. The induced currents are expressed using the A-phi formulation of the electric field where A is the magnetic vector potential and phi is the scalar potential function. It is assumed that A describes the primary magnetic vector potential that exists in the absence of the body. This assumption considerably simplifies the solution of the secondary magnetic fields caused by induced currents. In order to solve phi for objects of arbitrary conductivity distribution a three-dimensional (3-D) finite-element method (FEM) formulation is employed. A specific 7 x 7-coil system is assumed nearby the upper surface of a 10 x 10 x 5-cm conductive body. A sensitivity matrix, which relates the perturbation in measurements to the conductivity perturbations, is calculated. Singular-value decomposition of the sensitivity matrix shows various characteristics of the imaging system. Images are reconstructed using 500 voxels in the image domain, with truncated pseudoinverse. The noise level is assumed to produce a representative signal-to-noise ratio (SNR) of 80 dB. It is observed that it is possible to identify voxel perturbations (of volume 1 cm3) at 2 cm depth. However, resolution gradually decreases for deeper conductivity perturbations.     

Electromagnetic imaging for complex cylindrical objects

Electronic imaging of complex cylindrical objects with arbitrary cross sections was investigated, assuming an incident wave upon both penetrable inhomogeneous dielectric cylinders and perfectly conducting cylinders with known shape, and external measurements of the scanned field. By properly processing the scattered held measurements, the dielectric permittivity distribution of the scanned object can be reconstructed. A theoretical formulation was based on proper arrangement of the incident field directions resulting in a set of integral equations derived and solved by the moment method and the unrelated illumination method. Numerical results demonstrate the capability of the imaging algorithm. Good reconstruction results were obtained even in the presence of additive random noise. In addition, noise effects on the reconstruction results were investigated.     

Electromagnetic imaging of underground targets using constrainedoptimization

Several high-frequency electromagnetic techniques have been used in recent years to detect and identify buried objects. Post-processing of the collected data is performed in many of these techniques to obtain high-quality images of buried targets. Accurate reconstructions of the target's constitutive parameters can be obtained by casting the imaging problem in terms of an inverse electromagnetic scattering problem. A number of techniques have been put forth recently to invert the electromagnetic data to obtain such images. The authors use a frequency-domain Born iterative method to reconstruct images of shallow targets. The Born iterative technique requires successive solutions to a forward scattering problem followed by an inverse scattering problem at each iteration step. They use a finite-difference time-domain (FDTD) algorithm to solve the forward scattering problem and constrained optimization for the inverse problem. Two-dimensional simulated data for several canonical objects buried in the ground are obtained using the FDTD technique. The same FDTD code is also used in calculating the Green's function required for solving the constrained optimization problem. Lossy, inhomogeneous ground models are used in several simulations to illustrate the use of this technique for practical situations. The inversion process can be used to reconstruct images for many realistic dielectric contrasts for which a linear Born approximation fails. Moreover, it is also shown that a small number of measurements results in accurate reconstructions with this technique. Use of multiple frequencies is also investigated     

Electromagnetic geophysical imaging incorporating refraction and reflection

An accurate technique for remotely determining the internal structure of an object or underground environment would have a significant impact in mining, geoexploration, ultrasonics, and the life sciences. This process of resolving the intrinsic properties of an object or environment by the transmission of radiation or ultrasound through the unknown anomaly is known as reconstructive imaging or tomography. Several efforts have been made to apply imaging (reconstruction) methods to measurements taken between two boreholes on either side of an unknown geophysical structure. However, it became necessary, because of the nature of existing reconstruction methods, to assume a straight-line propagation path from source to receiver. This assumption is not valid in many important applications of geophysical imaging; thus it is desirable to develop a method to account for the radiation mechanisms of refraction and reflection in the unknown medium. An imaging scheme that explicitly incorporates refraction and first-order reflection in the reconstruction process is developed. Several examples of successful reconstruction of multicell underground environments are presented to demonstrate its accuracy.     

Electromagnetic imaging of two-dimensional perfectly conducting cylinders with transverse electric scattered field

Electromagnetic imaging of two-dimensional perfectly conducting cylinders using measured transverse electric scattered field is studied in this paper. The contours of cylinders are denoted by local shape functions /spl rho//sub i/=F/sub i/(/spl theta//sub i/) in local polar coordinates which are then approximated by closed cubic B-splines instead of trigonometric series. By using the boundary condition of vanishing tangential electric field on surfaces of perfectly conducting cylinders, a set of electric field integral equations governing the scattering problem is derived. The scattering problem is solved by a point-matching method with pulse basis and Dirac delta testing functions. The inverse problem is reformulated as an optimization problem and solved by a real-coded genetic algorithm with closed cubic B-splines local shape function. Numerical examples show good agreement between the true profiles and the reconstructed results.     

基于迭代多尺度深度网络的非线性逆散射成像方法

传统的迭代多尺度方法 (iterative multiscaling approach, IMA)在求解非线性电磁场逆散射问题时,可以自适应提高成像空间的分辨率,缓解逆问题的病态性,但容易陷入局部极小值且无法做到实时重构.文中提出了一种迭代多尺度深度网络,该网络结合传统IMA和深度网络的优势,将IMA展开成深度网络模型（命名为IMA-Net）.该方法迭代地执行一种感兴趣区域(regions of interest, ROI)提取算法,在不同尺度的ROI内构建目标函数,并将目标函数分解成若干个优化子问题,子问题的迭代更新过程映射到深度网络结构中,交替更新相关分量,求解出目标函数的最优值.实验结果验证了该方法的有效性和优越性,为目标实时重构提供了一个有效方案.     

Full-vector waveguide modeling using an iterative finite-differencemethod with transparent boundary conditions

We describe a finite difference solution technique for the full-vector waveguide equation based upon the alternating-direction-implicit (ADI) iterative method. Our technique accurately treats dielectric boundaries (including corners), requires minimal computer resources, and executes faster (by factors of 3-10) than other iterative approaches. In addition, we employ a transparent boundary condition that effectively removes the sensitivity of the calculated results to the size of the computational domain. This feature greatly facilitates the examination of modes near cutoff     

Electromagnetic scattering by a cylinder with an inhomogeneous sheath

This letter deals with a perfectly conducting infinitely long circular cylinder coated with a radially stratified sheath. Scattering of normally incident plane electromagnetic waves is considered. A certain profile of permittivity in the sheath is taken, leading to particularly simple results.     

Electromagnetic scattering by an inhomogeneous penetrable material with an embedded resistive sheet

This paper presents the method of moments (mom) formulation for the electromagnetic scattering by an inhomogeneous penetrable material with an embedded resistive sheet. Triangular surface facets and tetrahedral volume cells are used to discretize the scatterer allowing for greater flexibility in the geometric modeling of the material body. The formulation is very general in that it allows for a variety of material configurations : open or closed conducting surfaces, open or closed resistive (thin dielectric) surfaces, solid dielectric/ferrite material volumes, embedded conducting/resistive surfaces in material volumes, and partially embedded conducting/resistive surfaces (cladded materials). Results for a material coated resistive spherical shell and a material propeller blade are presented.     

LaAlO3SrTiO3界面导电性的近场成像

铝酸镧/钛酸锶(LaALO3/SrTiO3)异质界面被发现具有二维导电性并呈现出其他一些有趣的呈展现象.而此前, LaAlO3/SrTiO3异质界面的导电性主要都是通过电学方法进行表征.这里作者首次提出利用散射式扫描近场光学显微镜 (s-SNOM) 对LaAlO3/SrTiO3界面的导电性进行空间成像, 这为研究过渡金属氧化物异质界面体系的物理现象提供了一个新的手段.     

Removal of ring artifacts in micro-CT imaging using iterative morphological filters

In this paper, a novel technique based on morphological filtering is proposed for the detection and removal of ring artifacts with different patterns, e.g., sharp isolated rings with varying intensity, isolated light rings, and band rings, from the μ- and/or C-Arm CT images. To optimize the performance of our ring removal algorithm, three different types of iterative morphological filters (IMFs) are proposed here for different ring patterns. All the IMFs detect the positions of the defective detector elements using the mean curve constructed from the corrupted sinogram. However, the basic principle of sinogram correction differs. For the elimination of sharp intense rings, the IMF is directly applied on the raw projection data for each angle of view, whereas in other cases an estimate of the ring-noise-free mean curve is first obtained to remove the ring generating artifacts from the sinogram data through a normalization process. The effectiveness of the proposed IMFs is tested for a variety of μ-CT acquired images. Experimental results show that the IMFs perform better than the previously reported techniques in the removal of ring artifacts from real μ-CT images.     

Electromagnetic induction in thin sheet conductivity anomalies at the surface of the earth

Lateral variations in the Earth's conductivity complicate considerably the calculation of the electromagnetic response of the Earth to an external inducing field which is uniform and horizontal. Although analytic solutions have been found for a few simple two-dimensional models in which the conductivity varies in one horizontal direction only, it is necessary, in general, to resort to numerical methods. If the conductivity variations of interest are confined to a surface layer it is often possible to represent the Earth mathematically as a uniform conducting half-space covered by an infinitely thin sheet of variable surface conductance. This simplification effectively reduces by one the number of dimensions over which the field equations need to be integrated numerically. It is shown that for a two-dimensional model the horizontal component of the electric field satisfies an integral equation on the surface of the thin sheet, which can be solved numerically for arbitrary sheet conductance. The accuracy of the numerical procedure is confined by applying it to E- and B-polarization induction in two adjacent half-sheets and then comparing the solution obtained with known analytic solutions of the same problem. In three-dimensions the two horizontal components of the surface electric field satisfy a coupled pair of double integral equations which can also be solved numerically for an arbitrarily varying conductance of the surface sheet.     

A Double-sided Printed Dipole Array with an Electromagnetic Band-Gap Reflector

Electromagnetic band-gap (EBG) structures have unique properties in controlling the propagation of electromagnetic wave and have been applied to a wide range of electromagnetic devices design. In this paper, a double-sided printed dipole (DSPD) array backed by an EBG reflector is proposed for achieving a low-profile design as well as gain enhancement. Simulation results show that a reduction of more than 55% in antenna height can be obtained by placing the DSPD array over an EBG reflector rather than a perfect electric conductor (PEC) reflector. And the obtained gain of the antenna with an EBG reflector is about 1.9 dB higher than that with a PEC reflector at the operating frequency 2.77 GHz. The EBG reflector can be utilized to reduce a cavity-backed antenna height and enhance the antenna radiation efficiency. The design has a good potential application to antenna arrays with more elements in wireless communication.     

Electromagnetic radiation from a slotted-cylinder antenna with an inhomogeneous sheath

This letter deals with a perfectly conducting infinitely long circular cylinder coated with a radially stratified sheath. Radiation of electromagnetic waves from an infinitely long axial slot is considered. A certain profile of permittivity in the sheath is taken, leading to a particularly simple result.     

基于曲面片迭代物理光学法的电磁耦合计算

将基于平面片的迭代物理光学(IPO)方法推广到曲面片上,发展了基于曲面片的迭代物理光学(CIPO)方法。利用CIPO方法计算分析了薄层天线罩和罩内散射体之间的电磁耦合作用。研究表明:同基于三角面片的IPO方法相比,CIPO方法对于表面曲率较大的目标可获得更高的计算速度和精度。     

A three-dimensional iterative scheme for an electromagneticinductive applicator

An efficient iterative method for solving quasi-static electromagnetic field problems is presented. The electromagnetic field is generated by an inductive applicator and is represented as a superposition of two constituents, viz. a primary field in absence of the tissue configuration and a secondary field generated by the presence of the tissue. Then, for the secondary field a quasi-static approximation is employed. In the quasi-static field equations a relaxation function is introduced, such that the resulting equations can be solved iteratively. For a realistic three-dimensional model of a human hand numerical results are presented.