Chun Yu

Through-wall imaging (TWI) by radar: 2-D tomographic results and analyses

A two-dimensional nonlinear inverse scattering technique is developed for imaging objects in a multilayered medium that simulates the effects of building walls in the context of through-wall imaging (TWI). The effectiveness and capacity of the inversion algorithm and the feasibility of through-wall imaging is demonstrated via a number of numerical examples. It has been shown that using multifrequency data high-quality image reconstruction can be achieved with a limited array view.

Active Microwave Imaging II: 3-D System Prototype and Image Reconstruction From Experimental Data

A 3-D microwave imaging system prototype and an inverse scattering algorithm are developed to demonstrate the feasibility of 3-D microwave imaging for medical applications such as breast cancer detection with measured data. In this experimental prototype, the transmitting and receiving antennas are placed in a rectangular tub containing a fluid. The microwave scattering data are acquired by mechanically scanning a single transmit antenna and a single receive antenna, thus avoiding the mutual coupling that occurs when an array is used. Careful design and construction of the system has yielded accurate measurements of scattered fields so that even the weak scattered signals at S21 = -90 dB (or 30 dB below the background fields) can be measured accurately. Measurements are performed in the frequency domain at several discrete frequencies. The collected 3-D experimental data in fluid are processed by a 3-D nonlinear inverse scattering algorithm to unravel the complicated multiple scattering effects and produce high-resolution 3-D digital images of the dielectric constant and conductivity of the imaging domain. Dielectric objects as small as 5 mm in size have been imaged effectively at 1.74 GHz.

Reconstruction of 3D objects from multi-frequency experimental data with a fast DBIM-BCGS method

The objective of this work is to perform image reconstruction of 3D dielectric targets from multi-frequency experimental data by using a fast DBIM-BCGS method that combines the distorted Born iterative method (DBIM) and the stabilized biconjugate-gradient fast Fourier transform (BCGS-FFT) method. In this reconstruction technique, the BCGS-FFT method is used as a forward scattering method for solving the volume integral equations governing the 3D scattering problem; it provides both the predicted scattered fields due to 3D heterogeneous objects and the Frechet derivatives in the inverse scattering problem. The plane-wave source model and the point receiver model are used in the inversion procedure to invert the calibrated scattering data obtained from Institut Fresnels measurements. The multi-frequency experimental data are processed with the frequency-hopping approach to obtain high-resolution 3D images. The reconstruction of five different targets from the measured scattered fields verifies the capability and the effectiveness of the DBIM-BCGS method.

Microwave Imaging in Layered Media: 3-D Image Reconstruction From Experimental Data

A prototype microwave imaging system for imaging 3-D targets in layered media is developed to validate the capability of microwave imaging with experimental data and with 3-D nonlinear inverse scattering algorithms. In this experimental prototype, the transmitting and receiving antennas are placed in a rectangular tub containing a fluid. Two plastic slabs are placed in parallel in the fluid to form a five-layer medium. The microwave scattering data are acquired by mechanically scanning a single transmitting antenna and a single receiving antenna, thus avoiding the mutual coupling that occurs when an array is used. The collected 3-D experimental data in the fluid are processed by full 3-D nonlinear inverse scattering algorithms to unravel the complicated multiple scattering effects and produce 3-D digital images of the dielectric constant and conductivity of the imaging domain. The image reconstruction is focused on the position and dimensions of the unknown scatterers. Different dielectric and metallic objects have been imaged effectively at 1.64 GHz.

Inversion of multi-frequency experimental data for imaging complex objects by a DTA?CSI method

This paper deals with image reconstruction from measured multi-frequency scattering data for 2D inhomogeneous targets by using a technique combining the diagonal tensor approximation (DTA) and contrast source inversion (CSI) methods. In this algorithm, the diagonal tensor approximation is applied as a nonlinear approximation of the 2D domain integral equation for an inverse scattering problem, and is used as an initial guess for the nonlinear contrast source inversion. Taking advantage of the properties of fast convergence of the DTA operator and stable convergence of the CSI method, the combined technique makes image reconstructions more effective. Using this combined approach, the multi-frequency experimental data from Institut Fresnel have been processed, and excellent reconstruction results have been obtained for inhomogeneous targets.

Three-dimensional electromagnetic nonlinear inversion in layered media by a hybrid diagonal tensor approximation: Stabilized biconjugate gradient fast Fourier transform method

This paper presents an efficient three-dimensional nonlinear electromagnetic inversion method in a multilayered medium for radar applications where the object size is comparable to the wavelength. In the first step of this two-step inversion algorithm, the diagonal tensor approximation is used in the Born iterative method. The solution of this approximate inversion is used as an initial guess for the second step in which further inversion is carried out using a distorted Born iterative method. Since the aim of the second step is to improve the accuracy of the inversion, a full-wave solver, the stabilized biconjugate-gradient fast Fourier transform algorithm, is used for forward modelling. The conjugate-gradient method is applied at each inversion iteration to minimize the functional cost. The usage of an iterative solver based on the FFT algorithm and the developed recursive matrix method combined with an interpolation technique to evaluate the layered medium Greens functions rapidly, makes this method highly efficient. An inversion problem with 32 768 complex unknowns can be solved with 1% relative error by using a simple personal computer. Several numerical experiments for arbitrarily located source and receiver arrays are presented to show the high efficiency and accuracy of the proposed method.

Experimental and numerical investigations of a High-resolution 3D microwave imaging system for breast cancer detection

In this paper, we have developed a prototype 3D microwave imaging system for breast cancer diagnosis and screening. This presentation will give an overview of the hardware development, forward electromagnetic wave propagation models, nonlinear inverse scattering algorithms for this 3D microwave imaging system. This high sensitivity enables the 3D system to collect accurate scattering data from small phantom tumors.

2-D nonlinear image reconstruction for objects buried in layered media

In this paper, a contrast source inversion (CSI)-based nonlinear inversion method is developed for imaging reconstruction of buried objects in 2D layered media. First, the CSI method and relevant integral equation formulation is briefly described. Then some examples of the 2D inversion for objects buried in a multilayered medium are presented to illustrate the effectiveness of the method.

Reconstruction of 3-D dielectric objects from measured data

Electromagnetic inverse scattering has widespread applications in biomedical imaging, nondestructive evaluation, and subsurface sensing. However, in general the inverse scattering problem is nonlinear, ill-posed, and computationally demanding. This is especially true for three-dimensional inverse scattering problems, where there are few demonstrated inverse scattering methods. Furthermore, in many inverse scattering methods, the forward scattering methods are necessary, so the efficiency of forward scattering is also an important issue.

Image reconstruction from measured scattering data

We present our recent work on the image reconstruction of two dimensional scattering objects using the layered-medium contrast source inversion (CSI) method from the measured scattering data collected in June 2004 from Institut Fresnel. We focus on the TM case. Even though the Fresnel data sets were collected from the EM chamber that simulates a homogeneous background, it provided real data for validation test of our layered-medium contrast source inversion algorithm and procedure (when degenerating to a one layer medium).