Natalia K. Nikolova

Microwave Imaging for Breast Cancer

Microwaves and millimeter waves have been used extensively to image dielectric bodies. The application of microwaves in biomedical imaging and diagnostics, however, remains a field with many uncharted territories. This article is an overview on medical imaging using microwave imaging for breast cancer and its challenges, hopes, and outlook.

Near-Field Microwave Imaging Based on Aperture Raster Scanning With TEM Horn Antennas

The design, fabrication, and characterization of an ultrawideband (UWB) antenna for near-field microwave imaging of dielectric objects are presented together with the imaging setup. The focus is on an application in microwave breast tumor detection. The new antenna operates as a sensor with the following properties: 1) direct contact with the imaged body; 2) more than 90% of the microwave power is coupled directly into the tissue; 3) UWB performance; 4) excellent de-coupling from the outside environment; 5) small size; and 6) simple fabrication. The antenna characterization includes return loss, total efficiency, near-field directivity, fidelity, and group velocity. The near-field imaging setup employs planar aperture raster scanning. It consists of two antennas aligned along each others boresight and moving together to scan two parallel apertures. The imaged object lies between the two apertures. With a blind de-convolution algorithm, the images are de-blurred. Simulation and experimental results confirm the satisfactory performance of the antenna as an UWB sensor for near-field imaging.

Adjoint techniques for sensitivity analysis in high-frequency structure CAD

There is a revival of the interest in adjoint sensitivity analysis techniques. This is partly because current computer-aided-design software based on full-wave electromagnetic (EM) solvers remains too slow for the purposes of practical high-frequency structure design despite the increasing capacity of computers. The adjoint-variable methods for design sensitivity analysis offer computational speed and accuracy. They can be used for efficient gradient-based optimization, in tolerance and yield analysis. Adjoint-based sensitivity analysis for circuits has been well studied and extensively covered in the microwave literature. In comparison, sensitivities with full-wave analysis techniques have attracted little attention, and there have been few applications into feasible and versatile algorithms. We review adjoint-variable methods used in high-frequency structure design with both circuit analysis techniques and full-wave EM analysis techniques. A brief discussion on adjoint-based sensitivity analysis for nonlinear dynamic systems is also included.

Sensitivity analysis with the FDTD method on structured grids

We propose an adjoint-variable approach to design-sensitivity analysis with time-domain methods based on structured grids. Unlike conventional adjoint-based methods, it does not require analytical derivatives of the system matrices. It is simple to implement with existing computational algorithms such as the finite-difference time-domain (FDTD) technique. The resulting FDTD algorithm produces the response and its gradient in the design parameter space with two simulations regardless of the number of design parameters. The proposed method is validated by the adjoint-based FDTD analysis of waveguide structures with metallic boundaries.

Sensitivity analysis of scattering parameters with electromagnetic time-domain simulators

We propose an efficient adjoint-variable approach to the sensitivity analysis of S-parameters obtained from full-wave electromagnetic (EM) time-domain simulations. It allows the computation of the S-parameter derivatives with respect to the design variables with negligible overhead. No solutions of adjoint EM problems are needed. The computation is done as an independent post-process outside the solver. The sole requirement is the ability of the solver to export the field solution at user-defined points. Most commercial solvers have this ability, which makes our approach readily applicable to practical design problems. The approach is verified through the analysis of waveguide and antenna structures using commercial simulators.

An adjoint variable method for time-domain transmission-line modeling with fixed structured grids

We present a novel algorithm for efficient estimation of objective function sensitivities for time-domain transmission-line modeling (TLM) with nondispersive boundaries. The original electromagnetic structure is simulated using TLM. An adjoint TLM simulation that runs backward in time is derived and solved. The sensitivities of the objective function with respect to all designable parameters are estimated using only the original and adjoint simulations. Our approach is illustrated through the estimation of the sensitivities of objective functions with respect to the dimensions of waveguide discontinuities. A very good match is obtained between our sensitivity estimates and those obtained through the accurate and time-intensive central difference approximation.

TEM Horn Antenna for Ultra-Wide Band Microwave Breast Imaging

A novel TEM horn antenna placed in a solid dielectric medium is proposed for microwave imaging of the breast. The major design requirement is that the antenna couples the microwave energy into the tissue without being immersed itself in a coupling medium. The antenna achieves this requirement by: 1) directing all radiated power through its front aperture, and 2) blocking external electromagnetic interference by a carefully designed enclosure consisting of copper sheets and power absorbing sheets. In the whole ultra-wide band the antenna features: 1) good impedance match, 2) uniform field distribution at the antenna aperture, and 3) good coupling efficiency.

Antenna Optimization Through Space Mapping

We apply space mapping to antenna design for the first time. We exploit a coarse-mesh method of moments (MoM) solver as the coarse model and align it with the fine-mesh MoM solution through space mapping. We employ two plans: (I) implicit and output space mapping, and (II) input and output space mapping. We propose a local meshing method which avoids inconsistencies in the coarse model. The proposed techniques are implemented through our user-friendly space mapping framework (SMF) system. In a double annular ring antenna example, the S-parameter is optimized. The finite ground size effect for the MoM is efficiently solved by space mapping plan I and the design specification is satisfied after only three iterations. In a patch antenna example, we optimize the impedance using both plans in separate optimization processes. Comparisons are made. Coarseness in the coarse model and its effect on the space mapping performance are also discussed

A Study of Ultrawideband Antennas for Near-Field Imaging

A compact ultrawideband (UWB) antenna has been designed for UWB cancer detection systems. The numerical analysis and experimental characterization of a printed monopole antenna fed by a 50-Omega coplanar waveguide (CPW) and operating in a lossy coupling medium is presented. A detailed 3D model of the antenna and the biological environment has been constructed and simulated in XFDTD and HFSS. The near-field pattern of the antenna in the proximity of a multilayer human body model and the effect of the human body on the input matching of the antenna are studied. Experimental results show that the return loss is below -9.6 dB from 3.4 GHz to 9.9 GHz

Three-Dimensional Near-Field Microwave Holography Using Reflected and Transmitted Signals

A new 3-D holographic microwave imaging technique is proposed to reconstruct targets in the near-field range. It is based on the Fourier analysis of the wideband transmission and reflection signals recorded by two antennas scanning together along two rectangular parallel apertures on both sides of the inspected region. The complex scattering parameters of the two antennas are collected at several frequencies and then processed to obtain a representation of the 3-D target in terms of 2-D slice images at all desired range locations. No assumptions are made about the incident field and Greens function, which are derived either by simulation or by measurement. Furthermore, an approach is proposed to reduce the image artifacts along range. To validate the proposed technique, predetermined simulated targets are reconstructed. The effects of random noise, number of sampling frequencies, and dielectric contrast of the targets are also discussed.