Mehmet Cayoren

Transmission eigenvalues and the nondestructive testing of dielectrics

We show how transmission eigenvalues can be determined from electromagnetic scattering data and used to determine the presence of cavities in a dielectric.

Qualitative Microwave Imaging With Scattering Parameters Measurements

Microwave imaging (MWI) systems extensively employ vector network analyzers for microwave measurements due to their high availability and accuracy. This is in contrast to theoretical models, which are naturally formulated in terms of scattered electric field vectors. Accordingly, experimental verification of MWI methods requires an intermediate step where measured scattering parameters are converted to scattered electric fields. In parallel to recent research, which formulates the Born iterative method in terms of scattering parameters, we develop formulations of two closely related qualitative inverse scattering methods-the linear sampling method and the factorization method-directly in terms of scattering parameters to avoid the intermediate conversion step. To this aim, we introduce vector S-parameters and we extend the vector Greens function for S-parameters to the dyadic case. There are certain advantages of these formulations over their electric field counterparts. First of all, the resulting formulations inherently incorporate the antenna radiation characteristics. Moreover, they reduce the measurement time since they do not require any pre- or post-measurement process. We experimentally verified the presented novel formulations against multi-frequency measurements performed inside an anechoic chamber. Obtained results show that the proposed methodologies can accurately reconstruct the shape of the targets by directly exploiting multifrequency measurements in the imaging process.

Shape Reconstruction of Perfectly Conducting Targets From Single-Frequency Multiview Data

In this letter, we address the problem of reconstructing the shape of a perfectly conducting object illuminated by a set of plane waves at a fixed frequency. The proposed method is made up of two parts. In the first one, for each incident wave, the field in the vicinity of a scatterer is reconstructed by means of a regularized single-layer potential approach. In the second part, the reconstructed fields are simultaneously exploited to build a system of polynomial equations whose solution yields to the unknown contour. As shown by numerical examples, the method is effective, robust against noise on data, and provides satisfactory reconstructions for star-shaped scatterers.

Surface impedance based microwave imaging method for breast cancer screening: contrast-enhanced scenario

A new microwave imaging method that uses microwave contrast agents is presented for the detection and localization of breast tumours. The method is based on the reconstruction of breast surface impedance through a measured scattered field. The surface impedance modelling allows for representing the electrical properties of the breasts in terms of impedance boundary conditions, which enable us to map the inner structure of the breasts into surface impedance functions. Later a simple quantitative method is proposed to screen breasts against malignant tumours where the detection procedure is based on weighted cross correlations among impedance functions. Numerical results demonstrate that the method is capable of detecting small malignancies and provides reasonable localization.

Microwave subsurface imaging of buried objects under a rough air–soil interface

We consider subsurface imaging of buried objects under a rough air–soil interface and present a microwave imaging method that is capable of determining the geometrical properties of multiple objects without requiring any a-priori information on the objects. The theoretical background of the method relies on factorization of scattering operators and the locations of buried objects are qualitatively determined from limited aperture near-field measurements performed with a short antenna array moving over the investigated region. The numerical results demonstrate that the method can handle very rough interfaces, provided that they are exactly known, and accurately determine the locations of buried obstacles even while the measurements are still in progress, which in turn opens up the possibility of real-time operation.

Experimental Assessment of Linear Sampling and Factorization Methods for Microwave Imaging of Concealed Targets

Shape reconstruction methods are particularly well suited for imaging of concealed targets. Yet, these methods are rarely employed in real nondestructive testing applications, since they generally require the electrical parameters of outer object as a priori knowledge. In this regard, we propose an approach to relieve two well known shape reconstruction algorithms, which are the linear sampling and the factorization methods, from the requirement of the a priori knowledge on electrical parameters of the surrounding medium. The idea behind this paper is that if a measurement of the reference medium (a medium which can approximate the material, except the inclusion) can be supplied to these methods, reconstructions with very high qualities can be obtained even when there is no information about the electrical parameters of the surrounding medium. Taking the advantage of this idea, we consider that it is possible to use shape reconstruction methods in buried object detection. To this end, we perform several experiments inside an anechoic chamber to verify the approach against real measurements. Accuracy and stability of the obtained results show that both the linear sampling and the factorization methods can be quite useful for various buried obstacle imaging problems.

Reconstruction of surface impedance of an object located over a planar PEC surface

A method for the determination of inhomogeneous surface impedance of an arbitrary shaped cylindrical object located over a perfectly conducting (PEC) plane is presented. The problem is reduced to the solution of an ill-posed integral equation by the use of single layer representation which is handled by Truncated Singular Value Decomposition (TSVD). The total field and its normal derivative on the boundary of the object which are required for the evaluation of the surface impedance are obtained through Nystrom method. The method can also be used in shape reconstruction by using the relation between the shape of a PEC object and its equivalent one in terms of the surface impedance. The numerical implementations yield quite satisfactory results.

Experimental Microwave Imaging With a Novel Corrugated Vivaldi Antenna

In this communication, a novel corrugated Vivaldi antenna (CVA) is developed for microwave imaging (MWI) applications. In particular, different from the previous works, the lengths of the corrugations are independently optimized to reach the final design. It is shown that by letting more parameters to be optimized in the design process, we can obtain a more effective CVA, which has more suitable characteristics for MWI applications (i.e., higher gain, broader bandwidth) compared with the previous designs. Apart from that, the imaging performance of the proposed design is compared with a generic VA having the same size of the proposed antenna. As imaging algorithm, a recently introduced qualitative MWI technique, the scattering parameter-based linear sampling method (S-LSM), is employed. The similarity between exact shapes of the targets and the obtained qualitative results are compared with the help of the well-known Jaccard index. Experimental results show that the proposed CVA performs better than a generic VA in such real-world MWI problems.

Microwave Imaging of a Slightly Varying 2-D Conducting Object Through Generalized Impedance Boundary Conditions

We consider reconstructing the shape of a perfect electric conducting object and developed a novel imaging method based on generalized impedance boundary conditions (GIBCs). The method relies on selecting a fictitious surface encircling the unknown object in which the surface impedance is reconstructed from the scattered field measurements. Later, the shape reconstruction problem is cast into an equivalent problem where the distance variation between the fictitious impedance surface and the unknown target is determined from the reconstructed surface impedance. Since the performance of the method depends on the selection of the impedance surface, we present a selection criterion according to the validity conditions of GIBCs. The method is capable of reconstructing both convex and concave structures whose shape deviates, at most, a tenth of the wavelength from the impedance surface, as demonstrated with numerical results.

Surface impedance modeling of PEC targets: application to shape reconstruction

We consider the inverse electromagnetic scattering problem of retrieving the shape of an inaccessible, perfectly electric conducting target from a set of far field measurements and develop a new reconstruction method in which the unknown scatterer is modeled by means of a surface impedance. The basic idea is to approximate the unknown object through a circular impedance cylinder enclosed in it and equipped with a suitable inhomogeneous surface impedance. By doing so, the ill-posedness and the nonlinearity of the underlying problem are then handled separately, through two successive processing steps. In the first step, the inhomogeneous surface impedance of the auxiliary scatterer is reconstructed through the regularized analytical continuation of the measured far-field data. Then, the unknown shape is retrieved through the solution of a nonlinear optimization problem aimed at determining the spatial locations where the field outside of the impedance target vanishes, in agreement with the boundary condition arising onto the unknown target. As demonstrated with several numerical results, the method is robust against uncertainties on data and provides quite accurate results for targets with starlike boundaries having both convex and concave parts.