Adnan Koksal

Determination of optimum injected current patterns in electrical impedance tomography

The problem of finding the optimum current under different constraints in electrical impedance tomography is cast into a non-linear optimization problem. Optimum currents are investigated for a two-dimensional cylindrical body with a concentric or an eccentric inhomogeneity under the constraints of constant dissipated power and constant total injected current. For a concentric inhomogeneity, it is shown that the opposite drive results in a better distinguishability than the cosine current pattern under the constant-injected-current constraint. The results for the concentric case are extended to the eccentric case directly using the properties of the conformal transformation and of the constraints involved. Distinguishability and the minimum detectable object size achieved by the optimized currents are compared with the ones achieved by the cosine current pattern for conductivity distributions with the concentric and eccentric inhomogeneity.

Distinguishability analysis of an induced current EIT system using discrete coils

The distinguishability of a discrete coil induced current electrical impedance tomography system is analysed. The solution methodology of the forward problem of this system is explained. An optimization procedure using this forward problem solution is developed to find optimum currents that maximize the distinguishability. For the concentric inhomogeneity problem, it is shown that the coil currents can be optimized to focus the current density in any desired location, in the field of view. Optimum coil currents under the constraints of limited peak coil currents and limited total power are determined. Examples that demonstrate the performance of the system are presented.

Mutual impedance of parallel and perpendicular coplanar surface monopoles

One dimensional integral formulas are derived for mutual impedance of arbitrary size, coplanar, parallel, and perpendicular surface monopoles. The integrals in formulas are expressed as exponential integrals where possible. The mutual impedance expression for parallel monopoles is a summation of exponential integrals and one-dimensional integrals. For perpendicular monopoles, the mutual impedance is in closed form, containing exponential integrals only. The final expressions are in a form suitable for numerical computation. Since the expressions contain at most one-dimensional integrals, they can be utilized to reduce the matrix filling time in the moment method formulations, especially when inhomogeneous sectioning is preferred. Additionally, they can be used in rectangular surface patch modeling of conducting surfaces with edges which are at an angle to the surface patches, providing the angle is small. To this end, the expressions were utilized in the moment method analysis of linearly tapered slot antennas. Very good accuracy was obtained with a reduction in computer time. >

A quasi-static analysis for a class of induced-current EIT systems using discrete coils

A discrete coil EIT system is investigated for the general case of an eccentric circular inhomogeneity. The solution methodology of the forward problem of this system is explained. An optimization procedure using this forward problem solution is developed to find optimum currents that maximize the distinguishability. For an eccentric inhomogeneity problem, it is shown that the coil currents can be optimized to focus the current density in a region of interest. Optimum coil currents under limited peak coil currents constraint and limited total power constraint are obtained. Representative examples that demonstrate the performance of the system are presented.

Shifted-frequency internal equivalence

The shifted frequency internal equivalence (SFIE) theorem involving inhomogeneous regions is introduced and proven. For a lossless inhomogeneous region using a vector Greens theorem and potential formulation, it is shown that the frequency-domain electromagnetic field at frequency /spl omega/ inside the region can be obtained using a set of equivalent volume and surface currents radiating in free space and at the different frequency /spl omega//sub 0/. The equivalent currents thus obtained are functions of the two frequencies, electric- and magnetic-volume-type sources of the original problem, material parameters, and the original field phasors at /spl omega/, and they only exist inside the region and on its boundary. A direct application of this equivalence is that it can be used to construct an internal equivalence at a shifted frequency for electromagnetic scattering problems if data are needed in a band of frequency. /spl omega//sub 0/ can be kept constant while the incident field frequency changes and, as a result, full computation of fields at each different frequency for volume-type equivalent sources can be avoided.

Wire antennas optimized using genetic algorithm

In this work, wire antennas are designed to jam GSM frequencies using genetic algorithms. These antennas are designed to block communication at 3-band GSM frequencies. They are planned to be mounted on a vehicle and therefore are modeled on a ground plane. Jammer antennas designed in this work are composed of wires, placed on two square dielectric frames, perpendicular to each other. Genetic optimization routines are developed on MATLAB environment for the designs carried out in this work. Electromagnetic simulation program called SuperNEC, which analyses antennas by the method of moments, is used to determine the antenna performances and is called by the developed GA routines. The purpose of the antenna optimization is to obtain low VSWR values and omni-directional radiation pattern near ground at @q=[70^o 80^o 90^o] planes at all GSM frequencies, since the targets will be near ground for this application. Original and interesting antenna designs are obtained as the result of genetic optimization, and are presented in the paper.

Distinguishability for magnetic resonance-electrical impedance tomography (MR-EIT).

A distinguishability measure is defined for magnetic resonance-electrical impedance tomography (MR-EIT) based on magnetic flux density measurements. This general definition is valid for 2D and 3D structures of any shape. As a specific case, a 2D cylindrical body with concentric inhomogeneity is considered and a bound of the distinguishability is analytically formulated. Distinguishabilities obtained with potential and magnetic flux density measurements are compared.

Multi-frequency electromagnetic scattering from inhomogeneous rectangular waveguides using shifted frequency equivalence

The shifted frequency internal equivalency (SFIE) principle is applied to multi-frequency electromagnetic scattering problem from a two-dimensional inhomogeneous rectangular waveguide. As is well known, the surface integral formulation is only applicable to homogeneous structures, and in the volume integral formulation it is necessary to repeat the solution process by generating data at each incident field frequency. The author shows that, the volume interaction part of the data may be generated only at a single frequency and be utilized at other frequencies over a wide frequency band. The use of the SFIE principle for one-dimensional problems have demonstrated earlier. In this article, its extension to two-dimensional structures is presented.

Multi-frequency electromagnetic scattering from inhomogeneous bodies using shifted frequency equivalence

The surface integral formulation of EM scattering from a homogeneous body involves using internal and external equivalencies. Imposing the continuity of the fields at the boundary renders the unknowns defined at the surface. For an inhomogeneous body, no such surface formulation is available yet, and furthermore one has to repeat at the same volume integral equation solution at each frequency of interest. In this work, the shifted frequency internal equivalence is reported which allows the usage of the same data in numerical calculations, at least over a frequency band. Utilization of this equivalency is demonstrated using a simple inhomogeneous dielectric/magnetic slab example.

Current Injection Optimization for Magnetic Resonance-Electrical Impedance Tomography (MREIT)

Determining optimum current injection pattern is of interest in magnetic resonance-electrical impedance tomography (MREIT), since it helps in detecting smaller inhomogeneities within the body when total injected current into the body is limited. Based on this fact, for 2-D cylindrical body with concentric and cylindrical inhomogeneity, current injection optimization problem is analytically formulated, based on distinguishability definition for MREIT. The exterior penalty method is used to solve the optimization problem. In the second step, the same problem is considered for eccentric and cylindrical inhomogeneity and the best current injection pattern is obtained.