Nikolaos K. Uzunoglu

Radiation properties of microstrip dipoles

The fundamental problem of printed antennas is addressed. The printed or microstrip dipole is considered, and its radiation characteristics are investigated. The Greens function to the problem is obtained in dyadic form by solving the problem of a Hertzian dipole printed on a grounded substrate. Input impedance computations are presented, and the numerical solution for the Sommerfeld integrals is discussed.

Scattering from an eccentrically stratified dielectric sphere

In this paper the scattering from eccentrically stratified spheres is considered. For the case of a spherical inhomogeneity embedded inside a dielectric sphere, the method of separation of variables is used in conjunction with translational addition theorems for spherical vector waves. Analytical results are obtained when the difference in the dielectric constants of the two spheres is small by employing a special perturbation technique. Scattering properties such as distortion of the scattering patterns, variation of total and backscattering cross sections, and depolarization for randomly oriented scatterers are investigated. Methods of detection and identification of inhomogeneities are discussed.

In silico radiation oncology: combining novel simulation algorithms with current visualization techniques

The concept of in silica radiation oncology is clarified in this paper. A brief literature review points out the principal domains in which experimental, mathematical, and three-dimensional (3-D) computer simulation models of tumor growth and response to radiation therapy have been developed. Two paradigms of 3-D simulation models developed by our research group are concisely presented. The first one refers to the in vitro development and radiation response of a tumor spheroid whereas the second one refers to the fractionated radiation response of a clinical tumor in vivo based on the patients imaging data. In each case, a description of the salient points of the corresponding algorithms and the visualization techniques used takes place. Specific applications of the models to experimental and clinical cases are described and the behavior of the models is two- and three-dimensionally visualized by using virtual reality techniques. Good qualitative agreement with experimental and clinical observations strengthens the applicability of the models to real situations. A protocol for further testing and adaptation is outlined. Therefore, an advanced integrated patient specific decision support and spatio-temporal treatment planning system is expected to emerge after the completion of the necessary experimental tests and clinical evaluation.

Reflective properties of double-ring resonator system coupled to a waveguide

The response of a pair of directly interacting ring resonators, side-coupled to a waveguide, is investigated analytically for the first time. Owing to the direct interresonator coupling, counterpropagating waves are excited in each resonator, resulting in a complex filter behavior that can be totally reflective or transmissive at certain resonant frequencies. Using the scattering matrix formalism, we show that a variety of single- to four-peak reflection profiles can be obtained by appropriate tuning of the involved coupling coefficients. The predictions are validated by means of a full-wave integral equation analysis.

Analysis of the interaction between a layered spherical human head model and a finite-length dipole

The coupling between a finite-length dipole antenna and a three-layer lossy dielectric sphere, representing a simplified model of the human head, is analyzed theoretically in this paper. The proposed technique is based on the theory of Greens functions in conjunction with the method of auxiliary sources (MAS). The Greens function of the three-layer sphere can be calculated as the response of this object to the excitation generated by an elementary dipole of unit dipole moment. The MAS is then applied to model the dipole antenna by distributing a set of auxiliary current sources on a virtual surface lying inside the antenna physical surface. By imposing appropriate boundary conditions at a finite number of points on the real surface of the antenna, the unknown auxiliary sources coefficients can be calculated and, hence, the electric field at any point in space can be easily obtained. Numerical results concerning the specific absorption rate inside the head, the total power absorbed by the head, the input impedance, and the radiation pattern of the antenna are presented for homogeneous and layered head models exposed to the near-field radiation of half-wavelength dipoles at 900 and 1710 MHz. The developed method can serve as a reliable platform for the assessment of purely numerical electromagnetic methods. The method can also provide an efficient tool for accurate testing and comparison of different antenna designs since generalizations required to treat more complex antenna configurations are straightforward.

Towards functional noninvasive imaging of excitable tissues inside the human body using focused microwave radiometry

Focused microwave radiometry, aiming mainly in clinical applications at measuring temperature distributions inside the human body, may provide the capability of detecting electrical conductivity variations at microwave frequencies of excitable cell clusters, such as in the case of brain tissues. A novel microwave radiometric system, including an ellipsoidal conductive wall cavity, which provides the required beamforming and focusing, is developed for the imaging of biological tissues via contactless measurements. The measurement is realized by placing the human head in the region of the first focus and collecting the radiation converged at the second by an almost isotropic dipole antenna connected to a sensitive radiometer operating at 3.5 GHz. In order to compute the focusing properties of the ellipsoidal reflector, an accurate electromagnetic numerical analysis is developed using a semianalytical method. The experimental part of this study focuses on measurements of activation of the primary somatosensory (SI) brain area, elicited during the application of the cold pressor test, a standard experimental condition inducing pain. Analysis of the measured data from 16 healthy subjects suggests that this methodology may be able to pick up activation of the SI during the pain conditions as compared with the nonpainful control conditions. Future research is needed in order to elucidate all the interacting factors involved in the interpretation of the presented results. Finally, potential limitations to the generalization of our results and strategies to improve the systems response are discussed.

A 3D high-resolution gamma camera for radiopharmaceutical studies with small animals

The results of studies conducted with a small field of view tomographic gamma camera based on a Position Sensitive Photomultiplier Tube are reported. The system has been used for the evaluation of radiopharmaceuticals in small animals. Phantom studies have shown a spatial resolution of 2mm in planar and 2-3mm in tomographic imaging. Imaging studies in mice have been carried out both in 2D and 3D. Conventional radiopharmaceuticals have been used and the results have been compared with images from a clinically used system.

Optimal steady-state temperature distribution for a phased array hyperthermia system

A method is presented for the evaluation of optimal amplitude and phase excitations for the radiating elements of a phased array hyperthermia system, in order to achieve desired steady-state temperature distributions inside and outside of malignant tissues. Use is made of a detailed electromagnetic and thermal model of the heated tissue in order to predict the steady-state temperature at any point in tissue. Optimal excitations are obtained by minimizing the squared error between desired and model predicted temperatures inside the tumor volume, subject to the constraint that temperatures do not exceed an upper bound outside the tumor. The penalty function technique is used to solve the constrained optimization problem. Sequential unconstrained minima are obtained by a modified Newton method. Numerical results for a four element phased array hyperthermia system are presented.<<ETX>>

Sleep spindle detection using artificial neural networks trained with filtered time-domain EEG: A feasibility study

An artificial neural network (ANN) based on the Multi-Layer Perceptron (MLP) architecture is used for detecting sleep spindles in band-pass filtered electroencephalograms (EEG), without feature extraction. Following optimum classification schemes, the sensitivity of the network ranges from 79.2% to 87.5%, while the false positive rate ranges from 3.8% to 15.5%. Furthermore, due to the operation of the ANN on time-domain EEG data, there is agreement with visual assessment concerning temporal resolution. Specifically, the total inter-spindle interval duration and the total duration of spindles are calculated with 99% and 92% accuracy, respectively. Therefore, the present method may be suitable for investigations of the dynamics among successive inter-spindle intervals, which could provide information on the role of spindles in the sleep process, and for studies of pharmacological effects on sleep structure, as revealed by the modification of total spindle duration.

Photonic microresonator research and applications

Fundamental Principles of Operation and Notes on Fabrication of Photonic Microresonators.- Circular Integrated Optical Microresonators: Analytical Methods and Computational Aspects.- Polarization Rotation in Ring Resonators.- Series-Coupled and Parallel-Coupled Add/Drop Filters and FSR Extension.- Advanced Microring Photonic Filter Design.- Band-Limited Microresonator Reflectors and Mirror Structures.- Slow and Stopped Light in Coupled Resonator Systems.- Processing Light in Reconfigurable Directly Coupled Ring Resonators.- Microresonators with Active Tuning.- Performance of Single and Coupled Microresonators in Photonic Switching Schemes.- Single Molecule Detection Using Optical Microcavities.- Microfiber and Microcoil Resonators and Resonant Sensors.- Photonic Crystal Ring Resonators and Ring Resonator Circuits.- High-Q Photonic Crystal Microcavities.- Radial Bragg Resonators.- Photonic Molecules and Spectral Engineering.- Fundamentals and Applications of Microsphere Resonator Circuits.- MEMS-Tuned Microresonators.- Microresonators for Communication and Signal Processing Applications.