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Title: A MICROWAVE IMAGING APPROACH BASED ON AMPLITUDE-ONLY DATA FOR THE RECONSTRUCTIN OF THE ELECTROMAGNETIC FIELD INDUCED IN BIOLOGICAL PHANTOMS
Abstract: In this paper, a microwave imaging technique for the reconstructin of the electromagnetic field distribution inside exposed biological bodies from amplitude-only data is presented. The scattering problem, formulated in terms of integral equations, is recast into an optimization problem after defining a suitable cost function which is proportional to the difference between reconstructed and measured amplitudes of the scattered and of the incident electric fields in the near-field region. The field solution is obtained by minimizing the cost function by means of an hybrid simulated annealing-conjugate gradient based procedure, which, in principle, ensures the convergence to the global minimum, preventing the solution from being trapped in local minima. Moreover, the proposed approach prevents the problems due to inaccuracies in near-field phase measurements. The effectiveness of the microwave imaging approach is assessed by means of some numerical expamples (with synthetic input data) concerning a realistic cross-section of a biological phantom exposed to a TM electromagnetic illumination. The presence of noise on synthetic data is also considered and the dependence of the reconstruction accuracy on the signal-to-noise ratio investigated. Finally, in order to give some indications to further improve the prediction capability of the proposed approach, some preliminary guidelines are pointed out.
Author(s): Salvatore Caorsi, Emanuela Bermani, Andrea Massa
File Type: Journal Paper
Issue:Volume: 16      Number: 2      Year: 2001
Download Link:Click here to download PDF     File Size: 1321 KB

Title: A GRADED-MESH FDTD CODE FOR THE STUDY OF HUMAN EXPOSURE TO CELLULAR PHONES EQUIPPED WITH HELICAL ANTENNAS
Abstract: A FDTD code employing a graded mesh has been developed in order to make the FDTD technique suitable for studying celular phones equipped with helical antennas. The graded-mesh FDTD allows the evaluation of both the radiating properties of the helical antenna, and the power absorption inside the user's head. First, a canonical case has been studied to find the optimal values for the grading factor "q" and the best model for approximating the circular wire section. It has been found that "q" values up to 3 can be used without introducing significant errors in the field solution, and that a good model for the wire circular section is a five-cell cross. Then, the free space radiating properties of a phone equipped with a helical antenna (radiation impedance, radiation pattern, etc) have been evaluated, and compared with those predicted by MoM, showing good agreement. Finally, the power absorption in an anatomical model of the human head has been computed for a radiated power of 250 mW at the frequency of 835 MHz. Peak SAR values of .98 W/kg averaged over 1 g and 0.63 W/kg averaged over 10 g have been obtained with the phone held in contact with the ear.
Author(s): P. Bernardi, M. Cavagnaro, S. Pisa, E. Piuzzi
File Type: Journal Paper
Issue:Volume: 16      Number: 2      Year: 2001
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Title: EVOLUTION OF A SUITE OF COMPUTATIONAL MODELING TOOLS FOR MOBILE TELEPHONE DOSIMETRY STUDIES AT THE UNIVERSITY OF BRADFORD, U.K.
Abstract: The evolution of a set of software and skills for the computational simulation of the interaction of mobile telephones with the human body is reviewed. These have been developed and applied in the authors' research group over the past decade. The work has aimed at maximization of realism in the representation of the body components and the telephone through use of MRI images, use of resolutions down to 1mm, incorporation of a hand into the model, development of a rotation algorithm for the telephone, development of techniques for modeling helical antennas, and development of hybrid techniques to allow the Method of Moments to be used to model the telephone, combined with finite difference methods for the human tissues. Generic experience and specimen results are presented. It is argued that the policy of pursuing realism in the simulation, beyond the level that is sufficient on physical grounds, is essential to convince the non-specialist public of the trustworthiness of the work.
Author(s): Peter S. Excell, John A. Vaul
File Type: Journal Paper
Issue:Volume: 16      Number: 2      Year: 2001
Download Link:Click here to download PDF     File Size: 1082 KB

Title: Numerical Simulation of Experimental Investigation of thermal Effects of RF-Fields on Patients During MRI
Abstract: The application of radio-frequency electromagnetic fields during Magnetic Resonance imaging causes electric currrents and dielectric losses in the biological tissues of the patient during Magnetic Resonance procedures. The power deposition inside the human body leaddes to tissues temperature elevation. This phenomenon of whole body heating is unavoidable and of growing concern, since advanced imaging sequences as well as the trend towards high-field Magnetic Resonance scanners, lead to increased power depostion inside the human body. Therefore it becomes more and more important to incorporate the thermal aspect in the process of RF coil design. This paper presents a numerical model based on the finite element method to predict the thermophysiological responses in human subjects during Magnetic Resonance Imaging . The simulation model includes the specific absorption rate distributation inside an anatomically shaped model of the human body, the bioheat-transfer-equation, and a mathematical model of human thermoregulation. The model is applied to a real imaging situation and validated experimentally by temperature measurements. The comparison between experimental and numerical results shows good agreement.
Author(s): Frank Gustrau, Helmut Ermert
File Type: Journal Paper
Issue:Volume: 16      Number: 2      Year: 2001
Download Link:Click here to download PDF     File Size: 802 KB

Title: Investigation into the Accuracy, Efficiency and Applicability of the Method of Moments as Numerical Dosimetry Tool for the Head and Hand of a Mobile Phone User
Abstract: In this paper the Method of Moments (MoM) as numerical SAR assessment tool is investigated. Models for generic mobile phones operational at 900MHz and 1800MHz were developed. Simulations were performed using these phone models to investigate the accuracy and efficiency with which the MoM can predict field values inside lossy, dielectric test phantoms. Results obtained are compared to Finite Difference Time Domain (FDTD) simulations using the MoM were further performed using the generic phones, a parametric hand model and the preliminary standardised IEEE head phantom. Peak-SAR results are extracted from the MoM solutions and compared for different phone and hand positions, as well as with FDTD results where applicable.
Author(s): FJC Meyer, KD Palmer, U Jakobus
File Type: Journal Paper
Issue:Volume: 16      Number: 2      Year: 2001
Download Link:Click here to download PDF     File Size: 1503 KB

Title: Lanczos-Based Model-Order Reduction for Optimization and Control of Electromagnetically Induced Hyperthermia
Abstract: In this work, two procedures for the optimization of transient temperature fields in electromagnetically induced therapeutic hyperthermia are proposed. Both procedures employ numerical models of electromagnetic and heat transfer processes. The computational demands of the optimization procedures are mitigated by employing reduced-order numerical models obtained via the spectral Lanczos decomposition method (SLDM) in lieu of the original, high-order models. An open-loop optimization procedure based on quadratic programming (QP) is proposed that determines the time dependent RF power level necessary to reach therapeutic temperatures quickly without exposing healthy tissue to excesive temperatures. Additionally, a closed-loop optimization procedure is proposed based on linear-quadratic Gaussian (LQG) optimal control that employs feedback from temperature measurements such as those available from magnetic resonance thermography. The performance of both techniques is simulated on a realistic tissue model of the human trunk heated by an annular phased array (APA). It is shown that by optimizing the transient temperature fields in oncological hyperthermia, effective thermal dose can be increased for a fixed treatment time and level of risk to healthy tissue. Additionally, it is shown that in some cases the non-linear nature of the human thermoregulatory response (manifest as temperature dependent perfusion) can be compensated for by the proposed linear feedback controller.
Author(s): Marc E. Kowalski, Jian-Ming Jin
File Type: Journal Paper
Issue:Volume: 16      Number: 2      Year: 2001
Download Link:Click here to download PDF     File Size: 1373 KB

Title: CALCULATIONS OF EM INTERACTIONS WITH BIOLOGICAL TISSUE: MAGNETIC RESONANCE IMAGING AT ULTRA HIGH FIELD
Abstract: A finite difference time domain model is developed to simulate a TEM resonator for high frequency clinical magnetic resonance imaging applications. The coil-head electromagnetic interactions and their effects on the SAR in biological tissue are presented. The TEM resonator and the newly developed 18-tissue anatomically detailed human head are treated together as a single system. This is done through all the step of the model including excitation and numerical tuning. The treatment of the resonator and the head as a single system and the geometrical modeling of all the coil components including coaxial rods, shield, circular rings, and the excitation source(s), accurately account for the electromagnetic interactions between the coil and the tissue. As a result, close agreement was achieved between the FDTD results and MRI images obtained at 8 Tesla. The results show strong electromagnetic interactions between the coil in general, the excitation source(s) in particular, and the human head. As such, it is shown that the specific absorption rate (SAR) values are largely dependent on these interactions. Comparisons of linear, 2-port quadrature, and 4-port excitations with respect to SAR values were performed at 340 MHz (8 Tesla). The analysis reveals the advantages of using 4-port drive over the conventional 2-port quadrature excitation especially in reducing the SAR peak values.
Author(s): Tamer S. Ibrahim, Robert Lee, Brian A. Baertlein
File Type: Journal Paper
Issue:Volume: 16      Number: 2      Year: 2001
Download Link:Click here to download PDF     File Size: 876 KB

Title: A Model of finite electrodes in layered biological media: hybrid image series and moment method scheme
Abstract: The low-frequency electromagnetic field interaction with layered biological tissue is investigated for electrode array excitation. The problem may be reduced into a system of P (number of electrodes) Fredholm integral equations of the first kind for the electrodes' current distribution. We have shown that the kernel (Green's function) of each integral equation can be expressed by image series. This leads to a most effective inversion of the integral equation system via the moment method, since the moment matrix elements can be expressed explicitly by image series. The outlined procedure is simple to implement and allows estimation of the distributions of low-frequency potential, current, field and power within the multilayer tissue. It may serve as a simplified first-order prototype model for realistic biomedical problems where the dependence on the number of electrodes, tissue layers and their electrical properties must be accounted for. The model has been utilized for the calculation of the electrode array impedance matrix, potential fields, intra-muscular current distributions and isometric recruitment curves (IRC). The simulation results indicate that the IRCs are insensitive to the electrodes' size, however, the inclusion of the bone/fascia layer significantly increases the IRC slope. Furthermore, the simulation scheme, which can be readily implemented for the classification, calibration, verification and interpretation of reported numerical and experimental biomedical data, is also applicable in other problem areas such as geophysical prospecting and electrode grounding in power systems.
Author(s): Leonid M. Livshitsz, Pinchas D. Einziger, Joseph Mizrahi
File Type: Journal Paper
Issue:Volume: 16      Number: 2      Year: 2001
Download Link:Click here to download PDF     File Size: 842 KB

Title: A Heterogeneous Hybrid Computational Electromagnetics Formulation Including Conduction Current Crossing the Domain Boundary
Abstract: A heterogeneous hybrid computational electromagnetics method is presented, which enables different parts of a problem space to be treated by different methods, thus enabling the most appropriate method to be used for each part. The method uses a standard frequency-domain Method of Moments program and a Finited-Difference Time-Domain program to compute the fields in the two regions. The interface between the two regions is a surface on which effective sources are defined by application of the Equivalence Principle. An extension to this permits conduction currents to cross the boundary between the different computational domains. Several validation cases are examined and the results compared with available data. The method is particularly suitable for simulation of the interaction of a mobile telephone with the human body in cases where the antenna has a complex shape and the chassis is in contact with the body tissue.
Author(s): M A Mangoud, R A Abd-Alkhameed, P S Excell
File Type: Journal Paper
Issue:Volume: 16      Number: 2      Year: 2001
Download Link:Click here to download PDF     File Size: 541 KB

Title: EVALUATION OF MODELING ACCURACY OF POWER FREQUENCY FIELD INTERACTION WITH THE HUMAN BODY
Abstract: Heterogeneous voxel models of the human body are used in numerical computations of induced electric fields in tissues. Various numerical methods are used for electric and magnetic field sources under quasi-static conditions. Validation of the computational methods and detailed estimation of errors associated with the modeling are given. Analytic solutions for spheres (homogeneous and layered) and other simple models are used to evaluate modeling accuracy. Sources of inherent errors associated with voxel models (staircase approximation of smooth surfaces) are identified, and methods are presented that decrease the errors.
Author(s): T. W. Dawson, M. Potter, M. A. Stuchly
File Type: Journal Paper
Issue:Volume: 16      Number: 2      Year: 2001
Download Link:Click here to download PDF     File Size: 927 KB

Title: CALCULATIONS OF ELECTRIC FIELDS IN THE HUMAN KNEE PRODUCED BY A KNEE-THIGH ELECTRODE PAIR
Abstract: Calculations of the electric fields produced in the cartilage of the human knee by a knee-thigh electrode pair excited by a low-frequency voltage waveform are presented and discussed. The simulations were performed using a three-dimensional finite-difference frequency-domain technique based upon an equivalent circuit model for solving Maxwell's equations. The leg model used in these calculation was obtained from the anatomically segmented human-body model of Gandhi. The calculated average electric fields within the medial and lateral cartilage were found to be 2.92 V/m and 2.51 V/m respectively for a voltage excitation of one volt at a frequency of 1 kHz applied between the electrodes. A special truncation technique, which incorporates information from a physically larger coarse model in the excitation of a finer subsection of the model, was utilized to obtain higher resolution results in the region of interest.
Author(s): D. N. Buechler, D. A. Christensen, C. H. Durney, B. J. Simon
File Type: Journal Paper
Issue:Volume: 16      Number: 2      Year: 2001
Download Link:Click here to download PDF     File Size: 803 KB

Title: NUMERICAL ANALYSIS OF THE "FERRIS WHEEL" MICE EXPOSURE SYSTEM USING AN EFFICIENT CYLINDRICAL FDTD SCHEME
Abstract: The objective of this study is to develop a numerical tool for characterizing the "ferris wheel" exposure system employed in a long-term study of RF exposure on mice [5]. The "ferris wheel" is a radial cavity loaded with forty mice around its perimeter. In order to exploit the angular periodicity of the radial exposure system, we developed a Finite Difference in Time Domain (FDTD) code based on a cylindrical grid to analyze a single angular sector of the 40-mouse "ferris wheel". The singularity of the Maxwell equations in the cylindrical FDTD scheme was appropriately removed, and other suitable expedients were implemented to reduce simulation time and memory requiremnts. In order to estimate RF leakage, the actual openings in the radial cavity, needed to insert the mice, were considered as well. The FDTD scheme incorporates the perfectly matched layer (PML) unsplit formulation (anisotropic medium), as absorbing boundary condition. The code was validated with measurements, showing good agreement. A criterion for assessing the uniformity of the exposure is presented, together with results at 900 MHz and 1.8 GHz.
Author(s): P. Russo, A. Faraone
File Type: Journal Paper
Issue:Volume: 16      Number: 2      Year: 2001
Download Link:Click here to download PDF     File Size: 1077 KB