a simulation of electromagnetic field exposure to …...8th south-east asian congress of medical...

8th South-East Asian Congress of Medical Physics, Biophysics & Medical Engineering, Des 10-13, Bandung, Indonesia

A Simulation of Electromagnetic Field Exposure to Biological Tissues

Warindi1, Hamzah Berahim2, Suharyanto2, Sasongko Pramono Hadi2

1Department of Electrical Engineering, Faculty of Engineering, University of Mataram, Indonesia

2Department of Electrical Engineering and Information Technology, Faculty of Engineering, Gadjah Mada University, Yogyakarta, Indonesia

Abstract - The success application of electromagnetic based medical equipment is determined by mechanism and properties of the exposure and object. In this simulation, biological tissues are modeled as conductive medium. A Finite Element Method (FEM) is used as a tool to solve the models. Two assumptions are made, firstly the tissue is assumed as homogenous and the second as heterogeneous media. The results are graphical presentations shown the intensity of the electromagnetic field for both assumptions. From the experiment it is shown that the density of the field is smaller in the less conductive tissue.

Keywords - electromagnetic, biological tissues, FEM

I. INTRODUCTION

Exposure of electromagnetic to human body may occur unintentionally or intentionally. The unintended one comes from foreign electromagnetic source as side effect (e.g. radiations of high voltage transmission line, mobile phone, electrical machine etc). [1]. some study in this category have been widely presented, e.g. investigation the electric field intensity of the power line [2] [3] and a mobile phone operation [4]. In the other hand, electromagnetic field effects have established and developing uses in medicine. In medicine, electromagnetic can be categorized as nonionizing radiation (e.g. MRI, electroporation [5], [6] defibrillation [7], etc) and ionizing radiation (e.g. X rays, CT [8]).

The determination of the tissue composition is based on the fact that the electrical characteristics of the human tissue changes according to the relative amounts of fluid and tissues. Materials, such as blood or muscle, have a higher conductivity in comparison to bones or fat and a lung filled with air has a lower conductivity than a water filled one respectively. Obviously the tissue is a heterogeneous matter [9].

The success medical application is determined by the mechanism and properties of the exposure and the object [10], [11]. Because real measurement may not possible in the real condition therefore a simulation is needed. This paper presents a simulation study of exposing electromagnetic wave to biological tissues with emphasis on the effect of tissue composition (homogen or heterogen). It is done by exploitation of the finite element method to model exposing electromagnetic wave.

II. MATERIAL AND METHODS

A. BIOLOGICAL TISSUE MODEL

The general strategy is to simulate as simple a model of defibrillation as possible while still capturing the basic physics. In particular, we seek the effect of different assumption by comparing two approaches. The model could be made more realistic in many ways, but will make the solutions more complex.

In this simulation, the biological tissues are modeled as conductive medium (electrolyte). The electrical properties of the modeled tissues refer to conductivity σ in the reference paper [9], e.g. body fluid is 1.5 S/m, muscle is 0.34, Fat is 0.02424 S/m and Heart 0.1954 S/m.

Firstly the tissue is assumed as homogenous and the second as heterogeneous media. Model used by [9] and [10] which mainly an electronic equivalent of the biological system.

In first assumption, the homogenous tissue is assumed as conductivity of body fluid of 1.5 S/m. In the heterogeneous media uses combination of two tissues e.g. muscle with body fluid in its surrounding that has conductivity of 0.3 and 1.5 S/m respectively. The biological tissue is shaped as a square shape (body fluid) and a circle in the middle of it (muscle).

B. ELECTRODES

Real application such as defibrillation mostly uses circular shape [9]. The electromagnetic field emit from electrode. Two cylindrical electrodes with a distance and diameter of 10 cm and 2 cm are used. The voltage difference between electrodes is 1000 V.

Circle electrodes are used in the first and second experiments. For the third experiment, it uses square shape electrodes of (area of 2 x 2 cm) to investigate the effect of different electrodes shape to the electromagnetic intensity in the biological tissue.


C. FORMULATION

The simulation is conducted using numerical experiments program MATLAB finite element method (FEM). It refers to [2] and [12] the simulation methodology can be formulized as follows: a biological tissue with conductivity σ and a steady current is established. The current density J is related to the electric field E through:

J = σE (1)

Combining the continuity equation:

∇∙J = Q (2)

Where Q is a current source, with the definition of the electric potential V yields the elliptic Poisson's equation:

-∇·(σ∇V) = Q (3)

The partial derivative equation parameters are the conductivity σ and the current source Q. The Dirichlet boundary condition assigns values of the electric potential V to the boundaries. The Neumann boundary condition requires the value of the normal component of the current density (n (σ ∇(V))) to be known. It is also possible to specify a generalized Neumann condition defined by

n (σ ∇V) + qV = g (4) Where q can be interpreted as a film conductance for thin plates.

The electric potential V, the electric field, and the current density are all available for plotting. Other quantities to visualize are the current lines (the vector field of) and the equipotential lines of V. The equipotential lines are orthogonal to the current lines when σ is isotropic. Two circular metallic conductors are placed on a plane. The physical model for this problem consists of the Laplace equation

-∇·(σ∇V) = 0 (5)

For the electric potential V, the natural Neumann boundary condition on the outer boundaries is

0=∂∂

n

V (6)

Presented investigations were orientated at establishing the conditions, in which the determination of tissue current can be replaced by numerical calculations of two dimensional (2D) homogeneous model of biological tissue, accurately enough to evaluate the occupational exposure according to numerical simulations results.

Geometrical and electrical parameters (high, radius and electric conductivity corresponding to tissue) were appointed for the homogeneous and heterogeneous models of the tissues. This procedure permits simulation of ‘inside’ tissue

electromagnetic exposition. The calculations took into account the electrodes shape variation.

D. RESULTS

The results are graphical presentations that show the intensity of the electromagnetic field for both assumptions. The result for the first experiment is shown in the figure 1. It is an electric field distribution with equipotential shown as line curve. The left electrode is positive side and the right side is negative voltage.

Fig.1. The potential (contour lines) and electric field (arrows) distributions in and around the circular tissue during an exposing in a of a homogeneous media

With the same data except the assumption, the result for the second experiment is shown in the figure 2. The media is heterogeneous with conductivity of 1.5 S/m for the outer media as the majority portion and 0.3 S/m for the inner media as shown a circular media in the centre.


Fig.2. The potential (contour lines) and electric field (arrows) distributions in and around the circular tissue during an exposing in a heterogeneous media

By changing the shape of electrode to a square shape and with the other data remaining the same as the second experimentation, the result is presented in the figure 3.

Fig. 3. The potential (contour lines) and electric field (arrows) distributions in and around the circular tissue during an exposing of a heterogeneous media with square shape electrodes

The comparison of current distribution is shown in figure 4.

Fig. 4. The current counter lines distributions in and around the circular tissue during an exposing of a homogeneous media with circular shape electrodes

Fig. 5. The current counter lines distributions in and around the circular tissue during an exposing of a homogeneous media with circular shape electrodes

The summary of the experiments is presented in the table 1. Three parameters are shown to explain the comparisons i.e. electric field, potential, and current density. In the first line, it


shows only maximum electrical potential at the object. This occurs at circle which closes to the electromagnetic source. Second line shows the current density in the middle of the object. At the end this shows maximum electric field.

Table 1. Summary of the results

Parameter Exp. 1 Exp. 2 Exp. 3

Max Electrical potential, V (Volt) 400 610 645

Current Density at centre, J (A/cm) 280 85 90

Max. Electric Field, V/cm 221 185 232

E. DISCUSSIONS

First experiment (fig. 1) shows that using homogeneous approach gives linear distribution of the electric field. The object does not affect to the distribution of the field.

From the second experiment (fig. 2) using heterogeneous media assumption, the electric field potential is changing when touch the object because of different conductivity. It can be seen that the electric potential become higher, it means the impedance in the object is higher or the conductivity is smaller.

The total corresponding current density through object becomes smaller (table 1). The results of simulation indicate that assumption of biological tissue model with homogeneous structure and its electrical parameters, may not sufficient for estimation of current that flow through it. The heterogeneous approach is better for modeling the real condition.

From the third experiment (fig 3) it is shown that the density of the electric field is merely the same as second experiment but different direction in same part. The flat portion of electrodes that faces the object influences the concentration (directing) of the electric field.

The results of numerical experimentation have showed that use homogenous models have higher result that it should be. The heterogeneous model can be helpful to evaluate tissue exposure in the electric field and for supporting medical applications.

F. CONCLUSIONS

The electromagnetic exposure to biological tissues is depending on tissue medium properties. The results of computer calculations indicate that assumption of biological tissue model with homogeneous structure and parameters, may not sufficient for estimation of current that flow through it. The results of numerical calculation have showed that use heterogeneous models can be helpful to evaluate tissue

exposure in the electric field and for managing of safety and effectiveness of electro medical applications.

REFERENCES

1. Raines J K (1981) Electromagnetic field interactions With the human body: observed effects and theories. Greenbelt, Maryland

2. Hoang L H, Scoretti R, Burais N, & Voyer D, (2009) Numerical dosimetry of induced phenomena in the human body by three phase power line. IEEE Trans.Magnetics Vol 45,No. 3

3. Yuan Shangzun, Li Pengfei, Nie Ling (2008) Study on electromagnetic radiation of ultra-high voltage power transmission line. IEEE Int’l Conf Comp Sci & Inf Tech 978-0-7695-3308-7/08 DOI 10.1109/ICCSIT.2008.92

4. Boonwutiwiwat R, kulworawanichpong T, & Pao-La-Or P (2007) Electric Field Distribution Resulting from a Mobile Phone Human Interfacing With an Overhead Power Transmission Line. Proc. WSEAS Conf Power Systems, Beijing, China.

5. Krassowska W & Filev P D (2007) Modeling Electroporation in a Single Cell. Biophysical J 92: pp 404–417

6. Ellappan P, Sundararajan R (2005) A simulation study of the electrical model of a biological cell. J Electrostatics Vol 63: 297–307

7. Smaill B H, LeGrice I J, Hooks D A, Pullan A J, Caldwell B J & Hunter P J (2004) Cardiac structure and electrical activation: Models and measurement. Proc. Australian Phys & Phar Soc. 34: 141-149

8. Song X, Bai J (2008) An Improved Finite-element based Reconstruction Algorithm for Fluorescence Tomography. IEEE Congress on Image and Signal Processing 978-0-7695-3119-9/08 DOI 10.1109/CISP.2008.582

9. Beckmann L, Riesen D, Leonhardt S (2007) Optimal electrode placement and frequency range selection for the detection of lung water using Bioimpedance Spectroscopy. Proc. 29th IEEE Int’l Conf. EMBS Cité Internationale, Lyon, France, FrB06.4

10. Muzdeka S and Barbieri E (2005) Control theory inspired considerations of the mathematical models of defibrillation. Proc 44th IEEE Conf on Decision & Control, European Control Conf Seville, Spain, ThB13.4

11. Lai D, Cao T, Wu X, and Fang Z (2007) Optimization of Electrodes Displacement for Transthoracic Defibrillation: A Simulation Study on Spatial Configuration Pattern. Int’l J Bioelectromagnetism : 9, 1

12. Matworks (2009) Partial Diferential Equation Toolbox. Matwork, Inc. Address of the corresponding author:

Author: Warindi Institute: University of Mataram Street: Jl. Majapahit 62 City: Mataram Country: Indonesia Email: [email protected]

a simulation of electromagnetic field exposure to …...8th south-east asian congress of medical...

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