Simulation of Electromagnetic Field (EM) Focusing

Capability on Biological Tissue through the

Application of C-type Excitation Coil Screen

Zulkarnay Zakaria, Noor Alia Mohd Zain, Azian

Azamimi Abdullah, Sofi Yahya

Biomedical Electronic Engineering Dept

School of Mechatronic Engineering

Universiti Malaysia Perlis

Arau, Perlis, Malaysia

zulkarnay@unimap.edu.my

Ruzairi Abdul Rahim, Muhammad Saiful Badri

Mansor, Abdul Rahim Mohd Disar Dept. of Control and Instrumentation Engineering

Faculty of Electrical Engineering

Universiti Teknologi Malaysia

Skudai, Johor, Malaysia

ruzairi@fke.utm.my

Abstract – In Magnetic Induction Tomography (MIT) or

Electromagnetic Therapy system, excitation coil is applied with

AC source in generating the electromagnetic (EM) field which

then propagate and penetrate the object located in the region of

interest (ROI). However, instead to the target object, the fields

also propagate around the coil and create interference to the

nearby circuit which contributes noise to the system while at the

same time wasting the energy. This paper is focusing on the use

of C-type excitation coil screen in focusing the EM field to the

ROI and measure the penetration depth when different

frequency is applied to the generation system.

Keywords-component; Magnetic induction; excitation coil;

tomography; Electromagnetic therapy; screen.

I. INTRODUCTION

Magnetic Induction Tomography (MIT) [1] and

Electromagnetic Therapy [2] system are the example of

systems which used electromagnetic (EM) field as a source in

their applications. The EM fields generate by excitation coil,

then will propagate and penetrate the object located at the

region of interest (ROI). Due to general excitation coil

(without screen), while EM propagate to the ROI, there are

also some portion of the field which surround the coil and

interfere the system. Instead of contribute noise to the system

through the generation of heat, this phenomenon also can be

considered as wasting of EM energy.

Based on that motivation, Zakaria et al. (2011) [3],

Stawicki et al.(2009) [4] and Barba et al. (2009) [5] had done

their study on the excitation coil screen design in focusing the

EM field to the ROI while minimizing the interference effects

of the field to the electronic circuit system. Stawicki and

Barba had reported the used of cone-type screen while Zakaria

had compared the performance of C-type and Cone-type

screen in his research. Zakaria also had reported that both C-

type and Cone-type excitation coil screen are capable in

focusing the EM field to the ROI and at the same time

reducing the scattered EM field to the nearby system

especially the electronic circuitry.

II. FORMULATION OF EM FIELD

All EM field are assumed fulfilled the state of the art of

Maxwell’s equation that are:

∇ × � = −jω� (1)

∇ × = (� + jωε)� (2)

∇ ∙ � = � (3)

∇ ∙ � = 0 (4)

When EM field penetrate an object, where in this case is

biological tissue, eddy current is induced in the biological

tissue itself due to passive electrical properties (conductivity,

σ; permittivity, ε; and permeability,µ) of the tissue. However

in most cases of biological tissue cases, conductivity is

dominant compare to others, hence the equation becomes:

� = �� = −��(��� +���) (5)

where φ is scalar potential while A is vector potential and J is

the current density in a material.

Magnetic field strength at any point in the ROI due to

induced eddy current can be calculated through Biot-Savart

law that is:

� = ��∑ ���× !" #$ (6)

Base on the equation, the magnitude of the magnetic field

strength, � is also depends on the area or volume covered by

the field since dV is the small element of a volume. Deeper

penetration normally will involve volume. Related to that, the

penetration of the field inside the material (biological tissue) is

depends on the frequency used and is given by the formula:

% = &�' (

)*+*,-./0 ( 7)

2012 International Conference on Biomedical Engineering (ICoBE),27-28 February 2012,Penang

978-1-4577-1991-2/12/$26.00 ©2011 IEEE 598

Where d is penetration depth, ω is frequency, µ is

permeability, ε0 is free space permittivity, εr is relative

permittivity and tan δ is loss factor.

Related to that, thermal effect is another phenomenon exists

in biological tissue in relation with EM field. Wang et al.

(2009) [6] explained that, thermal effects are indirect

interaction with biological tissues due to the high RF

radiation, where in this condition, electromagnetic field

generates the heat and causing biological effect. The

absorption of EM energy within the biological tissues causing

the biological effect due to temperature rising. Tissue exposed

to EM fields will continue rising with temperature until the

heat absorption rate is balanced with the rate at which it is

dissipated. The temperature dissipation is due to conduction

with other tissue types, convection through blood perfusion

and radiation to the surroundings. This heating concept has

been applied as therapeutic technique in curing cancer [7]. On

the other hand, under localized (partial body) near-field

exposure conditions, the internal fields decay exponentially

with distance from the exposed external surface, and the rate

of decay depends on the conductivity of the tissue [8].

III. EXPERIMENTAL SETUPS

For this research, C-type excitation coil designs had been

considered with frequency are set to 0.5 kHz, 1 kHz, 5 kHz,

20 kHz, 50 kHz and 100 kHz which are the acceptable range

of a biological tissue imaging [9], number of turn is 10 with 1

A current flow and 10 AWG wire. The properties of skin and

muscle represent biological tissue are as in Table 1 [10]. The

thickness of skin and muscle are approximately 0.2 cm and 4.0

cm respectively.

Figure 1. C-type excitation coil design with biological tissue model in FEMM

environment

TABLE 1. ELECTRICAL PROPERTIES OF SKIN AND MUSCLE

BASED ON FREQUENCY

Frequency

(kHz)

Skin (Dry) Muscle

Conductivitty

(S/m)

Permittivitty Conductivitty

(S/m)

Permittivitty

100 0.00045128 1119.2 0.36 8089

50 0.00027309 1126.8 0.35 10094

20 0.00021417 1131.7 0.34 15521

5 0.00020117 1134.6 0.33669 52349

1 0.00020006 1135.6 0.32115 434930

0.5 0.00020002 1135.8 0.30972 1087500

IV. RESULTS AND DISCUSSIONS

Based on the simulation results, it is found that even with the

used of the electromagnetic screen on the excitaion coil, the

scatered field is still happened at the above, bottom and back

region of the screen when the appied frequency is 5 kHz and

below as shown in Fig. 3-5. However scattering effect is totally

eliminated when frequency 20 kHz and above are applied. In

term of magnetic field exposure to the tissue, 0.5 kHz

frequency has proven of providing more higher magnitude of

magnetic field compare to all others as in Fig. 9.

Figure 2. Mesh of the experimental model

Figure 3. Magnetic field strength distribution at 500 Hz

Coil

Aluminum

screen

A B C

0 1.2 cm 8.8 cm

599

Figure 4. Magnetic field strength distribution at 1 kHz

Figure 5. Magnetic field strength distribution at 5 kHz

Figure 6. Magnetic field strength distribution at 20 kHz

Figure 7. Magnetic field strength distribution at 50 kHz

Figure 8. Magnetic field strength distribution at 100 kHz

Figure 9. Magnetic field strength of distribution from 500 Hz to 100 kHz

On the penetration capability, higher frequency will have more

penetration depth with wide coverage of biological tissue area.

For a MIT system with the aim of imaging the whole body, the

frequency of higher than 100 kHz should be applied for the

purpose of giving more penetration depth in the body with at

least half of the body diameter. However for the

electromagnetic therapy purposes, application of frequency

within the range of 5 kHz - 20 kHz is enough as long as the

scattering effect is eliminated.

CONCLUSION

C-type excitation coil screen has been proven of minimizing

the scattered EM field and at certain frequency apply, it may

fully eliminating the scattering effects at the back, above and

below the screen area. For next stages, the design of the screen

needs to undergo some modifications, thus providing the

0.00E+00

5.00E-05

1.00E-04

1.50E-04

2.00E-04

2.50E-04

3.00E-04

3.50E-04

0.0

0

0.6

0

1.2

1

1.8

1

2.4

2

3.0

2

3.6

2

4.2

3

4.8

3

5.4

4

6.0

4

6.6

4

7.2

5

7.8

5

8.4

6

Ma

gn

eti

c fi

eld

str

en

gth

(B

), T

esl

a

B (Tesla) vs Distance (cm)

0.5k

1k

5k

20k

50k

100k

600

scattering EM field eliminating even in the application of very

low frequency.

.

ACKNOWLEDGMENT

This work is supported by the FRGS grant 9003-00248 by the

Ministry of Higher Education of Malaysia and Science fund

grant 06-01-06-SF0889.

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