Targeted Radiation Dipole Antenna using 3D Numerical Simulation

in Microwave Ablation

Hussein Alnassan

1,2

, Adrian Kastler

, Xia Wang

and Bruno Kastler

1,3

Laboratory I.4.S -EA 4268, Franche Comté Universty, Besançon, IFR 133, France

Electrical and Electronics Engineering Faculty, Aleppo University , Aleppo, Syria

Interventional Radiology Unit University Hospital Besançon, Besançon, France

Keywords: Microwave Ablation, Liver Tumors, Bone Microwave Ablation, Targeted Directional Radiation.

Abstract: Microwave ablation technology is being utilised in several medical applications for ablation therapy and

other applications. Microwave energy generates fast and high temperatures sufficient and capable to

produce coagulation necrosis. Theoretical models by numerical simulation of microwave ablation is a

distinct step in the implementation of system design, as well as in the results analysis before the ablation

procedure. Furthermore, these models play a role in design the microwave antennas. Classic microwave

ablation antenna around its radiating section applies electromagnetic field in tumors without worry about

near neural structures. This paper presents the temperature distributions of targeted radiation dipole antenna

model with active and non-active sides for microwave ablation at 2.45GHz at different powers and ablation

times. Temperature maps and SAR distributions around the radiating section show in two sides.

1 INTRODUCTION

Thermal ablation technology is an alternative to

surgical resection for destruction of several types of

tumors. Thermal ablations could be using many

energy sources, including laser, high intensity

focused ultrasound, radiofrequency and microwave.

While radiofrequency has been the most applied in

clinical thermal ablation, microwave ablation has

several advantages including, faster heating of large

tumors, less susceptibility to heat-sink effect and no

ground pads requirement.

Efficiency of minimally invasive percutaneous

thermal microwave ablation (MWA) has been

proved in treating several types of tumor including

the liver, lung, kidney (Kuang et al. 2007; Wolf et

al. 2008; Castle, Salas, and Leveillee 2011) and

recently bone (Kastler et al. 2013; Kastler et al.

2014). The phenomenon of denaturation is induced

by tissue temperature rising above 50°C which

causes coagulation and cells death in a matter of

minutes (Brace 2009).

Several coaxial-based antenna have been

designed for microwave ablation including the slot

antenna (Ito et al. 2004), dipole antenna (Hurter,

Reinbold, and Lorenz 1991), monopole antenna

(Labonte et al. 1996), cap-choke antenna (Lin and

Wang 1996), floating sleeve antenna (Yang et al.

2006), triaxial antenna (Brace et al. 2007), and

minimally invasive antenna (Cavagnaro et al. 2011).

Clinical application and several microwave

ablation devices have been reviewed in (Lubner et

al. 2010). Microwave energy at 915MHz or

2.45GHz induces a phenomenon known as dielectric

hysteresis to create heat generation (rotation of

water, proteins and other polar molecules in tissues).

Computational simulation using commercial

software (HFSS or COMSOL..) have been used to

predict the electromagnetic fields from the antenna,

temperature profile in tissue, and to integrate

electromagnetic and thermal solutions for

understanding how the ablation zones form (Chiang,

Wang, and Brace 2013).

Some tumors may be located in proximity of

neural noble structures which may be damaged by

the ablation process (nerves, vessels, biliary

ducts…). Therefore, in this paper, we design and

simulate a coaxial antenna with directive radiation

by varying the directive windows angle during the

ablation. The analyses were performed by 3D finite

element modelling. In order to compare the results in

terms of power and application time, we have used a

tissue temperature distribution model. The targeted

radiation in MWA has been presented in (Thaiwat et

al. 2011) for slot antenna in different open angles

Alnassan H., Kastler A., Wang X. and Kastler B..

Targeted Radiation Dipole Antenna using 3D Numerical Simulation in Microwave Ablation.

DOI: 10.5220/0005209900440048

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2015), pages 44-48

ISBN: 978-989-758-071-0

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

without however discussing lesion radius in the

active and non active side.

2 ANTENNA GEOMETRY

2.1 Structure

The goal was to design a coaxial antenna to realize

direction ablation on one side of antenna and prevent

it on the other side. The configuration and

dimensions of antenna geometry are presented in

Fig.1 and Table 1. To design our model, we chose

the coaxial dipole antenna model presented by yang

(Yang et al. 2006) without the sleeve as initial

antenna. We also were partly inspired by the work

presented in (“DIRECTIVE WINDOW ABLATION

ANTENNA WITH DIELECTRIC LOADING -

Patent - Europe PubMed Central” 2014). We have

inserted our antenna in a catheter with a dielectric

window at different lengths and angles opening. i.e.

8mm length and 300° angle. Our original idea was to

block the microwave energy by a metal catheter in

order to guide the wave to the desired directional

window for targeted tumoral ablating purposes.

We designed this model using Comsol 4.3

computer simulation for temperature distribution

around the antenna tip on two opposite sides (active

window side and non active side).

The model supposes that antenna is inserted in a

homogeneous liver block with diameter 50mm and

length 60mm. The dielectric insulator between inner

and outer conductors is Teflon with relative

permittivity equal to 2.03, and the dielectric window

is Ceramic with relative permittivity equal to 10 as

depicted in Fig. 2.

The parameters of material for the antenna and

liver are shown in Table 2. The liver parameters are

chosen in the static state without thermal effects on

the dielectric properties of tissue. All dielectric

properties value were taken from literatures

published static values.

2.2 Materials and Equations

Figure 1: Antenna top section with layers.

To simplify the design, all the conductors material of

the antenna were modelled using perfect electric

conductor (PEF) boundary conditions. The

microwave energy was chosen at 2.45GHz and

between power range 0-70W, with ablation duration

900 seconds.

We performed the simulation using the Finite

Element Method (FEM) Comsol 4.3 to solve our

Electromagnetic-Thermal problems. The mesh was

non uniform. A three dimension model was chosen

on platform Intel(R) Core(TM)2 Duo CPU P8400 @

2.26 GHz with RAM 4 GB.

The bioheat equation is represented in (1):







.































(1)

ρ Density of liver (kg/m



), C Heat capacity of

liver(J/kgk),K Thermal conductivity (W/mK), T

Temperature(°C),T



Temperature of blood

(°C),h



The convective heat transfer coefficient,Q



Energy from metabolic processes (W/m



),Q



External heat source (W/m



) (2).

SinceQ



is negligible, we excluded it from our

FE models. We also omitted T



from our

preliminary studies. The Specific Absorption Rate

SAR [W/kg] in tissue is calculated as a function of

position as follows (3):









/



 (1)

  



// (2)

E Electric field (V/m),σ Conductivity of liver

(s/m).

Table 1: Radius of antenna layers.

Layer Radius(mm)

Inner Conductor

0.256

Dielectric

0.84

Outer Conductor

1.1

Air Cavity

1.3

Catheter

1.5

Figure 2: Antenna side section in a liver block.

3 SIMULATION RESULTS

3D Temperature distributions are shown in Fig.3

from active side section at different powers and after

TargetedRadiationDipoleAntennausing3DNumericalSimulationinMicrowaveAblation

different application times we chose the temperature

level equal to 100°C.

The simulated lesion size is defined as the area

within the 50°C contour line, which corresponds to

actual ablation size. The Fig. 4 presents the

temperature distributions on active and opposite non

active sides of antenna tip at different powers after

900s and including contour 50°C. We can see non-

spherical lesion form around the antenna. These

lesions are in proportion with the power and

application time increase.

The temperature changes from active side and

opposite non active side at 10mm from antenna axis

at 0,300,600,900s and for powers 50, 60, 70W are

shown in Fig. 5. We see higher temperature level at

window side than the other side and the radius

increasing of lesion with the long duration of

ablation.

Table 2: Parameters of tissues.

Properties Values

(density of liver)

1060(kg/









(Heat capacity of liver)

3600(J/kgk)

K(Thermal conductivity of

liver)

0.56(W/mK)





(density of Blood)

1000(kg/









(Heat capacity of liver)

3639(J/kgk)





(Conductivity of liver)

1.69(S/m)





(Permittivity of liver)

43.3





(Permittivity of

dielectric)

2.03

Figure 3: three dimensions Temperature distributions from

top to down 70,60,50w and from left to right

900,600,300s.

Figure 4: Temperature distribution (a) 50W,(b) 60W,(c)

70W for 900s , 2 views.

Figure 5: Temperature value left, non active side right

active side (a)50W,(b)60W,(c)70W and ablation times

(300,600,900s).

BIODEVICES2015-InternationalConferenceonBiomedicalElectronicsandDevices

The temperature value increases with power

increases, and we see higher temperature peaks on

the active side comparing to the non active side.

Fig. 6 shows the SAR values along the

longitudinal direction for the non active side (green)

and active side (blue) of antenna in the lesion

regions. SAR values indicate there are differences

between the two sides and these values are

constrained.

Figure 6: SAR values non active side (green) , active side

(blue).

4 DISCUSSION AND

CONCLUSIONS

In this study we designed a novel dipole antenna

inserted in an asymmetric metallic catheter with a

dielectric window. We tested the simulation of this

MW ablation antenna achieving a directional

ablation in liver model by using a 3D Finite

Elements Method (FEM) which analyses the

temperature distribution. We chose a dipole antenna

because with 3mm diameter allowing high power

delivery and large lesion size. A metal catheter was

chosen for two goals: First to guide the generated

electromagnetic waves to the dielectric window,

Second: to obtain an air cavity through which water

may be circulated for cooling purpose and to prevent

backward heating.

Increasing power application times from 600 to

900s did not significantly contribute to temperature

induced changes, nor did it affect lesion radius on

both active and non active side.

The antenna we elaborated can selectively ablate

tissue on the active side. The lesion size on active

side can be controlled by increasing power and

ablation duration until 600s.

The disadvantage of our antenna is the relatively

large diameter 3mm, however still compatible for

percutaneous bone or laparoscopic microwave

ablation.

In this work dipole antenna model radiates a

directive electromagnetic field, the goal is to achieve

a selective ablation while avoiding damaging the

healthy tissue structures near targeted tumour.

According to temperature values and SAR

distributions levels between active side and non

active-side show promising results in targeted

ablation.

These simulation results are considered as a first

step before implementation of our device. May be

more computational modeling and experimental

searches are needed to understand the dielectric and

thermal tissue properties, as well as to increase the

lesion size.

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