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Thermal analysis of Microwave ablation by a coaxial open-tip antenna

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Introduction

Snoring is a one of the sleep disorder syndromes caused by the Obstructive Sleep Apnea (OSA) which causes serious breathing problems. This type of apnea occurs when your throat muscles intermittently relax and block your airway during sleep [1]. Many types of OSA treatments are available, we mention the mini-invasive snoring cure including the soft palate Radio-frequency-based surgery. Such safe technique has received increased attention due to its minimal postoperative pain and complications with a shorter recovery time.

RF Microwave ablation based on thermal therapies is an attractive solution for large soft tissues within short treatment period: it produces rapid heat generation from an electromagnetic Open-tip applicator, sufficient to trigger an instant tissue coagulation and necrosis which will be tightened and reduced in size. This tightening and reduction will directly prevent the tongue from blocking the airway while the patient is sleeping. Figure 1 shows an insertion of microwave applicator into soft tissue region for microwave emission at preset treatment time and power.

This process relies on an absorption power, time and temperature distribution of microwave energy. Estimation of those factors using simulation solver seems to be a promising way in microwave ablation system design. In this study, an FEM simulation of this process is presented, by an Open-tip antenna applicator introduced inside the soft palate tissue CAD model.

Illustration of the breathing operation (a-without and (b-with the OSA.[1] (c- Insertion of microwave antenna into soft palate region

Figure 1 - Illustration of the breathing operation (a-without and (b-with the OSA.[1] (c- Insertion of microwave antenna into soft palate region [2]

Simulation Setup

The introduced Applicator Design is a Coaxial antenna type providing a bending tip extremity to consider the anatomy of human mouth cavity and the curved region of the upper soft palate tissue. Figure 2 shows the modeled antenna design   and the dimensions are shown in Table 1.
Soft Palate model with the open-tip antenna
Figure 2 - Soft Palate model with the open-tip antenna
 
Table 1 - Dimensions of the studied model
Part Dimension (mm)
Diameter of inner conductor 0.912
Diameter of outer conductor 2.985
Diameter of dielectric 3.581
Length of antenna 45
Length of opened tip 15
Bending angle (degree) 75°

The Antenna solver of HFWorks is used, coupled to the thermal case for a working frequency of 2.45 GHz. The properties of the used materials are summarized in table 2.
Table 2 - Material properties
Material Relative permittivity Dielectric loss tangent Electrical conductivity (S/m) Thermal conductivity
 (W/m. K)
Copper 1 0 5.96E+7 401
Soft palate tissue 53.573 0 0.0125 0.5
Solid PTFE 2.03 0 0 0.4

Electromagnetic boundary conditions

- Wave port: The wave port boundary is applied to the dielectric input face of coaxial antenna.

Wave port boundary condition

Figure 3 - Wave port boundary condition
 
- Radiation: The Radiation boundary condition is used to truncate the open computation domain when analyzing antenna problems. In our case, it is applied to the outer faces of the studied tissue.

Thermal boundary conditions

- For an excitation power of P subscript i n end subscript equals 50 W applied to the input port at 2.45 GHz, a thermal boundary convection is applied to the outer faces of the Tissue part at the normal body temperature of 37°C and a convection coefficient set to 10 W divided by m ². C.
- Another BC temperature is applied to the outer faces of the outer conductor with a value set to 37°C.

Thermal convection boundary condition

Figure 4 - Thermal convection boundary condition
 

Mesh

In HFWorks, Mesh control refers to specifying different element sizes at different regions in the model. A smaller element size in a region improves the accuracy of results in that region. You can specify mesh control at faces, components and edges.
In this case, a fine mesh control was applied to antenna body like shown in the next figure of the meshed model.

 The meshed model
Figure 5 - The meshed model
 

Results 

HFWorks comes with an integrated thermal solver that allows to predict the thermal aspects of the studied High frequency design due to conductor and dielectric losses under different power excitation loads. For the case of studied Microwave ablation process, an excitation power of P subscript i n end subscript equals 50 W was used in heating the soft palate tissue. The simulation revealed the next results at frequency of 2.45GH:

(a-Electric and (b-Magnetic field distribution at 2.45GHz

Figure 6 - (a-Electric and (b-Magnetic field distribution at 2.45GHz
 

A steady state thermal analysis coupled to the antenna study gives the next temperature results: Figure 6 shows the temperature distribution in soft tissue due to heat generated from antenna at 2.45GH. Regions with highest temperature are focused along the perimeter of the opened-tip antenna part with a maximum value of 54°C. The coagulated zones of the studied tissue can be estimated from the obtained temperature distribution.

 (a-Fringe plot and (b- Iso-clipping plot of temperature distribution at 2.45GHz.
Figure 7 - (a-Fringe plot and (b- Iso-clipping plot of temperature distribution at 2.45GHz.
 

Figure 8 is showing the Heat flux density with a maximum distribution along the inner conductor of the antenna applicator.

 
 Heat flux distribution at 2.45 GHz
 
Figure 8 -  Heat flux distribution at 2.45 GHz

Conclusion

The increasing use of RF-based thermal therapies such as micro-wave ablation, radiofrequency ablation as an attractive modality makes it particularly suitable for destroying tumors in different biological tissues. For the studied process, a curved 2.45 GHz microwave applicator is investigated for effective energy deposition along soft palate tissue for snoring therapy. A thermal analysis is performed to analyze the coagulated tissue regions exposed to higher temperature distributions.

References

[1]. http://smileworksmeridian.com/treatments/sleep-apnea/
[2]. Jakawanchaisri, Wirote, et al. "FEM Analysis of Microwave Ablation for Snoring Therapy by Using Real Image." Proc ICBET 2012 (2012).