http://informahealthcare.com/ebm ISSN: 1536-8378 (print), 1536-8386 (electronic) Electromagn Biol Med, Early Online: 1–7 ! 2015 Informa Healthcare USA, Inc. DOI: 10.3109/15368378.2015.1048550

ORIGINAL ARTICLE

Reconfigurable tapered coaxial slot antenna for hepatic microwave ablation Neeru Malhotra1, Anupma Marwaha2, and Ajay Kumar3 DAV Institute of Engineering and Technology, ECE, Jalandhar, Punjab, India, 2Sant Longowal Institute of Engineering and Technology, ECE, Longowal, Sangrur, Punjab, India, and 3Beant College of Engineering and Technology, ECE, Gurdaspur, Punjab, India

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Abstract

Keywords

Microwave ablation is rapidly being rediscovered and developed for treating many cancers of liver, lung, kidney and bone, as well as arrhythmias and other medical conditions. The microwaves ablate tissue by heating it to cytotoxic temperatures. The microwave antenna design suffers the challenges of effective coupling and penetration into body tissues, uncontrolled power deposition due to applicator construction limitations affecting uniform heating of target region, and narrowband operation leading to mismatch for many patients and detrimental heating. To meet out the requirements of wideband operation and localized lesion reconfigurable linearly tapered slot interstitial wideband antenna has been proposed for working in the 1.38 GHz to 4.31 GHz frequency band. The performance of the antenna is evaluated by using FEM-based HFSS software. The slot height and taper height are reconfigured for parametric analysis achieving maximum impedance matching and spherical ablation zone without requiring any additional adjustable structures. The tapering of the slot in coaxial antenna generates current distribution at the edges of the slot for maximizing specific absorption rate.

FEM, HFSS, microwave ablation, tapered slot antenna, SAR

Introduction The microwave energy radiates into the tissue through an interstitial antenna which functions to couple energy from the generator power source to tissue. Due to the radiating nature of the antenna, direct heating occurs in a volume of tissue around the antenna. The use of minimally invasive antenna is recognized as a very promising technique in the field of microwave ablation (MWA) therapy for the treatment of small tumors, because a very thin antenna can be easily inserted inside the body and precisely localized using the advanced 3D imaging techniques and surgical robots (Maini et al., 2012). During the treatment, microwaves are applied directly to the tissues to produce rapid temperature elevation sufficient to produce immediate coagulative necrosis. The MWA heating causes temperatures higher than 60  C causing comparatively immediate cell death, while temperatures from 50 to 60  C will induce coagulation and cell death in a matter of minutes, depending upon temperature and previous thermal injury (Brace, 2010). The criteria for antenna design takes into consideration factors such as desired ablation zone size, reduced backward

Address correspondence to Mrs Neeru Malhotra, DAVIET, Jalandhar, ECE, DAVIET, Jalandhar, Punjab,, Jalandhar, India. E-mail: neeru_ins@ yahoo.com

History Received 26 November 2014 Revised 16 April 2015 Accepted 26 April 2015 Published online 3 July 2015

heating, treatment duration, and procedural invasiveness (Lubner, 2010). The antenna designs comprising of a linear element as monopoles, dipoles, and triaxial antennas have been reported (Brace et al., 2004; Gu et al., 1999; Hamada et al., 2000; Ito et al., 1990). In a recent study it was examined that increasing the insertion depth noticeably affected the current distribution, making it highly asymmetric. The power deposition becomes highly asymmetric and specific absorption rate (SAR) pattern produced by the antenna become more nonlocalized with increasing insertion depth. An increase in catheter thickness further decreases the peak current leading to decreased power localization and non-localized SAR patterns. Mario et al. (2011) designed coaxial antenna with single and double slots for microwave hyperthermia. The normalized SAR patterns show elongated thermal lesions. Bertam [Bertram et al., 2006] achieved higher degree of power localization by developing a double slot choked antenna. In the recent years genetic algorithms (GA) are being used by the researchers for developing optimized MWA antenna. Converse et al., 2009 optimize the design of a minimally invasive choke antenna using multiobjective GA to create near-spherical ablation zones of adjustable size by adjusting treatment durations and a sliding structure of the antenna. The authors also suggested a method for improving reflection coefficient for coaxial antennas (Wang et al., 2009).

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The antennas presented may be efficiently coupling input power into the tissue, with good broadside radiation patterns. However, they are relatively narrow band, radiation can be prone to hot-spots or elongated patterns due to additional structures requiring fine adjustment. The article presents linearly tapered slot antenna (TSA) for the minimally invasive

hepatic MWA that offers wideband operation in the range of frequencies from 1.38 GHz to 4.31 GHz. The interesting feature of the antenna is that it does not require any additional sliding choke type structure for providing optimized performance and automatically generates the best possible solution for selecting the design parameters of the antenna.

Figure 1. Design of linearly tapered slot coaxial interstitial antenna.

Figure 2. Axiallysymmetric antenna assembly.

Figure 3. Meshing diagrams.

Reconfigurable tapered coaxial slot antenna

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DOI: 10.3109/15368378.2015.1048550

Tapered slot antennas are conventionally well-behaved traveling-wave antennas producing high directivity and narrow beam width. The wide frequency band operation is obtained by producing symmetric E-plane and H-plane radiation patterns subject to the condition that antenna parameters including its total length, shape, dielectric thickness and slot dimensions are selected properly. The performance of the TSA is therefore optimized by reconfiguring the slot height, tapering angle and hence the tapering height. The antenna achieves more than 100% fractional bandwidth for various taper parameters. An analytical transmission line model can be used to optimize the input impedance of coaxial slot antenna, however, microwave ablation investigation requires the determination of heating pattern produced by the antenna (Brace, 2011). The simulated models serve as a quick, convenient and inexpensive evaluation to optimize promising antennas for prototyping. Therefore, numerical simulation on FEM-based HFSS software has been used in Table 1. Electromagnetic properties at 2.45 GHz. Name Relative permittivity, dielectric Relative permittivity, catheter Relative permittivity, liver Electrical conductivity, liver Relative permeability

Expression

Values

eps_diel eps_cat eps_liver sig_liver ("r)

2.03 2.6 42.6 1.69 (S/m) 1

Table 2. Thermal properties for thermal analysis. Name Thermal conductivity, liver Density, blood Specific heat, blood Blood perfusion rate Blood temperature

Expression

Values

K_liver Rho_blood C_blood Omega_blood T_blood

0.56 [W/(kgK)] 1000 (kg/m3) 3639 [J/(kgk)] 3.6  103 (1/s) 37 ( C)

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this study to analyze the influence of slot height size and tapering height on antenna efficiency, bandwidth and spatial heating pattern (ANSYS Inc., Canonsburg, Pennsylvania).

Numerical modeling In the microwave thermal therapy, the length of the linear coaxial antenna is in excess of the transverse diameter. The distribution energy of invasive system is achieved from the thermotherapic probe of powerful small antenna. Furthermore, it should have smaller volume and considerable intensity, and should be strongly radiative. Experimental investigations have discovered that the electrical and structural properties contribute to the overall performance of the radiating structure. Antenna’s parameters like thickness, dielectric constant of substrate and associated tangential loss, the permittivity variation with frequency and the temperature significantly affect the overall performance of the planar TSA under high-frequency operations. The temperature profile in body tissue during an ablation procedure requires the determination of heat transfer occurring in biological tissue which is characterized by Pennes bioheat equation (Yang et al., 2007) given by pc

@T ¼ r:ðkrTÞ þ b Cb !b ðTb  TÞ þ Qmet þ Qext @t

ð1Þ

where b is the blood density (kg/m3), Cb is the blood specific heat (J/kg.K), k is the tissue heat conductivity (W/m.K), !b is blood perfusion rate (kg/m3s), T is the final temperature, Tb is blood temperature, Qmet is the heat source from metabolism and Qext an external heat source. The external heating term for microwave ablation is given by  Qext ¼ jEj2 ð2Þ 2 where E is electric field intensity (Vm1) and s is the conductivity of tissue (Sm1).

Figure 4. Return loss, S11 (in dB) for varying slot distance.

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Figure 5. Return loss, S11 (in dB) with taper distance Ls ¼ 5.3 mm.

Reconfigurable tapered coaxial slot antenna

DOI: 10.3109/15368378.2015.1048550

Furthermore cautious examinations of both the antenna’s frequency-dependent reflection coefficient, S11 and specific rate pattern in tissue are indispensable parameters for the optimization of antennas for hepatic MWA. The SAR is defined as the power dissipation rate normalized by material density and can be defined mathematically as

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SAR ¼

E2 2

ð3Þ

where  is the density of the tissue [kg/m3]. The treatment of deep-rooted hepatic tumors needs the SAR pattern of an interstitial antenna to be exceedingly localized near the distal tip of the antenna. The Pennes equation (1) requires the specifications of both the initial and boundary conditions to be solved. The initial conditions for temperature is set at T ¼ 37  C which corresponds to the physiological temperature of the body tissue. The computational domain is limited to a part of the human tissue, so that it can be assumed that the heat exchange between parts of the same tissue does not take place and the boundary condition describing this process uses thermal insulation as follows: n: ðk: rTÞ ¼ 0 ð4Þ

Design and analysis Keeping in view the basic design of coaxial-fed interstitial slot antenna (Wu, 1995), the antenna designed here is composed of an inner conductor, a dielectric, outer conductor and catheter of radius 0.5 mm, 1 mm, 1.2 mm and 1.4 mm respectively. The antenna is enclosed in a catheter made of PTFE (polytetrafluoroethylene a biocompatible dielectric material) for hygienic purposes. Figure 1(a) shows the new design having a linear taper shaped slot cut on the outer conductor at distance, Ls from the short-circuited tip instead of the conventional ring shaped slot. The 3D model of the antenna is created in HFSS and optimetric feature is utilized in which the slot distance, Ls is varied in the range from 5 mm to 5.5 mm with increment of 0.05 mm considering nominal tapering profile. Further the slot height and the taper height are parameterized and allowed for variations in the simulation as can be seen in Figure 1(b). The slot height is linearly varied from 1 mm to 2 mm with an incremental step of 0.25 mm. For initializing optimetrics for taper angle, the taper height is chosen for simpler formulation and its value is varied from 0.25 mm to 0.5 mm in increments of 0.05 mm steps. The effective heating around the tip of the antenna is very important to the interstitial heating because the electric field becomes stronger near the slot so the field is required to be reflected back toward the slot. Multiple slotting (Hamada et al., 2000; Ito et al., 1990) was the possible solution to address this issue. Those antennas are however unbalanced coaxial-based antennas, which transmit the microwave power out of their tips. The active radiation region of the antennas is usually from the antenna tip all the way back to the end of the long tail. Thermal lesions hence created have elongated teardrop shapes due to backward heating problem. Further any impedance mismatch between the antenna and the surrounding medium also produces in unbalanced currents

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on the inner and outer conductors of the coaxial feed causing backward heating problem visible in the form of tail in many of the SAR patterns computed from simulations of MWA antennas. In the linearly tapered slot design change in the tapering profile is introduced to produce maximum impedance matching, causing increase in the gain maintaining small size of TSA antenna. The beam width, side lobe level and radiation pattern are the various parameters that depend upon the tapering profile of antenna. The current distribution at the edges of the slots generates maximum SAR (Yang et al., 2007). The needle tip of the antenna facilitates it to be inserted without requiring any insertion device into the targeted tumor tissue. Traditionally temperature probes are inserted separately at the critical locations to measure temperature in a target region. But an optical fiber temperature sensor can however be inserted along with needle tip antenna. Due to the mechanical properties of a needle the proposed antenna also supports the temperature probe to get inserted along with the antenna for temperature monitoring. The antenna is placed axisymmetrically with liver tissue. The external surface of the liver tissue acts as boundary for the computational domain. Scattering boundary condition is applied for the original radiating surface properties so that the total field formulation is applied for evaluating the field distribution. Thermal insulation conditions are assumed for the heat transfer problem. Figure 2 depicts the cross-sectional view of the geometry illustrating the axial symmetry of the model with appropriate boundary conditions. The port is subjected to the microwave source of 10 W set at the upper end of the coaxial cable. Table 2 shows the thermal properties for thermal analysis of antenna. Table 1 shows the material properties selected for implementing the antenna design. The simulation is executed at solution frequency of 2.45 GHz available freely for public use in the ISM band.

Figure 6. SAR distribution.

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Figure 7. SAR as a function of insertion depth.

Results The finite element method incorporates adaptive mesh refinement strategies that permits extensive mesh refinement iteratively in regions where the error is high, which increases the solution’s precision. HFSS recomputes the error, and the iterative process repeats until the convergence criteria are satisfied for final solution. Figure 3 shows the meshing diagrams for the antenna model. The parametric analysis was set up by initializing the optimetrics in HFSS software for varying the slot distance, taper profile in terms of taper height and slot height as variable parameters. The simulation was performed on Intel CoreÔ I7, 2.67 GHz processor with solution time of around 2.25 hrs for executing all the iterations (Santa Clara, California). The tapering angle gets changed if the taper height is automated for constant slot heights. The radiation characteristics of TSA are greatly influenced by these parameters. The return loss, S11 is plotted in dB for variations in slot distance, Ls considering the nominal values of taper height and slot height as depicted in Figure 4. It can be observed that as Ls changes from 5 mm to 5.5 mm, the S11 value reduces below 10dB and resonant frequency shifts toward lower frequency side. The peak minima of S11 is achieved for 5.5 mm slot distance but the resonant frequency however reduces to 2.39 GHz. The optimum value of Ls is therefore selected as 5.3 mm with S11 value of 28.7dB at 2.45 GHz operating frequency. To optimize the tapering profile of the slot, the slot height variations, Ws are given from 1 mm to 2 mm with incremental step of 0.25. The curves are demonstrated as in Figure 5 (a)–(e) for different taper heights in range from 0.25 mm to 0.5 mm. Wideband operation is visible from the plots with S11 less than 10dB in the wide range of frequencies extending from 1.38 GHz to 4.31 GHz at solution frequency around 2.5 GHz for all the cases. The improved reflection coefficient in wider band prevents the chances of heat transfer to the surrounding

Figure 8. Temperature distribution.

tissues and the backward heating problem. It can be observed from the plots that resonating frequency for the antenna shifts toward lower frequency with increasing slot height and taper height. The S11 value remains below 20dB at the center frequency and the value even shoots down to a peak minima of 31.4dB in the range. The taper dimensions of taper height ¼ 0.25 mm and slot height ¼ 2 mm result in the optimum S11 value of 28.7dB at the operating frequency of 2.45dB with BW of 2.39 GHz. The fractional bandwidth of more than 100% is achieved on account of the optimizing taper profile for the TSA as compared to narrowband operation for conventional ring slot coaxial antenna (Maini et al., 2012; Maini and Marwaha, 2013). Good SAR distribution is achieved due the rigorous parametric selection

DOI: 10.3109/15368378.2015.1048550

Reconfigurable tapered coaxial slot antenna

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Figure 9. Temperature plot as a function of insertion depth.

of the optimized taper slot antenna dimensions as depicted in Figure 6. Excellent power absorption is demonstrated in terms of SAR as a function of insertion depth in Figure 7. It is further evident from the surface plot of temperature distribution of Figure 8 that the antenna surface heat is well distributed near the tip with strong heat flux of spherical shape near the antenna. Surprising high temperatures are hence obviously achieved as shown in Figure 9. The linear tapering profile of the slot facilitates to divert the electric field toward the slot and hence prevents the reflected current from flowing along the antenna without introducing any additional sleeve (Yang et al., 2006) and choke segments (Longo et al., 2003) as practiced in earlier designs.

Conclusions The novel interstitial antenna design is proposed with linearly tapered slot reconfigured to achieve optimum dimensions of the taper slot which permits the antenna to exhibit wideband characteristics. The optimetric analysis is performed by varying multiple taper profile parameters for achieving good impedance matching with improved reflection coefficient and spherical lesion visible from SAR distribution and temperature measurements. The tapering of slot antenna obtains desired results without requiring any additional structures attached with coaxial antenna. The simulated antenna using HFSS software presents potentially good response in the wideband frequency band from 1.38 GHz to 4.31 GHz.

Declaration of interest The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

References Bertram, J. M., Yang, D., Converse, M. C., et al. (2006). Antenna design for microwave hepatic ablation using an axisymmetric electromagnetic model. BioMed. Eng. 5:15. Brace, C. L. (2010). Microwave tissue ablation: Biophysics, technology and applications. Int. J. Crit. Rev. TM Biomed. Eng. 38:65–78.

Brace, C. L., van der Weide, D. W., Lee, F. T., et al. (2004). Analysis and experimental validation of a triaxial antenna for microwave tumor ablation. IEEE MTTS Int. Microw. Symp. 3:1437–1440. Brace, C. L. (2011). Dual-slot antennas for microwave tissue heating: Parametric design analysis and experimental validation. Med. Phys. 38:4232–4240. Converse, M. C., Webster, J. G., Mahvi, D. M. (2009). An optimal sliding choke antenna for hepatic microwave ablation. IEEE Trans. Biomed. Eng. 56:2470–2476. Gu, Z., Rappaport, C. M., Wang, P. J., VanderBrink, B. A. (1999). A 21/4 Turn spiral antenna for catheter cardiac ablation. IEEE Trans. Biomed. Eng. 46:1480–1482. Hamada, L., Saito, K., Yoshimura, H., Ito, K. (2000). Dielectric-loaded coaxial-slot antenna for interstitial microwave hyperthermia: Longitudinal control of heating patterns. Int. J. Hyperthermia. 16: 219–229. Ito, K., Hyodo, M., Shimura, M., Kasai, H. (1990). Thin applicator having coaxial ring slots for interstitial microwave hyperthermia. Ant. Prop. Soc. Int. Sym. 3:1233–1236. Longo, I., Gentili, G. B., Cerretelli, M., Tosoratti, N. (2003). A coaxial antenna with miniaturized choke for minimally invasive interstitial heating. IEEE Trans. Biomed. Eng. 50:82–88. Lubner, M. G., Brace, C. L., Hinshaw, J. L., Lee, F. T., Jr. (2010). Microwave tumor ablation: Mechanism of action, clinical results and devices. J. Vasc Intervent. Radiol. 21:S192–S203. Maini, S., Marwaha, A., Marwaha, S. (2012). Finite element analysis for optimizing antenna for microwave coagulation therapy. J. Eng. Sci Technol. 7:462–470. Maini, S., Marwaha, A. (2013). Modeling and simulation of novel antenna for the treatment of hepatocellular carcinoma using finite element method. Electromagn Biol. Med. Informa Healthc. 32: 373–381. Rubio, M., Herna´ndez, A., Salas, L. (2011). Coaxial slot antenna design for microwave hyperthermia using finite-difference time-domain and finite element method. Open Nanomed. J. 3:2–9. Wang, P., Converse, M. C., Webster, J. G., Mahvi, D. M. (2009). Improved calculation of reflection coefficient for coaxial antennas with feed gap effect. IEEE Trans. Ant. Prop. 57:559–563. Wu, M. (1995). Analysis of current and electric field distributions of coaxial-slot antenna for interstitial microwave hyperthermia. J. Electromagn. Waves Appl. 9:831–849. Yang, D., Bertram, J. M., Converse, M. C., et al. (2006). A floating sleeve antenna yields localized hepatic microwave ablation. IEEE Trans. Biomed. Eng. 53:533–537. Yang, D., Converse, M. C., Mahvi, D. M., Webster, J. G. (2007). Expanding the bioheat equation to include tissue internal water evaporation during heating. IEEE Trans. Biomed. Eng. 54:1382–1388.

Reconfigurable tapered coaxial slot antenna for hepatic microwave ablation.

Microwave ablation is rapidly being rediscovered and developed for treating many cancers of liver, lung, kidney and bone, as well as arrhythmias and o...
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