PRACTICAL STUDY OF A COAXIAL-SLOT ANTENNA WITH SIMPLE MATCHING CIRCUIT FOR INTERSTITIAL HEATING

Kazuyuki SAITO1, Keiko MIYATA2, Hiroyuki YOSHIMURA2, and Koichi ITO1 1 Research Center for Frontier Medical Engineering, Chiba University

2 Faculty of Engineering, Chiba University 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

kazuyuki_saito@faculty.chiba-u.jp 1. Introduction In recent years, various types of medical applications of microwaves have widely been investigated and reported [1]. In particular, minimally invasive microwave thermal therapies using thin coaxial antennas are of a great interest. They are interstitial microwave hyperthermia [2] and microwave coagulation therapy (MCT) [3] for medical treatment of cancer, cardiac catheter ablation for ventricular arrhythmia treatment [4], thermal treatment of BPH (Benign Prostatic Hypertrophy) [5], etc. Up to now, the authors have been studying such thin coaxial antennas for the interstitial microwave hyperthermia.

Hyperthermia is one of the modalities for cancer treatment, utilizing the difference of thermal sensitivity between tumor and normal tissue. In this treatment, the tumor is heated up to the therapeutic temperature between 42 and 45 °C without overheating the surrounding normal tissues. We can enhance the treatment effect of other cancer treatments such as radiotherapy and chemotherapy by using them together with the hyperthermia.

The interstitial microwave hyperthermia is applied to the localized tumor by inserting thin microwave antennas into the targeted tumor. The authors have been studying coaxial-slot antenna, which is one kind of thin coaxial antennas. Moreover, we actually used our antennas for cancer treatments. Figure 1 shows the photograph taken during the treatment. In the all treatments, we could observe the actual effect of the hyperthermia.

Two major performances are required for the antennas for the interstitial heating. At first, it is necessary that the antenna generates a heating region only around its tip. Moreover, the shape of the heating region must be independent on the antenna insertion depth. Then, in order to get an effective power feeding to the antenna, we should realize the impedance matching of the antenna while keeping the localized heating pattern. We have already improved the shape of the heating pattern by optimizing the structural parameters of the coaxial-slot antenna. Therefore, here, we describe the improvement in the impedance matching of the coaxial-slot antenna.

In this paper, first, the structure of the coaxial-slot antenna and the simple matching circuit are explained. Then, the input impedance of the coaxial-slot antenna with matching circuit is shown. Finally, the effects of the matching circuit are explained by focusing on the heating performances.

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Fig. 1 Photograph of the patient during the treatment.

Proceedings of ISAP’04, Sendai, JAPAN

ISBN: 4-88552-207-2 C3055©IEICE - 797 -

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2. Structure of antenna We have been studying coaxial-slot antennas for interstitial heating. Figure 2 shows the basic structure of the considered coaxial-slot antenna [6]. This antenna is made of a thin semirigid coaxial cable, whose outer diameter is approximately 1.0 mm. The tip of the cable is short-circuited and several number of ring slots are cut on the outer conductor near the tip. We can control the heating pattern by changing the position and the number of slots. We have confirmed the possibility of generating a localized heating pattern by employing two slots, especially with Lts (distance from the tip to the upper slot) and Lls (distance from the tip to lower slot) set to 20 mm and 10 mm, respectively [7]. In addition, we also confirmed that the heating pattern of this coaxial-slot antenna is independent of the antenna insertion depth. We inserted the antenna into a catheter made of PTFE (polytetrafluoroethylene) for hygiene. The operating frequency is 2.45 GHz, which is one of the ISM (industrial, scientific, and medical) frequencies. Table 1 shows the structural parameters of the antenna.

In addition, we improved the input impedance of the antenna. Changing the position and number of slots allows a control of the antenna input impedance. However, it will influence the heating pattern of the antenna as previously explained. Therefore, we employed a matching circuit of simple structure at the position close to the feeding point. Figure 3 shows the detail of the matching circuit. This matching circuit consists of a matching slot and a metallic pipe. The edges of both sides of the metallic pipe are opened. We can adjust the input impedance of the antenna by changing the position of the matching slot (Lms: distance from the tip of the antenna, Lmp: distance from the center of the metallic pipe) and the length of the metallic pipe (Lp).

Table 1 Structural parameters of the coaxial-slot antenna. db (diameter of the antenna) [mm] 1.19 dc (diameter of the catheter) [mm] 1.79 tc (thickness of the catheter) [mm] 0.30 Lts (length from the tip to the center of the upper slot) [mm] 20.0 Lls (length from the tip to the center of the lower slot) [mm] 10.0 Wsl (width of the slot) [mm] 1.00

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Fig. 2 Coaxial-slot antenna with matching circuit. Fig. 3 Structure of the matching circuit. 3. Optimization of structural parameters of the matching circuit In order to improve the input impedance of the coaxial-slot antenna by changing the structural parameters of the matching circuit, we changed the position of the matching slot and the metallic pipe, namely, Lms and Lmp. Here, the length of the metallic pipe Lp was set to 30.0 mm. Figure 4 shows

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the variation of input impedance of the coaxial-slot antenna when Lmp was changed for three types of Lms. From this figure, Lms =99.0 mm and Lmp =-10.0 mm were found to give the optimized result showing the matching conditions is met (× mark in Fig. 4). Here, we obtained the reflection coefficient S11 with approximately -27 dB. Moreover, we investigated the input impedance of the coaxial-slot antenna with matching circuit under various conditions. Table 2 shows the reflection coefficient S11 as a function of the insertion depth Dt and the media, which are heated by the antenna. All results in Table 2 are less than -10 dB. Therefore, we may say that the matching circuit is effective under various situations at the treatment.

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Fig. 4 Input impedance of the coaxial-slot antenna with matching circuit (Dt =70 mm, 2.45 GHz).

Table 2 Reflection coefficients under various conditions at 2.45 GHz.

Insertion depth [mm] (biological tissue: muscle) S11 [dB]

30 -24.4 50 -14.5 70 -27.1

Biological tissue (Dt=70 mm) S11 [dB]

Liver (εr=43.0, σ =1.69 S/m) -27.6 Tongue (εr=52.6, σ =1.80 S/m) -28.2

4. Effect of the matching circuit In order to confirm the effect of the matching circuit, we calculated the temperature distributions around the antenna. The temperature rise in the biological tissue using the microwave energy can be calculated by solving the bioheat transfer equation shown in Eq. (1). Here, the SAR is a result of electromagnetic calculation by the FDTD method and acts as the heating source in the biological tissue.

( ) SAR2 ⋅+−−∇=∂∂ ρρρκρ bbb TTFcT

tTc (1)

Where T is the temperature [°C], t is the time [s], ρ is the density [kg/m3], c is the specific heat [J/kg·K], κ is the thermal conductivity [W/m·K], ρb is the density of the blood [kg/m3], cb is the specific heat of the blood [J/kg·K], Tb is the temperature of the blood [°C], and F is the blood flow rate [m3/kg·s].

Figure 5 (a) and (b) show the temperature distribution of the antenna with and without the

matching circuit. Here, the initial temperature, the input power of the antenna, and the heating time are set as 37 °C, 10.0 W and 300 s, respectively. In both cases, we can observe the localized heating

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region only around the tip of the antenna. Moreover, we can observe that the region of 42 °C in the Fig. 5 (a) is larger than that of Fig. 5 (b) in x direction because of the decrease in the mismatch loss. In addition, the maximum temperature in the Fig. 5 (a) is higher than that of Fig. 5 (b). From these results, we may say that the matching circuit realizes the improvement of the input impedance of the antenna while keeping the localized heating.

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(a) with matching circuit. (b) without matching circuit. Fig. 5 Temperature distributions around the coaxial-slot antenna.

5. Conclusions In this paper, we described the improvement of the coaxial-slot antenna for the interstitial microwave hyperthermia. First, we introduced the coaxial-slot antenna with a simple matching circuit. Next, we explained the input impedance of the coaxial-slot antenna with the matching circuit by the FDTD calculations and showed the temperature distributions around the antenna. As a result, generation of a localized heating region and improvement of the input impedance of the antenna were simultaneously realized. References [1] F. Sterzer, “Microwave medical devices,” IEEE Microwave Magazine, vol. 3, no. 1, pp. 65-70, 2002. [2] M. H. Seegenschmiedt, P. Fessenden, and C. C. Vernon (Eds.), “Thermoradiotherapy and thermochemotherapy,” Springer-Verlag, Berlin, 1995. [3] T. Seki, M. Wakabayashi, T. Nakagawa, T. Itoh, T. Shiro, K. Kunieda, M. Sato, S. Uchiyama, and K. Inoue, “Ultrasonically guided percutaneous microwave coagulation therapy for small carcinoma,” Cancer, vol. 74, no. 3, pp. 817-825, 1994. [4] R. D. Nevels, G. D. Arndt, G. W. Raffoul, J. R. Carl, and A. Pacifico, “Microwave catheter design,” IEEE Transactions on Biomedical Engineering, vol. 45, no. 7, pp. 885-890, 1998. [5] D. Despretz, J. C. Camart, C. Michel, J. J. Fabre, B. Prevost, J. P. Sozanski, and M. Chivé, “Microwave prostatic hyperthermia: interest of urethral and rectal applicators combination − Theoretical study and animal experimental results,” IEEE Transactions on Microwave Theory and Techniques, vol. 44, no. 10, pp. 1762-1768, 1996. [6] K. Ito, K. Ueno, M. Hyodo, and H. Kasai, “Interstitial applicator composed of coaxial ring slots for microwave hyperthermia,” Proceedings of International Symposium on Antennas and Propagation, pp. 253-256, 1989. [7] K. Saito, S. Okabe, T. Taniguchi, H. Yoshimura, and K. Ito, “Localized heating by using a coaxial-slot antenna with two slots for microwave coagulation therapy,” Digest of USNC/URSI National Radio Science Meeting, p. 422, 2001.

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