Novel Applicators for Local Microwave Hyperthermia Based on Zeroth-Order Mode Resonator Metamaterial

Hindawi Publishing Corporation International Journal of Antennas and Propagation Volume 2014, Article ID 631398, 7 pages http://dx.doi.org/10.1155/2014/631398 Research Article Novel Applicators for Local Microwave Hyperthermia Based on Zeroth-Order Mode Resonator Metamaterial David Vrba and Jan Vrba Department of Biomedical Technology, Faculty of Biomedical Engineering, Czech Technical University in Prague, Zikova 4, 166 36 Prague, Czech Republic Correspondence should be addressed to David Vrba; Received 17 January 2014; Accepted 11 March 2014; Published 9 April 2014 Academic Editor: Haider Raad Khaleel Copyright © 2014 D. Vrba and J. Vrba. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. It is demonstrated that a theory of zero-order mode resonator (ZOR) metamaterial (MTM) structure can be used for the development of a novel class of applicators for microwave thermotherapy, for example, for hyperthermia in cancer treatment or for physiotherapy. The main idea of creating such an applicator is to generate and radiate a plane electromagnetic (EM) wave into the treated biological tissue, at least in a certain extent. The main aim of this paper is to investigate whether an EM wave generated by ZOR MTM structure and emitted into the biological tissue can produce a homogeneous SAR distribution in the planes parallel to the applicator aperture and achieve a penetration depth approaching the theoretical limit represented by SAR distribution and penetration depth of an ideal EM plane wave. EM field distribution inside a virtual phantom of the treated region generated by the applicator that is based on the proposed ZOR MTM principle is investigated using a well-proven full-wave commercial simulation tool. The proposed applicator type shows both a low unwanted leaked electromagnetic field and a fairly homogeneous electric field in its aperture as well as in the virtual phantom of the treated region. 1. Introduction The main aim of this paper is to verify whether it is feasible to apply principles of ZOR MTM radiating structures in the design of efficient applicators for radiofrequency (RF)/ microwave (MW) thermotherapy, especially for hyperthermia cancer treatment and for physiotherapy. For this aim, MTM antennas inspired by those described in [1, 2] will be investigated and adapted with respect to requirements of effective hyperthermia treatment (i.e., we will study the homogeneity of SAR distribution in the treated region). In order to achieve an optimal waveform shape of the radiated EM wave, applicators have to be designed in a certain way. The best possible results can be obtained by comparing various shapes of waveforms of EM waves (e.g., plane wave, cylindrical wave, and spherical wave) for local and deep local treatment waveform of plane wave. This can ensure (i) the best possible value of the effective treatment depth; (ii) the best possible homogeneity of SAR 3D distribution (i.e., its distribution on the surface and in the whole volume of the area to be treated). SAR distribution inside the virtual phantom of the treated region, created using the proposed applicator, and the reflection coefficient of the proposed applicator are investigated using the well-proven full-wave commercial simulation tool COMSOL Multiphysics. Based on our previous experience [1–3], an excellent agreement between simulation and measurement results can be expected. That is why the conclusions made in this paper are based on the results of numerical simulations only. 2. MTM Zeroth-Order Mode Resonator The concept of MTM phenomenon was first comprehensively introduced by Veselago in 1968 [4]. In the aforementioned publication, he speculated on the existence of materials whose permittivity (𝜀) and permeability (𝜇) were simultaneously negative. He named these materials left-handed (LH), as the E, H, and k vectors of the studied EM wave formed a lefthanded triad if the wave propagated through such environment. The first experimental verification of MTM phenomena was performed by a research group at the University of California, San Diego (UCSD), in 2001 [5]. 2 International Journal of Antennas and Propagation CL LR Z CR LL Y (a) (b) Figure 1: (a) Infinitesimal element of MTM (b) and its equivalent circuit consisting of inherent series inductor 𝐿 𝑅 , shunt capacitor 𝐶𝑅 , artificially inserted series capacitor 𝐶𝐿 , and shunt inductor 𝐿 𝐿 . 3. Design of the Applicator Based on ZOR MTM Structure In this section, we will study the possibility of creating MTM based applicators for local microwave hyperthermia cancer treatment at a frequency of 434 MHz. The basic part of the proposed applicator will consist of the ZOR. The working idea of ZOR is based on a special case of resonance that can occur when the TL meets the conditions of the MTM phenomenon. The phase constant 𝛽 = 0 is at working frequency in this case, which implies infinite guided wavelength 𝜆 𝑔 = 2𝜋/|𝛽| along the MTM structure as well as zero phase shift (𝜃𝑚 = −𝛽𝑙 = 0) [11]. It is very important to note that this phenomenon enables creation of a very special kind of resonator whose physical length is completely independent of the classical resonance condition (i.e., required to be a natural number multiple of the half working wavelength in case of either open-circuited or short-circuited TL) [11]. The typical voltage wave distribution along the resonant length for the negative (𝑚 < 0) and zero (𝑚 = 0) resonances is shown in Figure 2. In the case of zeroth-order mode resonance (𝑚 = 0), the value of the voltage along the m=0 m = −1 m = −2 Voltage An infinitesimally short lossless transmission line (TL) section can be described by a simple equivalent circuit consisting of a series inductor 𝐿 𝑅 and a shunt capacitor 𝐶𝑅 [6]. The lossless MTM cell implemented in planar technology consists of a TL section with artificially inserted series capacitors 𝐶𝐿 and shunt inductors 𝐿 𝐿 (with subscript 𝐿 denoting its left-handed properties). The equivalent circuit of the MTM cell can be then represented by four-lumped elements (as shown in Figure 1) [6]. Several different EM radiating structures based on the MTM principle were introduced in the past [7–10]. Since the very beginning of the development, however, real implementation possibilities of such antennas, for example, in communication technology, have been very limited because of their poor radiation efficiency. MTM antennas with very good radiation efficiency were first presented in [1, 2]. m = −3 z 𝓁 0 Length Figure 2: Voltage distribution in case of open-circuited TL of length ℓ. Mode 𝑚 = 0 represents the ZOR with infinite guided wavelength. ZOR is constant. A more comprehensive description of ZOR properties can be found in [11]. The zeroth-order resonant frequency of the proposed ZOR MTM applicator is equal to (...truncated)