The cap-choke catheter antenna for microwave ablation treatment

A matched-dipole type catheter antenna is described for intracavitary and/or transluminal microwave treatment of diseases that may be responsive to thermal ablation therapy. This cap-choke antenna consists of an annular cap and coaxial choke design that can yield specific absorption rate (SAR) distributions in, and can produce heating of, tissue surrounding the distal end of the catheter antenna. The cap-choke antenna is simple in construction, provides the desired SAR distribution, and is efficient, i.e., the measured power reflection coefficient is 2% in tissue equivalent phantom modeling materials     

Simulation diagnostics of multiple discontinuities in a microwavecoaxial transmission line

Multiple discontinuities in microwave transmission lines can cause unusual reflection and transmission loss characteristics as functions of frequency. This article presents a method for developing models that simulate return loss and insertion loss data measured over a broad band of frequencies. The overall cable is modeled as a coaxial transmission line consisting of shunt susceptance discontinuities separated by line lengths. A nonlinear least-squares fit is then performed between theoretical data (from the model) and experimental data. When this method was applied to modeling discontinuities in a slightly damaged S-band antenna cable, excellent agreement between theory and experiment was obtained over a frequency range of 1.70-2.85 GHz     

An invasive microwave antenna for locally-induced hyperthermia for cancer therapy.

A microwave system has been developed and characterized for delivering heat directly into tumors. This system employs a microwave power source (3-10W) operating in the 500 MHz to 1.3 GHz frequency range, coaxial transmission line, and a monopole antenna. Absorbed power was measured in saline, in tissue equivalent phantoms, and in tumors in live and dead mice. Antennas were designed to operate at 500 MHz and 1 GHz, and the critical design parameters have been identified for this system. Analytical and experimental results obtained in our laboratory suggest that this system is capable of providing controlled temperature distributions appropriate for hyperthermia in animal tumors. Theoretical results predict that 3 GHz may be an optimum choice for this system in animal tumors of approximately 1 cm diameter; the microwave antenna system provides a heat distribution superior to that obtained using a resistance heater of similar dimensions. We propose that further development of this approach may overcome some of the problems associated with other systems which use external radiation sources, and implications for clinical application of this system are discussed.     

Microwave catheter design

A microwave antenna system for transcatheter ablation of cardiac tissue is investigated. A numerical model based on the finite-difference time-domain method incorporating a Gaussian pulse excitation has been constructed and frequency domain electric and magnetic fields are obtained through Fourier transformation. Results are presented for a coaxial line fed monopole catheter which is modified by the successive inclusion of a Teflon sheath outer coating, a terminating disk at the tip of the antenna, a sleeve choke, and a high dielectric constant cylinder surrounding the monopole antenna. The effects of these design features are characterized in terms of specific absorption rate (SAR) and return loss (RL). Numerical calculations are confirmed by comparing with the RL measurement of a Teflon-coated monopole containing a disk and choke     

新型左手传输线馈电微带阵列天线

提出了一种采用复合左右手传输线馈电的新型微带阵列天线.该天线利用左手传输线的相位超前特性来补偿传统的右手传输线所具有的相位滞后,从而保证了天线单元之间的同相位馈电,避免了因相位延迟导致的天线波束偏移,并进一步提高了天线的增益.仿真与实际测试证明:与同类型天线相比,该天线具有尺寸小、频带宽和馈电网络设计简单等优点,可在微波系统中实际使用.     

Design and Analysis of a 3-Arm Spiral Antenna

A novel 3-arm spiral antenna structure is presented in this paper. This antenna similar to traditional two-arm or four-arm spiral antennas exhibits wideband radiation characteristic and circular polarization. Advantages offered by the new design are two fold. Unlike the traditional spiral antennas the three-arm spiral can be fed by an unbalanced transmission line, such as a coaxial line or coplanar waveguide, and therefore an external balun is not needed at the feed point. Also by proper choice of arms' dimensions the antenna can be directly matched to any practical transmission line characteristic impedance and therefore external matching networks are not required. This is accomplished by feeding the antenna at the outer radius by a coplanar waveguide (CPW) transmission line and tapering it towards the center. The antenna can also be fed from the center using a coaxial or CPW line perpendicular to the plane of the spiral antenna. A full-wave numerical simulation tool is used to optimize the geometry of the proposed 3-arm spiral to achieve a compact size, wide bandwidth operation, and low axial ratio. The antenna is also designed over a ground plane to achieve a unidirectional radiation and center loading is examined that improves the axial ratio. Simulated results like return loss, radiation pattern, gain, and axial ratio are compared with those obtained from measurements and good agreements are shown. Because of its unique feed structure and compact size, application of the proposed 3-arm spiral antenna for wideband array applications is demonstrated     

一种组合馈线的设计

天线馈线是天线的重要组成部分,天线馈线设计的好坏对天线性能的影响非常大。在分析了造成某工程设计的L/S组合天线方向图畸变的原因之后,应用电磁波在金属导体上传输的趋肤效应和λ/4同轴短路传输线的阻抗特性,巧妙地设计了一种L/S双频段同轴组合馈线。经测试得到的方向图验证了该设计合理且简单有效。     

Finite-difference time-domain analysis of a stacked dual-frequencymicrostrip planar inverted-F antenna for mobile telephone handsets

We describe a stacked, dual-frequency microstrip planar inverted-F antenna (DF-PIFA) for mobile telephone handsets that can concurrently work in two frequency-bands, viz., those associated with the GSM and DCS 1800 systems operating at 0.9 GHz and 1.8 GHz, respectively. The proposed microstrip DF-PIFA is fed by a coaxial line, as opposed to two separated feed lines used in the conventional design. The design is carried out in a systematic manner and involves two steps. We begin with an initial configuration of the PIFA that is based on a standard design for a microstrip patch antenna fed by a coaxial line and is derived from an empirical approximation in conjunction with a transmission line model. Next, we employ a computer-aided design (CAD) tool, based on the nonuniform finite-difference time-domain (NU-FDTD) Maxwell solver, to optimize the performance characteristics of the DF-PIFA, including the return loss, the matching of the input impedance, and the far-field radiation patterns     

A minimally invasive antenna for microwave ablation therapies: design, performances, and experimental assessment

A new coaxial antenna for microwave ablation therapies is proposed. The antenna design includes a miniaturized choke and an arrowhead cap to facilitate antenna insertion into the tissues. Antenna matching and the shape and dimension of the area of ablated tissue (thermal lesion) obtained in ex vivo conditions are evaluated both numerically and experimentally, finding an optimal agreement between numerical and experimental data. Results show that the antenna is well matched, and that it is able to produce a thermal lesion with an average length of 6.5 cm and an average diameter of 4.5 cm in ex vivo bovine liver when irradiates 60 W for 10 min. Finally, the dependence of antenna performances on possible changes in the antenna's structure is investigated, finding an optimal stability with respect to manufacturing tolerances and highlighting the fundamental role played by the antenna's choke.     

Analysis of a horn-lens TEM antenna

Matching and radiation characteristics of a horn-lens TEM antenna are studied theoretically and experimentally in an ultra-wide frequency band. Theoretical results are obtained by means of electromagnetic modeling on the basis of the finite-element method. Antennas with different geometries of the lens and different methods of excitation of the TEM horn are considered. Direct excitation by a coaxial line and excitation with the use of a smooth transition from a coaxial line to a stripline are studied. The results of calculations are compared with experimental results.     

Influence of the magnetoelastic effect on the microwave magnetic properties of Fe-N thin films

The microwave permeability of nitrogen-doped iron films deposited on a flexible lavsan substrate has been experimentally investigated. The measurements were performed with the use of a coaxial transmission line. The coaxial samples were coiled from a flexible film in two different ways: with the deposited layer facing outward and with the deposited layer facing inward. It has been found that the measured frequency dependences of the permeability of the two samples strongly differ. A comparison is made between the microwave permeabilities measured in the coaxial transmission line and in a strip transmission line, in which planar samples were used. It is shown that investigation of the influence of the magnetoelastic effect on the magnetic properties at microwave frequencies can provide additional information on the magnetic structure of the material.     

Analysis of the characteristics of a polyconic antenna in a wide frequency band

A new type of the omnidirectional in the horizontal plane ultra-wideband antenna—a symmetric polyconic antenna—is proposed. Two design versions of the antenna are considered: (i) an antenna with hollow polycones and an antenna with polycones and flat bases. The finite-element method is used for modeling and optimization of the antenna parameters aimed at obtaining the maximum frequency band of matching an antenna with specified overall dimensions and a 50-Ω coaxial line. The integral-equation method is used to study the antenna patterns in a wide frequency band. The results of numerical modeling are compared with the results of a physical experiment.     

Microwave ablation with a triaxial antenna: results in ex vivo bovine liver

We apply a new triaxial antenna for microwave ablation procedures to an ex vivo bovine liver. The antenna consists of a coaxial monopole inserted through a biopsy needle positioned one quarter-wavelength from the antenna base. The insertion needle creates a triaxial structure, which enhances return loss more than 10 dB, maximizing energy transfer to the tissue while minimizing feed cable heating and invasiveness. Numerical electromagnetic and thermal simulations are used to optimize the antenna design and predict heating patterns. Numerical and ex vivo experimental results show that the lesion size depends strongly on ablation time and average input power, but not on peak power. Pulsing algorithms are also explored. We were able to measure a 3.8-cm lesion using 50 W for 7 min, which we believe to be the largest lesion reported thus far using a 17-gauge insertion needle.     

A coaxial antenna with miniaturized choke for minimally invasive interstitial heating

We present a new coaxial antenna for microwave interstitial coagulative therapy, working at 2450 MHz and endowed with a miniaturized sleeve choke in order to reduce back heating effects and make the system response less dependent on the antenna insertion depth into the tissue; the way the choke is implemented makes the overall transversal size minimum and allows small adjustments of the choke section length even during operation. We describe the main technical features of the antenna and show experimental results clearly proving the choke effectiveness. Numerical simulations well agree with experimental data, confirming the suitability of the proposed device for minimally invasive medical applications.     

Accurate characterization of planar printed antennas usingfinite-difference time-domain method

The finite-difference-time-domain method (FD-TD) is used to characterize complex planar printed antennas with various feed structures, which include coaxial probe feed, microstrip line feed, and aperture coupled feed structures. A coaxial probe model is developed by using a three-dimensional FD-TD technique. This model is shown to be an efficient and accurate tool for modeling coaxial line fed structures. A novel use of a dispersive absorbing boundary condition is presented for a printed antenna with a high dielectric constant. All the numerical results obtained by the FD-TD method are compared with experimental results, and the comparison shows excellent agreement over a wide frequency band     

射频同轴传输线的设计仿真与加工工艺

基于SU-8和BPN紫外负性感光胶,结合微电镀工艺加工制作射频同轴传输线,以实现射频器件信号的传输与耦合。首先确定在阻抗匹配情况下同轴传输线特性阻抗为50Ω的同轴传输线的具体尺寸,然后通过HFSS仿真软件对设计的结构进行模拟仿真。通过仿真结果验证设计的可行性,采用紫外光刻技术利用SU-8光刻胶做出内导体支柱,并用BPN光刻胶做出结构,对结构进行电镀。最后将BPN光刻胶剥离,即可得到射频同轴传输线。此方法制得的同轴传输线具有介质损耗小、辐射损耗小、无色散、带宽大和抗干扰强的优点,适用于高性能射频和微波电路。另外,它的制作工艺能与其他射频和微波器件及集成电路工艺兼容,便于与射频和微波电路集成。     

A floating sleeve antenna yields localized hepatic microwave ablation

We report a novel coaxial antenna for hepatic microwave ablation. This device uses a floating sleeve, that is, a metal conductor electrically isolated from the outer connector of the antenna coaxial body, to achieve a highly localized specific absorption rate pattern that is independent of insertion depth. This floating sleeve coaxial dipole antenna has low power reflection in the 2.4-GHz IMS band. Ex vivo experiments confirm our numerical simulation results. Index Terms-Ablation, coaxial aperture antennas, finite element methods, floating sleeve, microwave heating.     

Analysis of a novel expanded tip wire (ETW) antenna for microwave ablation of cardiac arrhythmias

A novel expanded tip wire (ETW) catheter antenna is proposed for microwave ablation for the treatment of atrial fibrillation (AF). The antenna is designed as an integral part of coaxial cable so that it can be inserted via a 6F catheter. A numerical model based on the rotationally symmetric finite-difference time-domain technique incorporating the generalized perfectly matched layer as the absorbing boundary condition has been utilized to accurately model the interaction between the antenna and the myocardium. Numerical and in-vitro experimental results are presented for specific absorption rate, return loss and heating pattern produced by the antenna. Both numerical modeling and in-vitro experimentation show that the proposed ETW antenna produces a well-defined electric field distribution that provides continuous long and linear lesions for the treatment of AF.     

Tissue dielectric measurement using an interstitial dipole antenna

The purpose of this study was to develop a technique to measure the dielectric properties of biological tissues with an interstitial dipole antenna based upon previous efforts for open-ended coaxial probes. The primary motivation for this technique is to facilitate treatment monitoring during microwave tumor ablation by utilizing the heating antenna without additional intervention or interruption of the treatment. The complex permittivity of a tissue volume surrounding the antenna was calculated from reflection coefficients measured after high-temperature microwave heating by using a rational function model of the antenna's input admittance. Three referencing liquids were needed for measurement calibration. The dielectric measurement technique was validated ex vivo in normal and ablated bovine livers. Relative permittivity and effective conductivity were lower in the ablation zone when compared to normal tissue, consistent with previous results. The dipole technique demonstrated a mean 10% difference of permittivity values when compared to open-ended coaxial cable measurements in the frequency range of 0.5-20 GHz. Variability in measured permittivities could be smoothed by fitting to a Cole-Cole dispersion model. Further development of this technique may facilitate real-time monitoring of microwave ablation treatments through the treatment applicator.     

A 2 1/4-turn spiral antenna for catheter cardiac ablation

To investigate the delivery of microwave energy by a catheter positioned inside the heart for ablating small abnormal regions producing cardiac arrhythmias, a 2 1/4-turn spiral catheter-based microwave antenna applicator has been developed. The antenna consists of the center conductor with continuous insulating material extending from the coaxial feed cable formed into a spiral antenna. The insulator completely isolates the center conductor from tissue. Phantom experiments were performed on homogeneous tissue equivalent medium. The reflection coefficient of the antenna at different frequencies and for different spiral lengths, the time course and temperature profile of an ablation, and the dosimetry of power versus temperature, all indicate that the high-power heating patterns from this antenna are both wider and deeper than with the other microwave antenna systems and radio-frequency electrodes.