SAR distributions in interstitial microwave antenna arrays with asingle dipole displacement

The use of interstitial microwave antenna array hyperthermia (IMAAH) as a treatment for cancer, in conjunction with radiation therapy and chemotherapy, has been investigated widely. The heating pattern produced by a coherently phased 915 MHz asymmetric antenna array displays the maximum power deposition in the array center. The authors report on the effect of variable insertion depth between antennas of an array on the heating patterns produced. The study of this heating behavior demonstrates a similar effect to that of the variably phased arrays, showing a shift of the heating peak towards the periphery of the tumor, offering a more simple method for the clinical treatment of such tumors     

FDTD analysis of power deposition patterns of an array ofinterstitial antennas for use in microwave hyperthermia

The finite-difference-time-domain (FDTD) method is used to calculate EM power deposition patterns in inhomogeneous tissue models of tumors. The radiated near-field patterns from an array of uniformly and step-insulated interstitial antennas were used as incident fields on the excitation planes to calculate the scattered fields and SAR patterns in tumors. A comparison of data shows that the FDTD solution, in this particular application, overcomes some of the modeling difficulties encountered in the method of moments. Results for some specific tumor geometries are also presented to show the effectiveness of the microwave interstitial heating techniques in treating large tumors. Other advantages of the FDTD method, such as improved accuracy in modeling dielectric interfaces and the ability to model large tumors, are illustrated     

Microwave hyperthermia induced by a phased interstitial antennaarray

An interstitial microwave antenna array for hyperthermia cancer treatment is investigated. The purpose is to generate both uniform and controlled nonuniform temperature distributions in biological tissue by modulating the phases of the signals applied to each antenna. The array has four antennas positioned on the corners of a 2 cm square. The distributions of absorbed power within the arrays are computed and then converted into temperature distributions through a heat conduction simulation. The temperature patterns over phantom muscle are presented in both the lateral plane (perpendicular to the antennas) and the axial plane (parallel to the antennas). It is found that by proper phase modulation of radiofrequency signals applied to each antenna, a uniform heating can be produced in the entire array volume     

Power density and temperature distributions produced by interstitial arrays of sleeved-slot antennas for hyperthermic cancer therapy

A graded-mesh finite-difference time-domain (FDTD) code, together with an alternate-direction-implicit finite-difference (ADI-FD) solution of the bioheat equation, are used for studying arrays of sleeved-slot antennas imbedded in a brain-equivalent phantom. The FDTD code allows efficient and accurate modeling of the fine structure of each antenna and of a sufficiently wide surrounding region. The ADI-FD solution of the bioheat equation allows evaluation of transient and steady-state temperature distributions in the brain-equivalent phantom with acceptable computational costs. The solution of the dosimetric-thermal problem in the volume irradiated by the antenna array permits the assessment of dimensions of the region where the temperature increase is above 43/spl deg/C (the threshold for an effective hyperthermia treatment) as a function of the array input power. Arrays made of three identical antennas placed at the vertices of equilateral triangles of 10-, 15-, and 20-mm sides have been studied. The temperature of 43/spl deg/C is reached in approximately 3 min in a deep-seated tumor region, from 10 to 40 mm in diameter, by applying input power levels between 2-32 W.     

Theoretical and experimental SAR distributions for interstitialdipole antenna arrays used in hyperthermia

Theoretical predictions and experimental measurements of power deposition in muscle tissue phantoms are compared for various arrays of microwave dipole antennas used for hyperthermia cancer therapy. The antennas are linear coaxial dipoles which are inserted into small nylon catheters implanted in the tumor volume. The specific absorption rate (SAR) patterns for a 2-cm square array of four 915-MHz antennas are presented for both resonant and nonresonant dipoles. Arrays of dipoles with lengths much shorter than the resonant half-wavelength have a far more reactive input impedance and a much smaller absolute SAR magnitude in the array center than is seen for arrays of resonant dipoles, and the maximum SAR shifts from the array center to the antenna surfaces. The absolute length of the volume heated by the small-diameter antennas with the longer half-wavelength was longer than that of the larger-diameter antennas. SAR distributions for 4-cm square arrays of eight and nine antennas fed with equal amplitude and phase are also compared. It is shown that much of the array volume has a power deposition less than 25% of the maximum SAR and that the distribution is nonuniform for both the eight- and nine-antenna configurations     

A uniformly valid loaded antenna theory

A quasi-static layered approximation is used to simplify the layered solution for insulated antennas to the solution of a generalized impedance boundary value problem, whose solution is expressed in terms of an integral. This integral applies to insulated antennas imbedded in a dense medium, insulated antennas imbedded in air (dielectric-coated antennas), and impedance-loaded antennas, all referred to as loaded antennas. The branch cut contribution for large distances is given by the Sommerfeld space wave formula. The physical transition of loaded antennas to bare antennas is investigated through the asymptotic evaluation of this integral. Simple uniform formulas for loaded antenna current are derived and generalized to cover the same range of validity as the integral. The direct calculation of the input admittance is consistent with the derived uniform formula for antenna current. For insulated antennas in a dense medium, the complete transmission line theory describes the antenna current through the transition to bare antennas     

SAR Patterns from an Interstitial Microwave Antenna-Array Hyperthermia System

In recent years, a number of groups have been investigating the use of interstitial microwave antenna-array hyperthermia (IMAAH) systems for the treatment of superficial and deep-seated tumors. A critical parameter in any hyperthermia system is the SAR (specific absorption rate) pattern, which gives information about the energy absorption in the tumor and surrounding tissue. In this paper, we compare the theoretical and measured values of the SAR distribution for an array of four 915-MHz antennas implanted on the corners of a 2-cm square array. The overall length of the antennas was assumed to be 7 cm theoretically and was either 6 or 7 cm for the measurements. In general, there was good agreement between the theoretical predictions and the experimental results. In particular, both theory and experiment demonstrated that the maximum SAR occurred in the junction plane of the antenna array and at the center of the square array. Similarly, both showed that the Iongitudinal extent of the heating pattern is about 5 cm for a single antenna, but closer to 3 cm for an array where the Iongitudinal extent is defined by the length in the longitudinal direction where the SAR is greater than 50 percent of the maximum. The experiments showed that the SAR patterns were quite reproducible, and that the use of double stub tuners to minimize the reflected power for each antenna resulted in poorer SAR patterns due to phase variations created by the tuners. However, the use of an attenuator with each antenna to equalize the maximum SAR from each antenna did improve the SAR distributions slightly.     

Heating patterns generated by phase modulation of a hexagonal arrayof interstitial antennas

In this paper, we investigate an array of six interstitial microwave antennas used for hyperthermia cancer treatment. The purpose is to generate both uniform and controlled nonuniform heating patterns in biological tissue by phase modulating the signals applied to each antenna. The array consists of six antennas positioned on the corners of a hexagon. The distance between two diagonal antennas is 4 cm. The distributions of absorbed power per unit mass within the array are computed, and then converted into temperature distributions through a thermal conduction simulation. The SAR and temperature patterns are presented in both the lateral plane (perpendicular to the antennas) and the axial plane (parallel with the antennas). By proper phase modulation of microwave signals applied to each antenna, a uniform heating pattern can be produced within the entire array volume. Also, a peripheral heating pattern may be generated around the array; again, by using the proper phase modulation. The modulation schemes for generating both types of heating patterns are discussed.     

Design optimization of interstitial antennas

To improve the performance of interstitial antennas for microwave hyperthermia, parameters such as the uniformity of the heating pattern, the depth of penetration, and the impedance matching properties must be optimized. We examined analytically and experimentally the radiation characteristics of multisection insulated antennas in conductive tissue. The effects of varying the diameters and lengths of the center conductors in the various sections of the antenna and the diameter and type of the insulation on the electromagnetic power deposition pattern and input impedance characteristics were examined. A new approximate numerical model which calculates the current distribution and the radiation characteristics of multisection insulated antennas was developed. The numerical predictions were verified in a qualitative way experimentally by mapping the various near- and far-field components of these antennas. Based on the obtained results, design tradeoffs are identified and quantified, and guidelines for optimum designs are specified. In particular, it is shown that an insulation-to-center-conductor diameter ratio between 1.5 to 2.0 is optimum for a uniform Teflon insulation, and that a multisection arrangement with the thinnest insulation near the antenna tip has superior performance compared with the uniform insulation or other multisection designs.     

Comparison of power deposition by in-phase 433 MHz andphase-modulated 915 MHz interstitial antenna array hyperthermia systems

The interstitial microwave antenna array hyperthermia (IMAAH) system produces a pattern of specific absorption rate (SAR) that is nonuniform within a 2-cm square array when driven in phase at 915 MHz. It was found that phase modulation makes the time-averaged SAR pattern significantly more uniform in planes perpendicular to the antennas. To drive antennas in phase at 433 MHz similarly improves SAR uniformity when the antennas are of resonance length     

Radiation patterns of dual concentric conductor microstrip antennasfor superficial hyperthermia

The finite difference time domain (FDTD) method has been used to calculate electromagnetic radiation patterns from 915-MHz dual concentric conductor (DCC) microwave antennas that are constructed from thin and flexible printed circuit board (PCB) materials. Radiated field distributions are calculated in homogeneous lossy muscle tissue loads located under variable thickness coupling bolus layers. This effort extends the results of previous investigations to consider more realistic applicator configurations with smaller 2 cm-square apertures and different coupling bolus materials and thicknesses, as well as various spacings of multiple-element arrays. Results are given for practical applicator designs with microstrip feedlines etched on the backside of the PCB antenna array instead of previously tested bulky coaxial-cable feedline connections to each radiating aperture. The results demonstrate that for an optimum coupling bolus thickness of 2.5-5 mm, the thin, flexible, and lightweight DCC antennas produce effective heating to the periphery of each aperture to a depth of approximately 1 cm, and may be combined into arrays for uniform heating of large area superficial tissue regions with the 50% power deposition contour conforming closely to the outer perimeter of the array     

共享孔径交错稀疏阵列天线互耦误差建模与校正

共享孔径交错稀疏阵列天线是实现多功能阵列天线的有效途径。现有的参数化互耦消除方法都是针对均匀阵列天线展开的,其研究的互耦矩阵都是规则的方阵,对共享孔径交错稀疏阵列天线的互耦矩阵模型并不适用。在充分考虑共享孔径交错稀疏阵列天线中子阵内互耦的“稀疏”和“方位依赖”的特殊性后,通过将常规的互耦矩阵扩展表示为“非方”的“增广互耦矩阵”来对交错稀疏阵列天线子阵内和子阵间的耦合效应进行建模,并通过“增广互耦矩阵”的参数化估计最终实现了共享孔径交错稀疏阵列天线互耦误差的建模与校正。仿真结果证实了所提方法的有效性和可行性。     

NUMERICAL SIMULATION OF ANNULAR PHASED ARRAY OF DIPOLE ANTENNAS USING THE FD-TD METHOD

The problem for calculating near fields of EM radiation systems by using the finitedifference time domain(FD-TD)method are discussed and the annular phased array of dipoleantennas has been simulated numerically by use of the FD-TD method.For a test run thenear field and current distribution of the single dipole antenna are calculated.The near fieldsof the annular phased array of dipole antennas in central region filled with deionized water arecomputed and the interaction of near fields with an anatomically-based inhomogeneous model ofhuman torso is considered as well.     

Optimal Polarized Beampattern Synthesis Using a Vector Antenna Array

Using polarized waveforms increases the capacity of communication systems and improves the performance of active sensing systems such as radar. We consider the optimal synthesis of a directional beam with full polarization control using an array of electromagnetic vector antennas (EMVA). In such an array, each antenna consists of $pgeq 2$ orthogonal electric or magnetic dipole elements. The control of polarization and spatial power patterns is achieved through carefully designing the amplitudes and phases of the weights of these dipole antennas. We formulate the problem in a convex form, which is thus efficiently solvable by existing solvers such as the interior point method. Our results indicate that vector antenna arrays not only enable full polarization control of the beampattern, but also improve the power gain of the main beam (over the sidelobes), where the gain is shown numerically to be linearly proportional to vector antenna dimensionality $p$. This implies that EMVA not only offers the freedom to control the beampattern polarization, but also virtually increases the array size by exploiting the full electromagnetic (EM) field components. We also study the effect of polarization on the spatial power pattern. Our analysis shows that for arrays consisting of pairs of electrical and magnetic dipoles, the spatial power pattern is independent of the mainbeam polarization constraint.      

The calculated and measured temperature distribution of a phasedinterstitial antenna array [invasive applicators]

The power deposition pattern of four antennas, positioned on the corners of a 2-cm square array with different driving phases, is computed under the assumption of negligible coupling between the antennas. The spatial SAR (specific absorption rate) distribution is calculated by modeling each interstitial applicator as an insulated, asymmetric dipole. For comparison with the heating patterns measured by a thermal video system, the calculated SAR distributions are converted into temperature patterns through an electric network simulation of the heating in artificial muscle tissue. At each nodal point of a grid in the thermal system, the absorbed microwave power (or SAR times density), thermal resistivity, heat capacitance, and temperature are simulated, respectively, as current source, electrical resistance, electrical capacitance and potential. Therefore, solving the equivalent electric network on a computerized simulation routine (SPICE) yields the temperature distribution. In both the axial and transverse planes, the resulting temperature distributions from the antenna array, with various driving phases, agree very well with the measured temperature patterns     

一种赋形阵列天线的优化设计

给出一种S 波段波束赋形阵列天线的优化设计过程,由带状耦合结构的印刷阵子天线并排组成16 元直线阵列。为使天线的辐射波束达到给定方向图的主瓣和副瓣要求,采用联合应用DFP 和BFGS 公式的变度量优化算法,对阵列天线各单元的馈电幅度和相位同时进行优化,并考虑阵列中各单元之间的互耦对阵列的影响,通过全波电磁仿真软件HFSS 进行了验证,所得的方向图与理论计算十分吻合。根据仿真优化的结果,制作了天线阵实物并进行了测试,得到的测试结果与仿真结果一致。     

Numerical annealing of low-redundancy linear arrays

An algorithm is developed that estimates the optimal distribution of antenna elements in a minimum redundancy linear array. These distributions are used in thinned array interferometric imagers to synthesize effective antenna apertures much larger than the physical aperture. The optimal selection of antenna locations is extremely time consuming when large numbers of antennas are involved. This algorithm uses a numerical implementation of the annealing process to guide a random search for the optimal array configuration. Highly thinned low-redundancy arrays are computed for up to 30 array elements. These arrays are equivalent to the optimal solutions that are known for up to 11 elements. The arrays computed for 12-30 elements have the fewest redundancies reported to date     

Design of 45-degree Linearly Polarized Substrate Integrated Waveguide-fed Slot Array Antennas

This paper presents a method of designing substrate integrated waveguide-fed (SIW-fed) slot array antennas. The design theory is based on the circuit model of slot and via as well as the reflection canceling. To prove the feasibility of this method, a 10-element K-band SIW-fed 45-degree linearly polarized slot array antenna with uniform power distribution is designed. By full-wave simulation, the antenna has a good impedance bandwidth of 7.5% and uniform power distribution. Besides, a maximum gain of 15.3dBi is obtained in the broadside and the cross polarization is suppressed below -23.5dB in the boresight. This type of SIW-fed slot array antennas can be a good candidate for microwave and millimeter-wave applications, especially for auto-motive collision-avoidance radar systems.     

The Effects of Driving Frequency and Antenna Length on Power Deposition Within a Microwave Antenna Array Used for Hyperthermia

The theory of the linear, insulated antenna embedded in an electrically dense medium is applied to microwave antennas used for hyperthermia cancer therapy. The pattern of power deposition is computed for a square array of four antennas with a side length of 3 cm under the assumption of no coupling among antennas. The driving frequency is set to seven values between 300 and 915 MHz, and the antenna halflength is set to three values: 3 cm, 6 cm, and the resonant value.     

Multiple-pattern linear antenna arrays with single prefixedamplitude distributions: modified Woodward-Lawson synthesis

The authors describe the design of linear antenna arrays that generate multiple radiation patterns by switching between different excitation phase distributions while maintaining a constant pre-established amplitude distribution. Examples are given of antennas with uniform or Gaussian amplitude distributions that, depending on their phase distribution, generate a sum pattern, a flat-topped beam or a cosec-squared pattern