Optimal power deposition with finite-sized, planar hyperthermiaapplicator arrays

Improved hyperthermia applicator technology is allowing finer spatial power resolution within the heated tissue volume. Effective utilization of these planar applicator arrays requires an understanding of the interrelationships between the lateral dimensions of the tumor and the applicators, the power field produced by the applicators, the amount of surface cooling, the tumor tissue blood perfusion, and the normal tissue blood perfusion. These interrelationships are investigated using three-dimensional power patterns and temperature fields produced by optimizing the power amplitudes of the individual applicators located within an array of small, but finite, planar applicators. Five major conclusions are obtained. First, optimization works and is effective in determining optimal power fields. Second, for optimal treatments the lateral dimensions of a single superficial applicator need to extend beyond the tumor boundary. Third, surface cooling is needed to reduce the high normal tissue temperatures at shallow depths. Fourth, finer power resolution becomes more important as the tumor size decreases, but, little improvement in the temperature field is achieved beyond a 3 x 3 array configuration. Fifth, increasing the normal blood perfusion rate can decrease the temperature on the tumor boundary if direct power deposition on that boundary is unavailable.     

The performance of a new 915-MHz direct contact applicator with reduced leakage

The technical feasibility of commercially developing a safe and effective direct contact diathermy applicator operating at the industrial, scientific, medical (ISM) frequency of 915 MHz is demonstrated. The basic design consists of a circular waveguide which is internally loaded with two orthogonal pairs of forward ridges to obtain circular polarization and two rear ridges with a probe to excite the guide. Two prototype designs are considered: the small applicator (15 cm diameter) has one annular choke covered with a 2.5-cm thick microwave absorber, and the large applicator (25 cm diameter) has two additional concentric chokes to limit leakage radiation. The performance of the applicators was evaluated in terms of the requirements of a ORH microwave diathermy test protocol to control stray radiation and deliver a thermally effective absorbed dose rate to simulated muscle tissue of a phantom with a 1-cm or 2-cm fat layer. The net power required to deliver a thermally effective 235-W/kg specific absorption rate (SAR) to such a planar phantom was determined. For this net power, leakage levels considerably less than 5 mW/cm2 (at 5 cm from applicator-phantom boundary) were obtained for the applicators in direct contact with the phantom. If a small spacing (1 cm) between these applicators and planar phantoms is introduced, the net power required to deliver an effective SAR to a phantom and the associated leakage can become excessive. For the small applicator, the required net power for inducing an SAR of 235 W/kg in muscle tissue of a 1-cm fat layer phantom is about 330 W and the leakage is about 120 mW/cm2. For a 2-cm fat layer phantom, these values are somewhat higher. For the large applicator, using a 1-cm fat layer phantom, the values are about 200 W and about 17 mW/cm2. Again, for a 2-cm fat layer phantom, these values are somewhat higher.     

Absorbed Power Distributions from Coherent Microwave Arrays for Localized Hyperthermia

Absurbed power distributions in a homogeneous muscle-like tissue due to a planar coherent array consisting of 16 small direct contact applicators at 434 and 915 MHz are calculated, assuming various relative phases and amplitudes are compared with that of a single aperture source at the same frequency with the same overall dimensions. The array applicator may offer improvement in field size or, when focused, a small improvement in penetration, but in practice the performance is very dependent upon bolus thickness. An additional advantage of the array applicator is the ability to modify absorbed power distributions during use by changing the amplitudes of individual applicators.     

A Comparison of the Annular Phased Array to Helical Coil Applicators for Limb and Torso Hyperthermia

The BSD annular phased array (APA) and miniature annular phased array (MAPA) have been compared to helical coil applicators designed for the trunk and thigh, respectively. All four applicators were tested using both cylindrical saline phantoms and a phantom-filled mannequin. Patterns of deposition were determined using both an implantable electric field probe and local values of the rate of temperature increase. Scattered fields were measured when the APA and trunk helix were used with the phantom-filled mannequin.     

Sustaining long linear uniform plasmas with microwaves using aleaky-wave (troughguide) field applicator

Linear, uniform, high density plasma sources can be realized using linear microwave field applicators. Such systems can be applied to process large surfaces of materials that can be made to move transversely across the plasma source. This paper details the design and operating characteristics of appropriate linear applicators that are of the leaky-wave type. A general analysis of longitudinally uniform leaky-wave-sustained discharges provides us with the generic features of the corresponding field applicators. On practical ground, we turn to troughguide structures which are well known in the field of antenna engineering; we consider periodically asymmetric troughguides terminated with a matched load and show experimentally that they can be designed to achieve the required linear uniform discharge. To assure the operation of the applicator at microwave powers of the order of one kW or more, we have devised and tested a new kind of ridge-waveguide mode transformers allowing one to feed microwave power to the applicator directly from a standard rectangular waveguide. The main advantages of the leaky-wave applicators are their relative simplicity of design and operation (no tuning elements), a good power match with the microwave generator and a good longitudinal uniformity of the sustained plasma     

Arrays of multielement ultrasound applicators for interstitialhyperthermia

Arrays of multielement ultrasound applicators for interstitial hyperthermia have been developed and tested both in vitro and in vivo. The system includes multielement applicators, a 64 channel RF driving unit, a power measuring unit, a 112 channel multisensor temperature measuring unit, and a water cooling unit. Ninety-five arrays of single-element and nine arrays of three-element ultrasound applicators were designed, built, and characterized by measuring transducer efficiency and ultrasound field distribution. Improved uniformity in the azimuthal direction was achieved by using multiple driving frequencies. In addition, production of ultrasound in a desired sector of the transducer was possible by selecting a suitable frequency. Both in vitro and in vivo experiments showed that 92% of monitored temperature points within the target volume of 30 mm x 30 mm x 35 mm achieved a therapeutic temperature rise (above 5 degrees C) when an array of five three-element applicators were used. These results indicated that the arrays of multielement ultrasound applicators have distinct advantages over present interstitial hyperthermia modalities in terms of the capability to control the temperature distribution with a large catheter spacing. As a conclusion, the feasibility of a practical arrays of multielement ultrasound applicators for interstitial hyperthermia was demonstrated.     

SAR CHARACTERIZATION OF FOCUSED PLANAR ARRAY OF WATER-LOADED MODIFIED BOX-HORNS FOR HYPERTHERMIA

A 4×4 planar array of modified box-horns as a microwave hyperthermia applicator is theoretically studied to characterize power deposition (SAR) in heating tissue (muscle) at 2450 MHz. A modified box-horn is a novel improved version of conventional box-horn in which horn exciting the box waveguide is flared in both E-and H-planes. Modified box-horn supports TE₁₀ and TE₃₀ modes. The amplitude distribution over the H-plane of the box-horn aperture is a closer approximation to the uniform distribution. It is proposed that the interior of the box-horn be filled with water to provide a better impedance match to biological tissue. By applying Fresnel-Kirchhoff scalar diffraction field theory, the expression for electric field in heating region is derived and distribution of specific absorption rate (SAR) in that region due to planar array of modified box-horns as direct contact applicator is evaluated at 2450 MHz. The results of modified box-horn array are compared with those of a single modified box-horn operating at the same frequency. Results demonstrate that planar array of modified box-horns offers improvement in SAR distribution and penetration depth. It is shown that by changing the phase and amplitude of excitation of the modified box-horns of the array, the relative amplitude and position of the hot spot can be changed. The present analysis is validated through the results obtained by plane wave spectral technique.     

Use of the impedance method to calculate 3-D power depositionpatterns for hyperthermia with capacitive plate electrodes

We have used the three-dimensional impedance method to calculate the power deposition in the human pelvic region due to a radio frequency current at 13.56 MHz applied through round or oval capacitive-plate applicators. We compare the energy deposited in several tumor sites when energy is applied using a variety of applicator sizes, locations, and boluses of various conductivities. We show detailed maps of the power deposition in these cases for selected regions of the body and suggest useful configurations of applicators for heating tumors in several locations in the body.     

Contact flexible microstrip applicators (CFMA) in a range from microwaves up to short waves

Contact flexible microstrip applicator (CFMA) is a new light-weight microstrip applicator type for superficial and deep local hyperthermia. Typical specimens are developed for operation at frequencies of 434, 70, 40, and 27 MHz. The main common features of CFMA, namely, their flexibility and light weight, as well as their aperture dimensions slightly depend on the operating frequency. Two antenna types are used in CFMAs: inductive antennas with a radiating plane electrical dipole at microwaves, and coplanar capacitive antennas, providing depression of the normal component of the electrical field in the very high-frequency (VHF) and high-frequency (HF) range. The flexibility of the applicators enables one to conform them with curved surfaces. In a bent state of the applicators there arises a focusing effect of energy deposition in deeper located tissues due to linear polarization of the irradiated electromagnetic (EM) field, inherent in CFMA. All CFMA are integrated with silicon water boluses which serve as a matching element, so as a skin cooling agent. Due to this and to the predominance of the tangential electrical component in the radiated EM field, no fat overheating effects are noticed, as a rule. The aperture of the developed applicators overlap the range 160-630 cm2 providing effective heating field sizes (EFSs) 64-400 cm2, respectively. The most bulky CFMAs with an aperture of (21 x 29) cm2 operating at the frequency of 434 MHz weigh 0.8 kg and 2.5 kg at 27 MHz. Phenomenological analysis of the radiating systems, as well as experimental evaluation of the applicators are presented. CFMAs operating at frequencies of 434 and 40 MHz are used in clinical practice. CFMA at 70 and 27 MHz are subjected to laboratory clinical investigations.     

Body conformable 915 MHz microstrip array applicators for largesurface area hyperthermia

The optimal treatment with hyperthermia of superficially located tumors which involve large surface areas requires applicators which can physically conform to body contours, and locally alter their power deposition patterns to adjust for nonuniform temperature caused by tissue inhomogeneities and blood flow variations. A series of 915 MHz microstrip array applicators satisfying these criteria have been developed and clinically tested. Clinical and engineering design tradeoffs for practical devices are discussed. Measurements taken in tissue equivalent phantoms and a summary of our clinical experiences with these microstrip arrays are presented.     

Y-Junction semiconductor laser arrays: Part II--Experiments

Laser arrays employingY-junctions to coherently couple the emitters in-phase have been fabricated by metalorganic chemical vapor deposition. The far-field patterns are stable and centered normal to the facet. They are typically multilobed in accordance with the diffraction pattern of multiple emitters. By the use of flared waveguides near the facets the near-field fill factor can be increased to greater than 80 percent. This modification concentrates the radiation in the central diffraction lobe. Power outputs of 200 mW CW have been obtained from a ten element array of 1.5μm wide emitters.     

Low-Frequency RF Twin-Dipole Applicator for Intermediate Depth Hyperthermia

In studies on heating deep-seated tumors, various attempts have been made to develop radiofrequency applicators and to confine the controlled volumes into limited sizes at variable useful depths. Results of the present investigation show that the conduction-current mechanism dominates the heating with magnetic dipoles working at frequencies as low as 27 MHz, and that two single-magnetic dipoles forming a loosely coupled pair (twin-dipole applicator) fed by low-frequency, in-phase currents, give a better performance than a single dipole of the same size, due to the phase coherence of the superimposing fields. A number of single-dipole and twin-dipole applicator working at 27 MHz have been developed and given the fundamental tests on phantoms simulating muscle and fat tissues. The results obtained show the feasibility of the proposed applicator to produce a penetration depth up to 7 cm and a power deposition pattern showing a well-defined maximum, which may undergo a controlled shift of a few centimeters in depth. Moreover, the surface overheating may be easily controlled. A circuit design is described that improves the matching and the uniformity of the power deposition pattern. A preliminary calculation in the quasi-static-fields approximation of the electric field induced by the twin-dipole applicator in air is also described.     

带电子聚焦的氧化锌场发射背光源

液晶显示需要低功耗、均匀度好的光源作为背光源。为了降低功耗,提高显示图像质量,未来的液晶显示需要实现对背光源进行时间与阵列调制。对比现有的液晶显示用背光源,研究制备了氧化锌场发射光源。该光源采用氧化锌纳米针作为场发射阴极,采用平面栅极作为门电极调制结构实现亮度的连续可调,通过带有氧化镁二次电子发射层的金属栅网对电子进行聚焦实现光源的均匀照度。实验结果表明,带电子聚焦的氧化锌场发射光源具有较低的开启场强(1.1V/μm),较小的电压调制区间(小于150V),较高的发光强度(大于1 000cd/m2),且基于电子聚焦结构的设计,实现了光源的均匀稳定照度,可以提高液晶显示的图像质量。带电子聚焦结构的氧化锌场发射光源,既可实现对发光的时间与阵列调制,同时能提高发光的均匀性,将可作为液晶显示的理想背光源。     

EM Local Heating with HF Electric Fields

Capacitor-plate applicators consisting of a pair of flat-plate electrodes, energized by a HP voltage, are utilized to induce heating inside a biological body for the purpose of hyperthermia cancer-therapy. In this paper, a theoretical analysis for such applicators is presented. A pair of coupled integral equations for the unknown total induced electric field inside the body and the charge density on the electrodes is derived and solved numerically for several different cases. The distribution of the specific absorption rate (SAR) of energy inside the body is obtained for each case. The body-electrode coupling is taken into account. A theoretical scheme for synthesizing proper potential distributions on two arrays of subelectrodes for inducing a desirable SAR ditribution inside the body is also developed. Using such arrays of subelectrodes, the excessive heating at the fat layer of the body may be avoided. An experiment has been conducted to test the theory.     

A Whole Body Thenmal Model of Man During Hyperthermia

A whole body thermal model of man has been developed to predict the changes in regional temperatures and blood flows during hyperthermia treatments with the miniannular phased array (MAPA) and annular phased array (APA) applicators. A model of the thermo-regulatory response to regional heating based on the experimental and numerical studies of others has been incorporated into this study. Experimentally obtained energy deposition patterns within a human leg exposed to the MAPA were input into the model and the results were compared to those based upon a theoretical deposition pattern. Exposure of the abdomen to the APA was modeled with and without the aberrant energy deposition that has been described previously. Results of the model reveal that therapeutic heating (42°C) of extremity soft tissue sarcomas is possible without significant systemic heating. Very high bone temperatures (50°C) were obtained when the experimental absorption pattern was used. Calculations show that systemic heating due to APA exposure is reduced via evaporative spray cooling techniques coupled with high-velocity ambient air flow.     

表面波放电狭缝天线辐射电磁场的三维数值分析

针对平板型表面波放电等离子体源,建立了表面波放电狭缝天线辐射电磁波模型,对狭缝天线辐射电磁场分布进行了三维数值计算,并与表面波电磁场进行对比分析,讨论了平板型表面波放电机理。结果表明:整个狭缝天线阵激发的电磁场是每个狭缝天线激发电磁场的线性叠加;狭缝天线阵直接激发的电磁场强度在临近波导壁面处很大,并且随着空间距离的增大迅速衰减;狭缝天线阵直接激发和表面波的电场均远大于各自的磁场,分析电、磁场对带电粒子的力作用时可以忽略磁场力的作用;表面波电磁场远大于狭缝天线阵直接辐射的电磁场,强电磁场范围也远大于狭缝天线阵直接激发的强电磁场范围,等离子体有增强电磁场强度、扩大强电磁场范围的作用。     

平面数字阵列雷达的子阵级波束形成算法

大规模平面数字阵列雷达因为其优点得到广泛应用,但其全自适应处理实现困难,实际中广泛采用部分自适应处理,子阵结构就是其中的一种重要方法。该文针对有幅度加权的均匀平面阵,以各子阵噪声输出功率相等为原则进行非均匀邻接子阵划分,并通过归一化算法保证各子阵噪声功率相等,进而对平面阵子阵结构的波束形成性能进行了研究。计算机模拟仿真结果表明,该子阵结构的二维相扫和一维相扫的自适应方向图保形良好,可以达到与全自适应相接近的干扰抑制性能。     

Effect of Mutual Coupling on a Mobile-Radio Maximum Ratio Diversity Combiner With a Large Number of Branches

The effect of mutual coupling on a space diversity combined signal from an optimum resistive load network lessens when the antenna spacingdand the number of elementsMincrease. Whendis greater than 0.2λ, the cumulative distribution curves of anM-branch signal with mutual effects included show only a small difference compared to an array with no coupling, and are close to the cumulative distribution curve of an array withMindependent branches. For the average SNR of an array with 100 branches obtained from an optimum resistive load network to be within -3 dB of the SNR from independent branches the required antenna spacing is at least 0.25λ, for an in-line array and 0.5λ, for a planar array. Since a planar array is much smaller than an in-line array for equal performance, the planar array is a more desirable configuration.     

Direct computation of ultrasound phased-array driving signals froma specified temperature distribution for hyperthermia

This paper presents a new method which obtains ultrasound hyperthermia applicator phased-array element driving signals from a desired temperature distribution. The approach combines a technique which computes array element driving signals from focal point locations and intensities with a new technique which calculates focal point locations and power deposition values from temperature requirements. Temperature specifications appear here as upper and lower bounds within the tumor volume, and a focal point placement algorithm chooses focal patterns capable of achieving the temperature range objective. The linear algebraic structure of the method allows rapid calculation of both the phased-array driving signals and an approximate temperature field response. Computer simulations verify the method with a spherical section array (SSA) for a variety of temperature specifications and blood perfusion values. This scheme, which applies to any phased-array geometry, completes an essential step in both treatment planning and feedback for hyperthermia with ultrasound phased-array applicators.     

The effect of illumination on Gunn oscillations in epitaxial GaAs

Coherent Gunn oscillations have been observed in long samples of epitaxial GaAs grown by both the solution and the vapor-transport methods on semi-insulating GaAs substrates. Samples of low carrier concentrations which do not exhibit Gunn oscillations, and samples which oscillate incoherently in the dark, oscillate coherently when illuminated. Illumination is observed to lead to an improvement in the peak current-to-valley current ratio.