Analysis of the helix as an RF applicator for hyperthermia

A coaxially layered bone-muscle-fat-skin model of the human upper arm has been used to analyse the helix as an indicator for RF hyperthermia. Calculated values of wave-length for propagation in the system agree well with experimental results. Optimisation of the applicator is discussed.     

Induction of hyperthermia using an intracavitary multielementultrasonic applicator

In this paper, the possibility of inducing controlled hyperthermia in rectal or vaginal wall tumors using an intracavitary ultrasonic applicator was investigated. A computer model that took into account the thermal and ultrasonic properties of tissues and surface cooling was used to optimize the transducer parameters to obtain desirable temperature distributions for different perfusion situations in the tumor. Also, an applicator that consisted of a cylindrical array of five independently controllable ultrasonic transducers was developed. This array was then tested in degassed water to determine the functional characteristics. This same applicator, modified to include water cooling of the tissue surface, was tested in vivo in dogs. The temperature distributions were found to be promising and with modifications this approach will be used in clinical treatments of suitable tumors.     

Power deposition properties of a travelling-wave applicator forinterstitial hyperthermia

An interstitial hyperthermia applicator is described that is capable of providing uniform heating over an extended range of depths. The applicator utilizes lossy two-wire transmission lines that are terminated in p-i-n diodes that may be biased on or off. A pair of these transmission lines within a single applicator allows a variety of symmetrical and asymmetrical patterns to be produced. Theoretical predictions of the applicator's performance are provided that are based on empirically determined transmission line characteristics     

Dielectric-loaded lens applicator for microwave hyperthermia

The authors describe a lens applicator which can concentrate the E-plane field in a lossy medium and is constructed of an easily fabricated, dielectric-filled waveguide. Since the microwave field distribution on the aperture can be varied by using different sizes of dielectric material, the heating pattern inside the medium can be changed, allowing the configuration of a compact applicator that can heat portions deeper within the human body. Heating experiments on a model simulating human muscle have shown that an applicator with an aperture of 150*100 mm/sup 2/ achieves a maximum heating depth of over 80 mm, results that are compatible with the deep, localized heating required by hyperthermia.<>     

An insulated dipole applicator for intracavitary hyperthermia

Specific absorption rate (SAR) patterns for an insulated sleeve dipole applicator, operating at 433 MHz, were measured by infrared thermography. The applicator was modeled using classical transmission-line theory, and experimental and theoretical results were compared. In general, agreement between measured and calculated SAR values was good. However, at the antenna feedpoint, the measured values were appreciably higher than the calculated values. This indicates that inclusion of junction effects would improve the model     

A multiloop concentric hyperthermia applicator with enhancedpenetration depth

The electromagnetic field deposition into a three-layer cylindrical human-body model by a multiloop concentric hyperthermia applicator is investigated analytically. The multiloop radiator axis is taken to be coincident with the cylinder axis. A technique based on the method of separation of variables is used to determine the axisymmetric field in every point. In order to compute the imposed specific absorption rate in the tissues, a numerical integration technique is utilized. Numerical results are presented for several loop geometries at a 13-MHz operation frequency. The possibility of obtaining improved in-depth heating in comparison with conventional single-magnetic-loop applications is investigated. It is shown that significant enhancement of the penetration depth can be obtained by using a simple phased-magnetic-loop system     

A spherical-section ultrasound phased array applicator for deeplocalized hyperthermia

Computer simulation shows that a new ultrasound phased-array with nonplanar geometry has considerable potential as an applicator for deep localized hyperthermia. The array provides precise control over the heating pattern in three dimensions. The array elements form a rectangular lattice on a section of a sphere. Therefore, the array has a natural focus at its geometric center when all its elements are driven in phase. When compared to a planar array with similar dimensions, the spherical-section array provides higher focal intensity gain which is useful for deep penetration and heat localization. Furthermore, the relative grating-lobe level (with respect to the focus) is lower for scanned foci synthesized with this array (compared to a planar array with equal center-to-center spacing and number of elements). This could be the key to the realization of phased-array applicator systems with a realistic number of elements. The spherical-section array is simulated as a spot-scanning applicator and, using the pseudo-inverse pattern synthesis method, to directly synthesize heating patterns overlaying the tumor geometry. A combination of the above two methods can be used to achieve the desired heating pattern in the rapidly varying tumor environment.     

An eccentrically coated asymmetric antenna applicator forintracavitary hyperthermia treatment of cancer

In this paper a model based on transmission line theory is used to predict the behavior of an eccentrically coated asymmetric antenna applicator for use in intracavitary hyperthermia. Theoretical results for the heating rate (HR) of the applicators are compared to experimental results. The experimental results were obtained at City of Hope National Medical Center using four different 915-MHz applicators, each with a different antenna size and eccentricity of the coating. A parameter delta is defined where delta < 1.0 is a thin wire approximation; delta is primarily a function of the eccentricity of the coating, the antenna diameter, and the coating diameter. It is found that when delta approximately less than 0.5, the theoretical model works well. In particular, it predicts the directivity due to the eccentricity of the coating. However, as this eccentricity is increased or as the antenna diameter is increased (delta approximately greater than 0.6), the model no longer accurately predicts directivity. Thus, the model that can be used to predict the HR profiles for an eccentrically coated asymmetric antenna only when delta approximately less than 0.5.     

A patch antenna design for application in a phased-array head and neck hyperthermia applicator

In this paper, we describe a specifically designed patch antenna that can be used as the basis antenna element of a clinical phased-array head and neck hyperthermia applicator. Using electromagnetic simulations we optimized the dimensions of a probe-fed patch antenna design for operation at 433 MHz. By several optimization steps we could converge to a theoretical reflection of -38 dB and a bandwidth (-15 dB) of 20 MHz (4.6%). Theoretically, the electrical performance of the antenna was satisfactory over a temperature range of 15 degrees C-35 degrees C, and stable for patient-antenna distances to as low as 4 cm. In an experimental cylindrical setup using six elements of the final patch design, we measured the impedance characteristics of the antenna 1) to establish its performance in the applicator and 2) to validate the simulations. For this experimental setup we simulated and measured comparable values: -21 dB reflection at 433 MHz and a bandwidth of 18.5 MHz. On the basis of this study, we anticipate good central interference of the fields of multiple antennas and conclude that this patch antenna design is very suitable for the clinical antenna array. In future research we will verify the electrical performance in a prototype applicator.     

A new coaxial TEM radiofrequency/microwave applicator for non-invasive deep-body hyperthermia

At the moment great efforts are being made to develop non-invasive heating systems which produce controlled local or regional deep-body hyperthermia. Electromagnetic interference techniques (10-80 MHz) with several separated applicators or with multiple applicators can produce deep-body heating. In our institute a coaxial frequency-independent TEM radiofrequency/microwave applicator has been designed. This applicator can produce a theoretically optimal interference maximum in the centre of the body. The applicator is very simple to construct and inexpensive. To test the design with the equipment available, a scaled prototype of the TEM applicator has been developed. The prototype has a diameter of 20 cm and operates at 434 MHz. The applicator has been tested on several phantom materials. The measured absorbed power distributions are in good agreement with the calculated theoretical distributions. The theoretical calculations of the absorbed power distributions of a 10-80 MHz TEM deep-body applicator are very encouraging.     

New type of compact electromagnetic applicator for hyperthermia in the treatment of cancer

The design and performance of a novel, inexpensive, light-weight, electromagnetic applicator are described. Experimental models have been constructed for operation at frequencies in the range 22 to 900 MHz.     

Unexpected physical phenomena indicated by FDTD modeling of theSigma-60 deep hyperthermia applicator

We investigate the numerical convergence properties of two-dimensional (2-D) and three-dimensional (3-D) finite-difference time-domain (FDTD) models of the BSD-2000 Sigma-60 annular phased array used for deep hyperthermia. The FDTD modeling data indicate unexpected physical phenomena for the case of Sigma-60 excitation of an elliptical tissue phantom embedded in a circular water bolus. These phenomena include: (1) high-Q energy storage; (2) electromagnetic (EM) mode flipping within the water bolus/phantom; and (3) whispering-gallery transmission of energy to the opposite side of the phantom relative to the exciting dipole pair. We conclude that these phenomena substantially impact the FDTD numerical modeling of this system, and further conclude that the whispering-gallery effect can impact clinical applications of the Sigma-60     

Experimental and numerical investigation of feed-point parameters in a 3-D hyperthermia applicator using different FDTD models of feed networks

Experimental and numerical methods were used to determine the coupling of energy in a multichannel three-dimensional hyperthermia applicator (SIGMA-Eye), consisting of 12 short dipole antenna pairs with stubs for impedance matching. The relationship between the amplitudes and phases of the forward waves from the amplifiers, to the resulting amplitudes and phases at the antenna feed-points was determined in terms of interaction matrices. Three measuring methods were used: 1) a differential probe soldered directly at the antenna feed-points; 2) an E-field sensor placed near the feed-points; and 3) measurements were made at the outputs of the amplifier. The measured data were compared with finite-difference time-domain (FDTD) calculations made with three different models. The first model assumes that single antennas are fed independently. The second model simulates antenna pairs connected to the transmission lines. The measured data correlate best with the latter FDTD model, resulting in an improvement of more than 20% and 20 degrees (average difference in amplitudes and phases) when compared with the two simpler FDTD models.     

A clinical water-coated antenna applicator for MR-controlled deep-body hyperthermia: a comparison of calculated and measured 3-D temperature data sets

A magnetic resonance (MR)-compatible three-dimensional (3-D) hyperthermia applicator was developed and evaluated in the magnetic resonance (MR) tomograph Siemens MAGNETOM Symphony 1.5 T. Radiating elements of this applicator are 12 so-called water coated antenna (WACOA) modules, which are designed as specially shaped and adjustable dipole structures in hermetically closed cassettes that are filled by deionized water. The WACOA modules are arranged in the applicator frame in two transversal antenna subarrays, six antennas per subarray. As a standard load for the applicator an inhomogeneous phantom was fabricated. Details of applicator's realization are presented and a 3-D comparison of calculated and measured temperature data sets is made. A fair agreement is achieved that demonstrates the numerically supported applicator's ability of phase-defined 3-D pattern steering. Further refinement of numerical models and measuring methods is necessary. The applicator's design and the E-field calculations were performed using the finite-difference time-domain (FDTD) method. The calculation and optimization of temperature patterns was obtained using the finite element method (FEM). For MR temperature measurements the proton resonance frequency (PRF) method was used.     

Chemical activation using an open-end coaxial applicator.

This article gives an overview of a novel, experimentally simple and versatile method of activation of chemical processes with microwaves without resorting to an oven. It is based on the use of a microwave antenna, namely an open-end coaxial dipole applicator immersed in an ordinary reaction vessel. Accounts of the apparatus at 2450 MHz and of the procedures adopted in reactions of organic synthesis, extraction of essential oils from plants and photo catalytic mineralization of liquid pollutants are given, discussing the necessary safety measures. The method is of practical interest for scientific and industrial applications. Thus this article is meant for a broad audience of scientists, engineers and technicians interested in new technologies for chemistry.     

On the determination of an optimum microwave diathermy frequency for a direct contact applicator

An approximate theoretical analysis is presented for determining the relative heating in a two layer fat-muscle medium due to a dielectric loaded dipole-corner reflector applicator in direct contact with the finite fat layer. The results indicate that convenient applicator sizes, equivalent to those now used at 2450 Mc/s, operating at a frequency in the neighborhood of 750 Mc/s provide the maximum ratio of relative heat per unit volume in the muscle as compared to that in the fat. Experimental data taken with the aid of an electrical model of the fat-muscle layers shows reasonable agreement with the results of the approximate analysis.     

Microwave impedance matching strategies of an applicator supplied by a bi-directional magnetron waveguide launcher.

It is shown that a bi-directional waveguide launcher can be used advantageously for reducing the reflection coefficient mismatch of an input impedance of an applicator. In a simple bi-directional waveguide launcher, the magnetron is placed in the waveguide and generates a nominal field distribution with significant output impedance in both directions of the waveguide. If a standing wave is tolerated in the torus, which connects the launcher and the applicator, the power transfer from the magnetron to the applicator can be optimal, without using special matching devices. It is also possible to match the bi-directional launcher with two inductance stubs near the antenna of the magnetron and use them for supplying a two-input applicator without reflection.     

Development of a high-speed solenoid valve-investigation of theenergizing circuits

The electrical circuits developed for the purpose of energizing the solenoid valve are examined. These circuits, called the dummy coil method, where energy stored in a dummy coil in the form of a magnetic field is transferred to an energizing solenoid, and the pre-energize method, where the solenoid is electrically energized in advance by taking advantage of the solenoid's attraction force characteristics, are discussed. The results of experiments show that the pre-energize method is highly effective in speeding up the operation of solenoid valves. These experimental results are discussed along with the results of calculations conducted using a mathematical model