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     

Evaluation of a semi-cylindrical solenoid as an applicator forradio-frequency hyperthermia

A modified solenoid having a semi-circular cross section is evaluated as an applicator for radio-frequency heating of biological materials. The operating efficiency of this applicator and the distribution of the electric field established within a simulated biological sample are determined for a number of modes of excitation. It is shown that uniform heating to a substantial depth is possible at efficiencies in excess of 80 percent.     

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.     

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.<>     

Design and experimental evaluation of an intracavitary ultrasoundphased array system for hyperthermia

For evaluating the feasibility of treating prostate cancer, a 64-element linear ultrasound phased array applicator for intracavitary hyperthermia was designed and constructed. A 64-channel ultrasound driving system including amplifiers, phase shifters, and RF power meters was also developed to drive the array. The design of the array and driving equipment are presented, as are the results of acoustical field measurements and in vitro perfused phantom studies performed with the array. Several techniques for heating realistically sized tumor volumes were also investigated, including single focus scanning and two techniques for producing multiple stationary foci. The results show that the operation of the array correlated closely with the theoretical model. When producing a single stationary focus, the array was able to increase tissue temperature by 12°C in vitro in perfused phantom. With some minor improvements in array design, intracavitary phased arrays could be evaluated in a clinical environment     

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     

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     

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.     

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 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.     

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.     

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     

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.     

Low-frequency RF hyperthermia. IV. A 27 MHz hybrid applicator forlocalized deep tumor heating

A 27 MHz dual-device applicator of novel design, which is aimed to heat noninvasively with improved safety tumor masses at depth, is proposed. A substantially localized temperature gain is obtained by superimposing two delocalized low RF frequency and phase-coherent current distributions, which are launched to constructively interfere over a limited region emcompassing the tumor volume. An hybrid applicator (HA) has been developed, integrated one capacitive and one inductive heating device, and has been assessed on a 20 cm diameter cylindrical fat-muscle phantom. The interference pattern is characterized by a deep broad SAR maximum and by the disappearance of the central null SAR value typical of single inductive devices. An 80% SAR useful therapeutic volume (UTV) of a near-cylindrical shape of about 800 cm3 is obtained with a penetration of 6-8 cm for the phantom surface, with a noncritical axial length of approximately 21 cm. The UTV may be somehow controlled in size and penetration. These results are obtained with the tissue-like medium surrounding the UTV heated uniformly and safely to a temperature pedestal below the therapeutic temperature with about half RF power values to each of the heating devices.     

Cancer therapy with localized hyperthermia using an invasive microwave system.

A system has been developed for producing local hyperthermia in small animal tumors by means of an invasive needle microwave antenna. Thermal distributions produced by this system using 1 GHz microwaves have been characterized in mammary adenocarcinomas (MTG-B) implanted in thighs of C3H mice. Therapeutic efficacy by hyperthermia was demonstrated by comparing MTG-B diameters in mice following control, sham or hyperthermia treatment. It is proposed that this system could be employed clinically to provide very local hyperthermia in deep-seated tumors.     

A three-dimensional iterative scheme for an electromagneticinductive applicator

An efficient iterative method for solving quasi-static electromagnetic field problems is presented. The electromagnetic field is generated by an inductive applicator and is represented as a superposition of two constituents, viz. a primary field in absence of the tissue configuration and a secondary field generated by the presence of the tissue. Then, for the secondary field a quasi-static approximation is employed. In the quasi-static field equations a relaxation function is introduced, such that the resulting equations can be solved iteratively. For a realistic three-dimensional model of a human hand numerical results are presented.     

A three-dimensional iterative scheme for an electromagneticcapacitive applicator

An efficient iterative method for solving quasi-static electromagnetic field problems is presented. A relaxation function is introduced in the quasi-static field equations. Then, the resulting equations can be solved by iteration. The method is similar to the one of solving a Laplace equation by computing the stationary state of a diffusion equation. Next, for a radially layered configuration the numerical results are compared with the results from an existing integral equation method. Subsequently, for a realistic three-dimensional model of a human knee numerical results are arrived at.     

Localized hyperthermia in the treatment of malignant brain tumors using an interstitial microwave antenna array

An interstitial microwave antenna array hyperthermia (IMAAH) system was evaluated in a series of acute experiments for its ability to generate localized hyperthermic fields in the normal adult and developing dog brain. Using a single microwave antenna, the maximum diameter of heating greater than or equal to 42°C was 1.3 cm. Using four microwave antennas, temperatures greater than or equal to 42°C were obtained over a 2.5 cm cross-sectional diameter. Normal gray matter cannot be heated above 42.2°C for 60 min without causing acute damage to cortical neurons. Edema formation can occur at temperatures as low as 42°C in both gray and white matter. Disruption of myelin tracts in white matter is apparent at temperatures of 43.0-43.5°C. The IMAAH system generates significantly higher and more variable temperature distributions in virally induced experimental brain tumors than in the developing dog brain at the same power levels.