Youji Kotsuka

Fundamental investigation on a weakly magnetized ferrite absorber

Investigated are the matching characteristics of a magnetized ferrite absorber, which applies a weakly static magnetic field H/sub dc/ perpendicular to the sintered ferrite surface. Applying a static magnetic field to the ferrite enables the matching thickness of the absorber to be reduced from 8 mm in the absence of H/sub dc/ to 3 mm under 750 gauss. The matching frequency can also be changed broadly from 0.1 to 0.9 GHz by controlling ferrite thickness and H/sub dc/, simultaneously. These are well explained in terns of an nonsaturated permeability rather than tensor permeability. A thin magnetized ferrite absorber is realized using a ferrite with a large value of imaginary part of permeability when real part takes the value near 1.0. Matching frequency characteristics, whether broad or narrow, correlates closely with the slant of the curve for real part of permeability taking the value near 1.0 in the present frequency.

Development of ferrite core applicator system for deep-induction hyperthermia

To achieve deep-induction hyperthermia, a ferrite core applicator system has been developed. The clinical goal is to produce a temperature rise of 7.5/spl deg/C at 10 cm tissue depth. Deep heating becomes possible by introducing an auxiliary electrode to control an eddy current. The auxiliary electrode has been designed to optimize the eddy current with respect to the magnetic flux density. The optimization was performed by solving the fundamental equation using the finite element method (FEM). A flexible auxiliary electrode, which can be used for clinical treatments, has been manufactured. Heating tests have been conducted with the new applicator system, which operates at 4.0 MHz. The experimental results demonstrate that the system is capable of producing a temperature rise of 7.5/spl deg/C at 10 cm depth, without heating an overlaying fat layer.

Development of inductive regional heating system for breast hyperthermia

In response to demand for clinical use, a simple noninvasive regional heating applicator system for breast hyperthermia has been developed using ferrite cores. Since the breast is positioned between a pair of ferrite cores, it is possible to regionally heat it without considering the dimension of the breast. To find a method of controlling the heating position horizontally and vertically, magnetic-held distributions are analyzed using the three-dimensional finite-element method. Theoretical analyses suggest that a conductive thin plate and a novel eddy current absorber are effective for controlling the maximum heating position. A new applicator system operates at a frequency of 4 MHz and a maximum output power of 600 W. Heating tests using an agar phantom and rabbits show a temperature rise of more than 8/spl deg/C at a depth of 8 cm after heating for 10 min without heating fatty tissue.

New wireless thermometer for RF and microwave thermal therapy using an MMIC in an Si BJT VCO type

A new wireless thermometer is proposed for thermal therapy that uses a silicon bipolar transistor voltage-controlled oscillator (Si VCO), based on three-dimensional monolithic-microwave integrated-circuit (MMIC) technology. A theoretical analysis of heating characteristics was conducted using the finite-element method. The result suggests the MMIC thermometer may be used for RF/microwave thermal therapy in a strong electromagnetic wave circumstances. The fundamental characteristics of an Si VCO MMIC are examined experimentally. As a result, the VCOs oscillation frequency (around 4.35 GHz) is extremely linear against temperature in the range from 30/spl deg/C to 50/spl deg/. Tests using a phantom material show that a prototype thermometer can be detected at depths of more than 8 cm of soft tissue.

A novel microwave absorber with surface-printed conductive line patterns

A novel microwave absorber with surface-printed conductive line patterns is proposed. The advantage of this absorber is to be able to control the matching frequency both toward a higher frequency or a lower frequency region and to offer twin-peaks characteristic using a conventional single material. The matching characteristics are investigated particularly for making a slim absorber by FDTD analysis and experiments. A slim absorber of 2 mm thickness at 2.45 GHz is presented by computer-aided design.

Novel Right-Handed Metamaterial Based on the Concept of “Autonomous Control System of Living Cells” and Its Absorber Applications

A novel right-handed metamaterial similar to an “autonomous control system of a living cell” and capable of responding to electromagnetic waves is proposed with its fundamental configurations. Using microwave absorbers as an example, the possibility of equivalently controlling their material constants are investigated both theoretically and empirically. These absorbers are composed of unit cells based on the concept of an autonomous controllable metamaterial (ACMM). The theoretical approaches used to obtain ACMM-based absorber design data are revealed. In order to improve the absorber matching characteristics, the principle of obtaining the broadband matching characteristics with an absorption band of 1.3 GHz at 4.25 GHz is clarified. In addition, the methods for improving the matching degradation of the oblique incidence of a TM wave are investigated. For the TM wave oblique incidence, a new ACMM-based absorber configuration comprising unit cells with inductive fins is proposed. At an incident angle of 45°, a matching characteristic of -30 dB is achieved in the case of the TM wave. These ACMMs are controlled by using only two bias feeder wirings for PIN diodes.

Broadband EM-wave absorber based on integrated circuit concept

A broadband EM-wave absorber based on the structural concept of microwave integrated circuit is proposed. Following the fundamental principle and construction, the method of converting the actual microwave circuit into the integrated circuit type absorber is clarified. The problem of fine control of the resistance on the circuit is resolved by changing the configuration of a unit circuit element in a cross shape. Broadband matching characteristics with an absorption band 1.9 times as broad as a conventional Salisbury screen absorber with resistance and capacitance, and 2.6 times as a typical Salisbury absorber are obtained. The thickness of the present absorber is further reduced by introducing a ferrite material, while keeping almost the same broadband matching frequency.

Development of double-electrode applicator for localized thermal therapy

A double-electrode applicator for localized thermal therapy is developed by breaking through the construction principle of the capacitive applicator. To make the beam of the electric field narrow, a ferrodielectric material is introduced as the sub-electrode. The optimum configuration of the double electrode and the material constant in each part of this applicator are investigated theoretically and experimentally. Good localised deep-heating characteristics are obtained at an operating frequency of 13.56 MHz and an output power 450 W by choosing a ferrodieletric material (BaTiO/sub 3/) with a relative permittivity of 6000 as the sub-electrode.

A method of effective use of ferrite for microwave absorber

To meet the demands of various types of EM-wave absorber, a method of effective use of conventional ferrites for microwave absorber applications is investigated both by FDTD analysis and experiments. The effects of various kinds of parameters are clarified, particularly for permeability. A new matching characteristic is realized simply by punching out small holes in the conventional rubber ferrite without producing a new ferrite material in the region of the microwaves.

Method of improving EM field distribution in a small room with an RF radiator

A method of controlling the field distribution in a room is proposed to improve the electromagnetic (EM) environment in a relatively small room compared to the wavelength of an RF radiator such as a medical treatment device. For this purpose, this paper investigates a method of realizing both a region of weak and strong electric fields in the room by adjusting the material constant of the room or by changing the shape and arrangement of ceiling, walls, etc. This idea is based upon the principle that an electric field concentrates on materials with a large permittivity. The electric field distribution is analyzed using the finite-element method to understand the effects of concentrating the electric field by taking into account many factors as parameters. To examine these in a qualitative investigation, electric field distribution is verified using a reduced-scale model. Further, on the basis of these concepts, a shielded room is constructed. It is clarified that the electric field distribution in the room is effectively controlled using the method proposed.