BACKGROUND OF THE INVENTION Until now, many techniques to stimulate nerves of muscles of the human body have been proposed. One of the examples is the paper, "Magnetic stimulation of the human brain", published by A. Barker in Journal of Physiology, vol. 369, pp 3 in 1985. In the paper, an experimental result of stimulating human brains by a magnetic stimulation coil is introduced.

Before the magnetic stimulation technology was introduced in the medical field, the electric stimulation method was used to stimulate human brains. Since the electric stimulation method was very invasive in that electrodes have to be attached to the brain after opening the skull, the electric stimulation method has not been widely used in the brain stimulation.

However, after the first publication about the brain

stimulation using the magnetic stimulation coil, many researches on the magnetic stimulation of human nerves and muscles have been carried out. In 1991, R. Jalinous published a paper, "Technical and practical aspects of magnetic nerve stimulation"in Journal of Clinical Neurophysiology, vol. 8, no. 1, pp 10-25, in which many technical aspects of the magnetic stimulation technology were described in detail. In the paper, applying a very intense electric current pulse, higher than several thousands of amperes, to a stimulating coil in a very short time period, less than 1 msec, to stimulate the nerves or muscles in the human body is reported.

This large coil current can make the magnetic field as large as several teslas around the stimulating coil. Since such a high current pulse is applied to the magnetic stimulation coil, the energy delivered to the coil can be as large as 500 Joules per each current pulse and the power can be as large as 5MW.

Since the magnetic stimulation technology consumes such a large power in the stimulation of the human body, the stimulation repetition frequency had been limited to 1-2Hz in the past. Therefore, the magnetic stimulation technology had been used only for diagnosis of diseases in nervous system with few exceptional uses in treatments.

Recently, there were several reports on applications of the magnetic stimulation to treatment areas. One of the examples is the paper,"Non-invasive treatment system for urinary incontinence using a continuous magnetic stimulation", published by N. Ishikawa et al. in Proceedings of 19th Int.

Conference IEEE/EMBS (Chicago), pp 2096-2099 in Oct. 1997. In the paper, N. Ishikawa et al. reported that magnetic

stimulation of pelvic floor muscles and bladder nerves with the pulse repetition frequency of 5-30Hz can be used to treat urinary incontinence. In addition to the urinary incontinence treatment, there were publications reporting the efficacy of the magnetic stimulation in the treatment of deprementia.

When the magnetic stimulation technology is applied to treatments, it is usual to apply magnetic pulses with the pulse repetition frequency of several tens of Hz. With such a high pulse repetition frequency, it is inevitable to have high power consumption and excessive heat generation in the stimulating coil.

There were also many techniques to supply high power electric current to the stimulating coil. U. S. Patent Nos.

5,766, 124 and 5,769, 778 are among the examples. Here, to solve the problem of excessive heat generation in a stimulating coil, the inventors have to either reduce the power consumption in the stimulating coil or cool the stimulating coil efficiently. Reducing the power consumption in the stimulating coil while maintaining the magnetic field strength generated by the coil will be the better way to solve the problem.

There are techniques to improve the stimulating coil efficiency in power consumption by optimizing the coil shape.

For example, stimulating coil shapes suitable for urinary incontinence treatment were introduced in U. S. Patent Nos.

6,179, 769 and 5,725, 471, and stimulating coil shapes suitable for stimulating brain and peripheral nerves were introduced in U. S. Patent Nos. 4,056, 0974 and 4,994, 015. In summary, it is very essential to improve the coil efficiency and to use an efficient coil cooling method in the magnetic stimulation

with the pulse repetition frequency of several tens of Hz for treatments.

SUMMARY OF THE INVENTION The present invention is to solve the problems in current magnetic stimulation technology described above, and to provide a stimulating coil to increase the magnetic field strength per unit current fed to the stimulating coil while lowering the heat generation of the stimulating coil.

To achieve the purpose described above, the stimulating coil according to the present invention comprising: (a) a coil made of metallic conductor; and (b) a magnetic mirror made of high magnetic permeability material.

BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings, in which: Fig. 1 is a diagram of a magnetic stimulation coil using a magnetic mirror according to an embodiment of the present invention; Fig. 2 is a diagram for explaining the operating principle of the magnetic mirror shown in Fig. 1; Fig. 3 is a diagram for explaining the operating principle of the magnetic mirror shown in Fig. 1; Fig. 4 is a diagram showing the magnetic mirror made of

laminated ferromagnetic plates to reduce eddy currents; Fig. 5 is a diagram for explaining the performance of a magnetic stimulation coil using the magnetic mirror according to an embodiment of the present invention; Fig. 6 is a diagram showing the performance comparison between the stimulating coils with and without the magnetic mirror effect; Fig. 7 is a diagram showing a shape of the magnetic mirror suitable for a 8-shaped stimulating coil; Fig. 8 is a diagram showing a shape of the magnetic mirror suitable for a curved'stimulating coil; Fig. 9 is a diagram for explaining a method efficiently cooling the heat generated in the stimulating coil; Fig. 10 is a diagram showing a magnetic mirror and a stimulating coil which are applicable to an urinary incontinence treatment device; Fig. 11 is a diagram showing a magnetic mirror and a stimulating coil which are applicable to an physical therapy device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS There will now be described an embodiment of the present invention with reference to the accompanying drawings.

The gist of the present invention is to improve the power consumption efficiency of a stimulating coil for stimulating nerves, muscles, and bones of the human body.

In Fig. 1, a preferred stimulating coil shape is shown.

The stimulating coil consists of a coil 1 made of metallic conductor and a magnetic mirror 2 made of high magnetic

permeability material. In Fig. 1, the magnetic mirror has a circular shape, however, the magnetic mirror can have another shapes as described thereinafter. It is preferred that the coil 1 and the magnetic mirror 2 are positioned to meet as closely as possible. The more closely the coil 1 is positioned to the mirror 2, the larger the magnetic mirror effect and the smaller heat generation in the stimulating coil as described in hereinafter. In the conventional technologies, the magnetic stimulation coil consists of only a coil without a magnetic mirror.

Electric current I is fed to the stimulating coil from an external power supply. Since the peak amplitude of the current supplied to the stimulating coil is as large as several hundreds or several thousands of amperes, it is usual to use high voltage capacitors to charge electric charges supplied from the external power supply and discharge the charged electric charges to the stimulating coil instantaneously. In this case, the capacitor and the stimulating coil form a LC resonant circuit, and the resonant frequency is about several KHz. When electric current flows along the coil, magnetic field is generated. The magnetic field can penetrate the tissues of the human body since most of biological tissues are magnetically transparent. When a stimulating coil is used to stimulate tissues of the human body, the stimulating coil can have various shapes. We have shown a circular coil, the most frequently used coil, in Fig.

1, the present invention applies to the coil with an arbitrary shape.

In Fig. 2, basic principle of the present invention is illustrated. For the purpose of explanation, we assume that

the coil consists of an infinitely long wire 3. In Fig. 2, the coil wire points perpendicularly to the paper. Around the wire, there is a magnetic material 4 parallel to the wire with the distance d. It is assumed that the magnetic material has an semi-infinite shape. In the present invention, magnetic material means the substance which as high magnetic permeability. That is, when an external magnetic field is applied to the magnetic material, strong magnetization appears inside the magnetic material making dense magnetic flux inside the material. There are many kinds of magnetic materials, such as ferrite, silicon steel, permalloy, permendure, metallic glass, and powder iron. Magnetic permeability of air is about 4x3. 14x10". The magnetic permeability of ferrite is several hundred times larger than that of air, and the magnetic permeability of silicon steel is several thousand times larger than that of air.

When a magnetic material is located around the coil wire 3 as shown in Fig. 2, inventors of the present invention have to consider the magnetic field generated by the magnetization inside the magnetic material as well as the magnetic field generated by the wire. If the magnetic material has an infinite size and the magnetic permeability of the material is much higher than that of air, the magnetic field around the wire can be calculated as if there is an imaginary wire 5 instead of the magnetic material carrying the same current as that of the wire. The magnetic field calculation method in such a way is called image method or mirror method. The mirror method can be found in most electromagnetism texts.

In Fig. 3, the principle of the mirror method is schematically illustrated. Only perpendicular magnetic field

component exists on the surface of a magnetic material. That is, the magnetic flux vectors meet the magnetic material perpendicularly on its surface. The left side of Fig. 3 shows such a shape using magnetic flux lines 6. The shape of magnetic flux vectors generated around a wire with magnetic material is same as the shape of the magnetic flux vectors 7 of the case that there are a wire without magnetic material and another imaginary wire located at the distance 2d from the wire carrying the electric current of the same amplitude and direction. This is illustrated in the right side of Fig.

3. When the permeability of the magnetic material is not infinite, the shape and the position of the imaginary wire are changed. However, its disagreement with the case of infinite permeability can be neglected, since the permeability of ferrite or silicon steel is several hundred times larger than that of air. If the wires of Fig. 2 and Fig.

3 are regarded as stimulating coils, it can be found that there is additionally an imaginary stimulating coil inside the magnetic material. If the distance, d between the magnetic material and a coil is zero, a stimulating coil and a imaginary coil are overlapped each other, and the effect that the same shape of a magnetic field as that of a stimulating coil is added can be obtained. In other words, the effective strength of a magnetic field can be increased twice by using just one stimulating coil and a magnetic mirror.

When the material of a magnetic mirror is chosen, the high permeablity and the ability to reduce an eddy current should be considered. Theoretically, the fact that a imaginary coil is made inside the magnetic material requires

the assumption that its electrical conductivity is zero, or the applied current is a direct current. The current pulse of a few hundreds psec is usually applied to a coil stimulating the human body. Therefore, if the electrical conductivity of the magnetic material is not zero, an eddy current is generated in magnetic material. Since the eddy current is generated in the opposite direction of the coil current, it reduces the magnetic field strength. Though ferrite has a very low conductivity, its permeablity is not high. Since the saturation magnetic flux density of ferrite is 0.4-0. 5 teslas and a stimulating coil makes magnetic flux density of 1-2 teslas, ferrite is not a proper material. In order to avoid a magnetic saturation, the present invention uses silicon steel, not ferrite. Silicon steel has some advantages of its low cost and high saturation magnetic flux density over 2.0 teslas. However, the high conductivity of silicon steel can make a large eddy current. To reduce a eddy current, the present invention uses laminated silicon steel plates as shown in Fig. 4. Since silicon steel plates are coated with an insulator, eddy currents cannot flow between plates, but can flow inside a plate. Therefore, the generation of eddy currents is restrained and the reduction of the magnetic field generated by a imaginary coil becomes very small.

In Fig. 2 and Fig. 3, the infinite size of the magnetic material is assumed. In order to analyze the effect of the magnetic mirror made of a magnetic material with a finite size, the model as shown in Fig. 5 is constructed. The size of the magnetic material plate 11 with a circular shape is recommended to be a little bit larger than that of the circular coil 10 in Fig. 5. The circle coil can be made of

enameled wires. Using a finite element method (FEM), we carried out a simulation to obtain magnetic flux vectors about the model of Fig. 5. Inventors of the present invention used the simulation software provided by Vector Field Corporation for an electro-magnetic analysis using an FEM.

Fig. 6 shows the simulation result. The solid and dotted lines of Fig. 6 represent the strength of the magnetic flux density of the cases with and without a magnetic mirror, respectively. In the simulation, the width of the coil, Wl=5mm, the thickness of the magnetic mirror, W2=15mm, the inner radius of the coil, Rl=10mm, the outer radius, R2=55mm, the outer radius of the mirror, R3=75mm were used respectively. The magnetic material was made of silicon steel mentioned in the above. In this case, the inductance of the stimulating coil was assumed to be 50pH. The performance comparison should be carried out under the condition of the same inductance value. The current flowing along the coil is a shape of a damped sinusoidal wave and the its frequency depends on the inductance of the coil. A stimulating coil with a magnetic mirror can have about twice larger inductance than the one without it. Therefore, inventors of the present invention designed the coil without a magnetic mirror so that its inductance is 50lah. The current flowing along the coil was 2, OOOA.

The result of Fig. 6 illustrates the values along the center axis of the parallel plane 5mm distant from the coil.

This result shows that the magnetic flux density in the center of the coil increases from 0.92 teslas to 1.36 teslas by using a magnetic mirror. The increase rate of the magnetic field in this case is upto about 48%.

On the other hand, inventors of the present invention performed the practical experiment to verify the previously mentioned simulation result. All inductances of the used stimulating coils were adjusted to 50pH. The inner and outer radii of the coil were 10mm and 60mm, respectively. And the thickness of the magnetic mirror was 15mm and its radius was 75mm. The magnetic mirror was made of laminated silicon steel plates. The thickness of silicon steel plates was lmm. The plates were properly cut and stacked. When the current of 2, 000A was fed to the stimulating coil, the measured result showed that the magnetic flux density in the center of the coil with the magnetic mirror was about 33% larger than that of the case without it. The difference between 48% increase of the simulation result and 33% of the experimental result is due to the eddy current generated inside the magnetic mirror.

Even though the desirable conductivity of the magnetic mirror is ideally zero, that of a silicon steel plate is very high. Therefore, even the laminated magnetic mirror cannot remove all eddy currents. In order to reduce the effect of the eddy current as much as possible, the plate should be very thin.

Another simulation was carried out to analyze the effect of the thickness of the magnetic mirror. The result showed that the effects were almost same for the thickness, of 15mm, 20mm, and 25mm. It means that the effect of the magnetic mirror may be saturated, when its thickness gets over a critical value. On the contrary, if its thickness gets under a critical value, the saturation of magnetic flux density inside the mirror may occur and the effect is reduced.

However, it is inevitable to use a thin mirror for some applications, and the reduction of the effect should be accepted.

The additional advantage of the magnetic mirror is to reduce the heat generated in the stimulating coil. When the number of turns of the stimulating coil increases, its electrical resistance gets larger. The magnetic mirror can reduce the number of coil turns and the proportional resistance, and finally the heat generated in the coil can be reduced. The following table 1 shows the measured temperature of the stimulating coil surface, when the pulse duration and the frequency of the applied current were 50011 sec and 50Hz, respectively. The current pulse was a bipolar pulse and its amplitude was 2, 000A.

Table 1. The surface temperature of the stimulating coil initial after after after after after After state 1 min. 2 min. 3 min. 4 min. 5 min. 6 min. Without the 21.2 59.8 87.8 98.4 98.2 100.2 114.2 magnetic mirror With the 21. 2 34.2 47.2 52.4 67.2 77.8 80. 8 magnetic mirror The table 1 represents that the surface temperature of the stimulating coil can fall down by using the magnetic mirror. If the temperature rise of the coil is reduced, the higher frequency of the stimulating pulse can be used and the capacity of the cooler chilling the coil can be reduced.

The shape of the stimulation coil is not always circular.

Fig. 7 illustrates the 8-shaped coil 14 which is widely used. This 8-shaped coil is composed of two circular coils.

It is widely used for selectively stimulating a specific area, since the magnetic flux can be focused at the place where two circles meet each other. The magnetic mirror 15 can be also applied for this 8-shaped coil. In this case, the shape of the mirror may not be circular and a little bit larger size than that of the 8-shaped mirror is recommended. The shape of the magnetic mirror can be varied according to the shape of the stimulating coils. Even though the circular shape of the mirror is the most desirable, a rectangle or a polygon larger than the coil can be used in consideration of manufacturing convenience. And the magnetic mirror can be applied to the coil with a curved surface, too. The curved surface had better be parallel to the coil surface.

Fig. 8 illustrates the curved surface of the magnetic mirror 17 which is suitable to stimulate a human brain.

In order to efficiently cool down the heat generated in the stimulating coil and the magnetic mirror, the opposite side of the magnetic mirror can be made uneven. Fig. 9 illustrates an uneven shape of the magnetic mirror 20 which is made by stacking magnetic material plates 18,19 with different widths. This is an efficient structure to cool down the magnetic mirror.

Fig. 10 illustrates the shape of the stimulating coil and the magnetic mirror which are applicable to the treatment of urinary incontinence. In the urinary incontinence treatment using the magnetic stimulation, a patient is usually seated at a chair and the stimulating coil 21 and the magnetic

mirror 22 are generally installed inside a seat 23.

Fig. 11 illustrates the shape of the stimulating coil and the magnetic mirror which are applicable to a physical therapy. Since the stimulating coil for a physical therapy are generally attached to the structure like an artificial arm, it is desirable to attach both of the coil and the magnetic mirror to an artificial arm 24.

The stimulating coil and the magnetic mirror can be closely adhered each other in the various ways using nonconductive glue, thread, ribbon, etc. And it is possible to insert the material with high thermal conductivity between the coil and the mirror in order to cool down the heat generated in the coil through the mirror.

As explained in the above, the present invention can provide the stimulating coil with the magnetic mirror, which can reduce the required electric power and the heat generation by enhancing the efficiency of the stimulating coil. The stimulating coil is used to stimulate the human body, magnetically for the function diagnosis of nerves and muscles, urinary incontinence treatment, physical therapy, the pain relief, obesity treatment, etc.

The present invention has been described in terms of preferred embodiments. However, it should be understood that the present invention is not limited in its application to the specific embodiments. Those skilled in the art will recognize that various modifications and variations may be made without departing from the spirit and scope of this invention, as defined in the following claims.

< References > 1. A. T. Barker et al. ,"Magnetic stimulation of the human brain", Journal of physiology, Vol. 369, pp 3,1985 2. R. Jalinous, "Technical and practical aspects of magnetic nerve stimulation", Journal of Clinical Neurophysiology, Vol. 8, No. 1, pp 10-25,1991 3. N. Ishikawa et al. ,"Non-invasive treatment system for urinary incontinence using a continuous magnetic stimulation", Proceeding 19th Int. Conference IEEE/EMBS pp 2096-2099,1997.

10.

4. U. S. Patent No. 5,766, 124, entitled"Magnetic stimulator for neuro-muscular tissue" 5. U. S. Patent No. 5,769, 778, entitled"Medical magnetic non-convulsive stimulation therapy" 6. U. S. Patent No. 6,179, 769, entitled"Magnetic stimulus type urinary incontinence treatment apparatus" 7. U. S. Patent No. 5,725, 471, entitled"Magnetic nerve stimulator for exciting peripheral nerves, 8. U. S. Patent No. 4,994, 015, entitled"Magnetic stimulator coils" 9. U. S. Patent No. 4,056, 097, entitled"Contactless stimulus transducer"