WO2001078829A2

WO2001078829A2 - Magnetic nerve stimulator utilizing vertically overlapping cores

Info

Publication number: WO2001078829A2
Application number: PCT/US2001/011992
Authority: WO
Inventors: Kent R. Davey
Original assignee: Neotonus, Inc.
Priority date: 2000-04-12
Filing date: 2001-04-12
Publication date: 2001-10-25
Also published as: WO2001078829A3; AU2001253422A1

Abstract

An apparatus for excitation of peripheral nerves which is made of a pair of vertically overlapped arc-shaped cores. The cores are brought together with one leg of one arc-shaped core vertically overlapping one leg of another arc-shaped core in a central region. The resulting common central leg of the two arc-shaped cores is wound by a coil and the return path for the flux is split between the non-common legs of the two arcs. In one preferred embodiment, each of the arc-shaped cores spans an angle of approximately 210°. The vertically overlapping core is preferably made of a highly saturable material.

Description

Magnetic Nerve Stimulator Utilizing Vertically Overlapping Cores

Related Applications

The present application is a continuation-in-part of U.S. Patent Application Serial No. 09/001,782, filed December 31, 1997 (pending), and claims all rights of priority thereto.

Background of the Invention and Description of the Prior Art

A nerve cell can be excited in a number of different ways, but one direct method is to increase the electrical charge within the nerve, thus increasing the membrane potential inside the nerve with respect to the surrounding extracellular fluid. One class of devices that falls under the umbrella of Functional Electrical Stimulation (FES) realizes the excitation of the nerves by directly injecting charges into the nerves via electrodes which are either placed on the skin or in vivo next to the nerve group of interest. The electric fields necessary for the charge transfer are simply impressed via the wires of the electrodes.

FES is accomplished through a mechanism which involves a half-cell reaction. Electrons flow in wires and ions flow in the body. At the electro-electrolytic interface, a half-cell reaction occurs to accomplish the electron-ion interchange. Unless this half-cell reaction is maintained in the reversible regime, necrosis will result — partially because of the oxidation of the half-cell reaction and partially because of the chemical imbalance accompanied by it.

The advantage of FES is that the stimulation can usually be accomplished from extremely small electrodes with very modest current and voltage levels. The disadvantage however, is that it involves half-cell reactions. Most rehabilitation programs using FES place the electrodes directly on the skin. A conductive gel or buffering solution must be in place between the electrodes and the skin surface. Long term excitation of nerve or muscle tissue is often accompanied by skin irritation due to the current concentration at the electrode/skin interface. This problem is especially aggravated when larger excitation levels are required for more complete stimulation or recruitment of the nerve group.

By contrast, magnetic stimulation realizes the electric fields necessary for the charge transfer by induction. Rapidly changing magnetic fields induce electric fields in the biological tissue; when properly oriented, and when the proper magnitude is achieved, the magnetically induced electric field accomplishes the same result as realized by FES, that of transferring charge directly into the nerve to be excited. When the localized membrane potential inside the nerve rises with respect to its normal negative ambient level of approximately -90 millivolts (this level being sensitive to the type of nerve and local pH of the surrounding tissue), the nerve "fires."

As opposed to FES, magnetic excitation has the attractive feature of not requiring electrode skin contact. Thus, stimulation can be achieved through the clothing that is being worn. This overcomes the objection of inconvenience and preserves the patient's dignity. Secondly, because there is no direct contact, stronger excitation levels can be realized without undue additional skin irritation. A contribution offered by the present invention is the ability to achieve higher levels of focusing of the magnetic field and thus stimulation within the patient. Commensurate with this greater level of focusing comes some flexibility in the number of possible applications that might be targeted. Also accompanying the focusing is a higher level of power efficiency. Typically, the devices being designed by the methods outlined in this invention reduce the magnetic reluctance path by a factor of two. This reluctance reduction ti'anslates into a diminution of the current by the same factor and a fourfold reduction in power loss. Magnetic stimulation of neurons has been heavily investigated over the last decade. Almost all magnetic stimulation work has been done in vivo. The bulk of the magnetic stimulation work has been in the area of brain stimulation. Cohen has been a rather large contributor to this field of research (See e.g., T. Kujirai, M. Sato, J. Rothwell, and L. G. Cohen, "The Effects of Transcranial Magnetic Stimulation on Median Nerve Somatosensory Evoked Potentials", Journal of Clinical Neurophysiology and Electro Encephalography, Vol. 89, No. 4, 1993, pps. 227 - 234.) This work has been accompanied by various other research efforts including that of Davey, et al. (See, K. R. Davey, C. H. Cheng, C. M. Epstein "An Alloy - Core Electromagnet for Transcranial Brain Stimulation", Journal of Clinical Neurophysiology, Volume 6, Number 4, 1989, p.354); and that of Epstein, et al. (See, Charles Epstein, Daniel Schwartzberg, Kent Davey, and David Sudderth, "Localizing the Site of Magnetic Brain Stimulation in Humans", Neurology, Volume 40, April 1990, pps. 666-670). The bulk of all magnetic stimulation research attempts to fire nerves in the central nervous system.

The above described techniques, however, provide a low level of magnetic field concentration. Therefore, there is a substantial interest in the art for a magnetic nerve stimulator which would allow a greater concentration and focusing of the generated magnetic flux. The greater concentration and focusing will provide more efficient penetration into the body and, therefore, better nerve stimulation. Additionally, the present invention provides a magnetic nerve stimulator utilizing a core of highly saturable material, in contrast with the prior art air cores.

Summary of the Invention

It is an object of the present invention to provide a core design for a magnetic nerve stimulator allowing a greater concentration and focusing of magnetic flux. It is an object of the present invention to provide a core design for a magnetic nerve stimulator allowing a more efficient penetration of the generated magnetic field into the body.

Other objects, advantages and features of this invention will be more apparent hereinafter.

The above enumerated objects are accomplished by an apparatus for excitation of peripheral nerves which includes a vertically overlapping magnetic core constructed of two arc-shaped cores brought together to vertically overlap each other in a central region. The resulting common central leg of the two arc-shaped cores is wound by a coil and the return path for the flux is split between the two arcs.

In one preferred embodiment, each of the arc-shaped cores spans an angle of approximately 210°. Generally, an arc shaped core spanning an angle less than 360 degrees may be used with the invention. For example, a 180 degree core is very convenient for using the material efficiently since two cores can be constructed from every mandrel. A core having a larger angle (e.g. 210-220 degrees) can also be used. In addition, the vertically overlapping core is made of a magnetic material, preferably one having high saturability. Between alternate core designs having two adjacent C-shaped cores, two overlapping thinner cores and two overlapping wider cores, it has been found that the use of overlapping wider cores with a smaller inner diameter provides superior performance.

Brief Description of the Drawings

A full understanding of the invention can be gained from the following description of the preferred embodiment when read in conjunction with the accompanying drawing in which:

Figure 1 is a perspective view of primary cores, useful in the preferred embodiment for incontinence therapy, in which Figure 1(a) is a perspective view of a relative thin-legged core for magnetic stimulation of peripheral nerves, and Figure 1(b) is a perspective view of a core with relatively wider legs for magnetic stimulation of peripheral nerves;

Figure 2 is a schematic diagram of a magnetic field arrow plot for an embodiment of the core layout, in accordance with the present invention;

Figure 3 is a schematic diagram of the primary search coil planes and their currents, in which Figure 3(a) features a horizontal plane search coil, and Figure 3(b) features a vertical plane search coil;

Figure 4 is a schematic diagram of the preferred embodiment of the invention with preferred dimensions;

Figure 5 is a perspective view of the present invention utilizing a horizontal flat test search coil;

Figure 6 is a perspective view of the present invention utilizing a vertical flat test search coil to better reflect field penetration into the body;

Figure 7(a) is a front perspective view of two C-shaped cores placed adjacently to form the equivalent of a single core for excitation of peripheral nerves;

Figure 7(b) is a side perspective view of two thinner legged C-shaped cores vertically overlapping each other to form a single common, center core for use in magnetic nerve stimulation;

Figure 7(c) is a side perspective view of two wider legged C-shaped cores vertically overlapping each other to form a single common, center core for use in magnetic nerve stimulation; and

Figure 8 is a schematic diagram of a chair used for treatment of incontinence with two C-shaped cores placed adjacently to each other and having a single coil winding.

Detailed Description of the Preferred Embodiment and the Drawings

For the treatment of incontinence using magnetic nerve stimulation, it is necessary to stimulate the pelvic floor muscles. As discussed in prior U.S. Patent No. 5,125,411 and U.S. Patent Application Serial No.09/001,782 filed December 31, 1997 (the disclosures of which are fully incorporated herein by reference), such a stimulation is achieved by concentrating magnetic flux directly up the vaginal cavity, focusing in the direction of a patient's bladder muscles.

Three suitable core designs capable of realizing this objective are shown in Figure 7. Each of the cores are constructed by combining two individual "C" shaped cores each spanning an angle of approximately 210°. In alternate embodiments, any arc-shaped core spanning an angle less than 360 degrees may be used with the invention. For example, a core spanning 180° is very convenient for using the material efficiently since two cores can be constructed from every mandrel. A core comprised of two cores each of which spans a larger angle (e.g. 210-220 degrees) can also be used. A generic-use core that might be used at various locations around the body can span an angle in the range of approximately 180- 220°.

hi accordance with the invention, the legs of the cores are brought together in a central, common region, with various configurations being possible. The legs of the individual cores are adjacent in the first core design as shown in Fig. 7 (a), to form a common, centrally located leg. The core legs are vertically overlapping each other in the second and third core designs, as shown in Figs. 7 (b) and (c). The resulting common central leg of the two "C" cores is wound by a coil which is stimulated using an electrical circuit such as that disclosed in U.S. Patent No. 5,725,471, and the return path for the flux is split between the two "C"s. The cores themselves are located proximally and distally under a seat upon which a patient sits during the treatment, as shown in Fig. 8. It has been found that, between the different configurations, the vertically overlapping or stacked designs of the legs (Figs. 7(b) and 7(c)) provide a stronger magnetic field emanating from the central hub. Such overlapping designs produce a greater concentration and focusing of the magnetic flux.

Dimensions for two alternative vertically overlapping core configurations are shown in Figure 1. The primary distinction between the two configurations is that one utilizes a pair of wider core legs, each of the wider-legged cores having a 2 inch inner diameter, while the other ("the thinner core") utilizes a pair of cores each having an inner diameter of 3 inches. Both cores are preferably constructed from a pair of cores with outside diameter of 6 inches. The height is the same for both cores.

Shown in Figure 2 is a magnetic field arrow plot for the thinner core. A strong B field comes out of the central hub and returns into the non-common leg. According to Lenz's law, if a loop of wire is provided, the number of arrows going through that loop per unit of time will dictate the induced E field in that loop. Loops that lie in the x-y plane will register zero induced voltage, since the arrows never penetrate the loop. Small exceptions to this occur in the fringe region where the field bends away from the core as shown in the lower arrow plot. Only loops that lie in the x-z and y-z plane will register induced voltage. The x-z plane loop corresponds to a plane tangential to the core faces, and when used in connection with the incontinence chair, the seat's bottom surface. The y-z plane corresponds to the region occupied by the patient receiving the incontinence treatment in the chair. This plane is especially important for getting a picture of the penetration field depth.

To determine if the magnetic field stimulator is operating properly, a rectangular field test coil can be positioned over the incontinence core stimulator as shown in Fig. 5, and as further disclosed in U.S. Patent Application Serial No. 09/137,209 filed August 20, 1998, the disclosure of which is fully incorporated herein by reference. The core 1 is excited by the coil current in the winding 2. The field so produced induces a voltage in the field test coil 3. The coil is positioned over the core in a plane parallel to the plane of the core faces 6. A simple voltmeter is then used to measure the voltage induced in the test coil. This simple test provides a rapid assessment of whether the core and coil are operating properly. The voltage is simply compared to that expected from a test model serving as the bench test model. Alternatively, to measure the induced E field from the stimulator that would better reflect field penetration into the body, a vertical coil that catches flux traveling from one pole to the next can be provided, as shown in Fig. 6 and as further disclosed in U.S. Patent Application Serial No. 09/501,245, filed February 10, 2000, the disclosure of which is fully incorporated herein by reference. When main core 1 is excited with exciting coil 2, it induces a voltage in the search coil 4 reflecting operation of both the core and the exciting coil. Coil 4 delivers a voltage that is the time rate of change of all the magnetic flux passing through it. It therefore provides a mechanism for assessing to some extent penetration into the body. The coil lies in a plane perpendicular to that of the core faces 6.

The two primary search coil planes are shown in Fig. 3. On the horizontal x-z plane, shown in Fig. 3(a), the currents primarily run in the z direction, up on the left side and down on the right side. The vertical y-z search coil, shown in Fig. 3(b), also shows the currents running primarily in the z direction, up on the right and down on the left. The bulk of the current crowds close to the core.

To determine the best possible configuration of the core and coil, the following experiment was conducted. Suppose Coil A is 11 turn #8 circular coil, Coil B is 9 turn #8 ellipsoidal coil, Coil C is 11 turn #10 coil, and Coil D is 9 turn #10 circular coil. There are four coils and two cores. For each possible configuration, the induced voltage was measured using a 1.5" by 4" coil wrapped over 1.5" PVC pipe. The coil was spaced off the core using the spacer block presently employed for incontinence. This block is approximately 1/8" thick. Both the horizontal and vertical induced voltage are measured in each case, as shown in Figure 3.

The results are shown tabulated in Table I - Table V. Table I. Thinner Core with vertical search coil induced voltage (mv) orientation

Table II. Wider Core with vertical search coil induced voltage (mv) orientation

Table III. Thinner Core with horizontal search coil induced voltage (mv) orientation

Table IV. Wider Core with horizontal search coil induced voltage (mv) orientation

Table V Inductance of the coils with the cores.

The following observations were made from the above experiment: 1) Changing from #8 to # 10 results in a smaller induced E field due to the higher I²R loss in the wires. 2) When compared against the same coil with meaningful data, the wider cores provide superior results to the thinner cores. Results in Column A for the wider core might be confusing since the 11 turn #8 wire bundle did not fit in the slot. Therefore, a larger inner diameter might be more appropriate. 3) Reducing the number of turns results in an- increase in the induced field. This is a questionable trade off since it happens at the expense of exciting the nerve with a higher frequency. Furthermore, the current in the coil increases; with that increase, comes more parasitic losses. Therefore, it is probably better to use the larger number of turns (11). 4) The ellipsoidal winding has a poorer performance than the circular winding. All four sides of the winding contribute to the B field. Therefore a tight wrap is the goal for the winding.

Based on the following experiment, the preferred embodiment of the incontinence core is the embodiment shown in Fig. 1 inset (b), but with a slightly larger inner diameter to allow for 11 turns of #8 wire in a tight wrap. A modification to achieve this objective is shown in Figure 4. The modification uses a 1.5" extrusion depth, a 6" outer diameter, and a 2.4" inner diameter, which was determined from [1] below. The #8 wire is 0.25" in diameter. A double run of an 11 turn winding with a 50% packing factor requires a diameter:

The turns on the winding wrap, in the preferred embodiment, should be as tight as possible.

The preferred embodiment of the invention may also employ the 1.5" by 4" coil wrap on end (i.e. in a vertical orientation) to quality test the units. The vertical orientation, shown in Figs. 3(b) and 6, provides a better yield for the field penetration into the body. The horizontal field measures only surface fields on the plane of the coil. Either is sufficient to determine whether the unit is functioning. The vertical search coil orientation, however, will reflect whether the field is properly penetrating to depths required for pelvic floor stimulation.

As disclosed in U.S. Patent No. 5,725,471, the disclosure of which is fully incorporated herein by reference, the core should preferably be constructed using highly saturable material. Since the magnetic fields desired typically reach 1.5 Tesla or higher, it is desirable to use materials which saturate at or above 1.5 Tesla. One suitable material, for example, is vanadium permendur. Other suitable materials include the metallic glasses (i.e. metglass), permalloy, supermalloy, powdered iron, and silicon irons or silicon steels, in particular, 3% grain oriented steel (magnesil). Ferrite can also be used, although it is not preferred, due to the fact that it saturates at 0.5 T. These materials can be obtained, for example, from Magnetics, Inc. in Butler, Pennsylvania. Incontinence cores used in the preferred embodiment of the present invention are constructed of vanadium permendur.

Having described this invention with regard to specific embodiments, it is to be understood that the description is not meant as a limitation since further variations or modifications may be apparent or may suggest themselves to those skilled in the art. Although the invention is described with respect to the incontinence treatment, the disclosed invention may also be used to stimulate other groups of nerves. It is intended that the present application cover such variations and modifications as fall within the scope of the appended claims.

Claims

I claim as follows:

1. An apparatus for excitation of peripheral nerves, comprising a. a vertically overlapping core, said vertically overlapping core comprising at least two arc-shaped cores, each of said arc-shaped cores spanning an angle of less than three hundred sixty degrees (360°), said vertically overlapping core further comprising a highly saturable material; b. an excitation coil, said coil at least partially wrapped around said vertically overlapping core; and c. electric current means connected to said excitation coil to create a current flow in said excitation coil that causes said vertically overlapping core to generate a magnetic field.

2. An apparatus as claimed in Claim 1, wherein each of said arc-shaped cores comprises an inner diameter, said inner diameter being less than three inches (3").

3. An apparatus as claimed in Claim 1, wherein each of said arc-shaped cores comprises an iimer diameter, said inner diameter being within the range between about two and three inches (2"-3").

4. An apparatus as claimed in Claim 1, wherein each of said arc-shaped cores comprises an inner diameter, said inner diameter being two inches (2").

5. An apparatus as claimed in Claim 1, wherein each of said arc-shaped cores comprises an inner diameter, said inner diameter being 2.4".

6. An apparatus as claimed in Claim 1, wherein each of said arc-shaped cores defines an arc of approximately two hundred and ten (210) degrees.

7. An apparatus as claimed in Claim 1, further comprising a seat for a user to sit upon, said vertically overlapping core being located underneath said seat with the flux of said magnetic field generated by said vertically overlapping core being found above said seat.

8. An apparatus as claimed in Claim 7, wherein said magnetic field produced by said vertically overlapping core is focused in the area of about a user's bladder muscles when the user sits on said seat.

9. An apparatus as claimed in Claim 1, wherein said highly saturable material is vanadium permendur.

10. An apparatus as claimed in Claim 1, wherein said arc-shaped cores comprise faces, said faces forming a plane.

11. An apparatus as claimed in Claim 10, further comprising a search coil for testing of said magnetic field generated by said apparatus, said search coil being positioned over said vertically overlapping core in a plane parallel to said plane of said faces of said arc-shaped cores.

12. An apparatus as claimed in Claim 10, further comprising a search coil for testing of said magnetic field generated by said apparatus, said search coil being positioned over said vertically overlapping core in a plane perpendicular to said plane of said faces of said arc-shaped cores.