GB2271931A

GB2271931A - Magnetic stimulator for medical use

Info

Publication number: GB2271931A
Application number: GB9222703A
Authority: GB
Inventors: Benjamin Israel Sacks; Noel Rudolf; Eric Robert Laithwaite
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-10-29
Filing date: 1992-10-29
Publication date: 1994-05-04
Also published as: GB9222703D0

Abstract

A diagnostic device is in the form of a magnetic stimulator for e.g. non-invasive techniques for intracranial magnetic stimulation, in which the magnetic pulse is generated by a plurality of coils 20, 21, 22 in a co-axial array supplied with electrical energy in such a form as to allow the coils to bring a transient magnetic pulse to focus (f) at a distance (d) "outboard" of the plane of the coils P1 P2 and able to be varied by a neuro-surgeon. The coils may be axially movable, and may be co-planar. A cup 23 containing the coils may be mounted on a stand or held in the hand. A ferromagnetic insert may be used. <IMAGE>

Description

SPECIFICATION DIAGNOSTIC DEVICE The present invention relates to diagnostic devices and in particular to magnetic stimulators that are useful to make localized non-invasive studies for inter alia transcranial magnetic stimulations of the human motor cortex and peripheral nerves focally in 'remote' internal locations in both medical and veterinary studies.

Descnntion of the Prior Art Non-invasive techniques according to Cohen, L.G. et al (1990) for transcranial electrical stimulation of human motor cortex have been developed in recent years; see Merton, P.A. et al (1980), Marsden, C.D. et al (1981), Rossini, P.M. et al (1985).

However, transcranial electrical stimulation may be painful. Barker, A.T. et al (1985) have described a less painful method of transcranial stimulation of the brain involving the use of a brief magnetic pulse. This technique is useful for studying central motor conduction velocities, see Barker, A.T. et al (1986) and Hess, C.W.

et al (1986).

Biblioaranhv Barker, A.T., Jalinous, R. and Freeston, l.L. Noninvasive magnetic stimulation of human motor cortex. Lancet, 1985, ii: 1106-1107.

Barker, A.T., Freeston, I.L., Jalinous, R. and Jarratt, J.A. Clinical evaluation of conduction time measurements in central motor pathways using magnetic stimulation of the human brain. Lancet, 1986, i: 1325-1326.

Cohen, L.G., Roth, B.J., Nilsson, J., Nguyet Dang, Panizza, M., Bandinelli, S., Friauf, W. and Hallett, M. Effects of coil design on delivery of focal magnetic stimulation. Technical considerations. Electroencephalog raphy and clinical Neurophysiology, 1990, 75: 350-357, Elsevier Scientific Publishers Ireland, Ltd.

Hess, C.W., Mills, K.R. and Murray, N.M.F. Measurement of central motor conduction in multiple sclerosis by magnetic brain stimulation, Lancet, 1986, ii: 355-358.

Marsden, C.D., Merton, P.A. and Morton, H.B. Maximal twitches from stimulation of motor cortex in man. J. Physiol. (Lond.). 1981, 312: 5P.

Merton, P.A. and Morton, H.B. Stimulation of the cerebral cortex in the intact human subject. Nature, 1980, 285: 227.

Rossini, P.M., Marciani, M.G., Caramia, M., Roma, V. and Zarola, F. Nervous propagation along 'central' motor pathways in intact man: characteristics of motor responses to 'bifocal' and 'unifocal' spine and scalp non-invasive stimulation.

Electroenceph. clin. Neurophysiol., 1985, 61: 272-286.

A typical array of prior art coils for magnetic stimulation is shown in the paper by Cohen, L.G. et al (1990) on page 351; it is their figure 2. All of these six stimulator coils have the disadvantage, according to Cohen L.G. et al, that their operation is less than ideal in this, that the stimulation cannot be accurately targeted on a small region of the cerebral cortex or other parts of the central nervous system for the magnetic field drowned progressively when measurements were made more distant from the plane of the coil, and the magnetic field was not focused, but tended to diverge.

It is a primary feature of the present invention to advance the prior art and to reverse this serious disadvantage by increasing the ability of the neuro-surgeon to focus the magnetic field from a magnetic stimulator at a variable distance from the place of the coils.

A diagnostic magnetic stimulator embracing principles of the present invention and useful for making examinations of a non-invasive character for inter alia transcranial magnetic stimulation may comprise a plurality of electrical coils coaxially arranged and suitably provided with electrical energy thereby to produce a focused transient magnetic field at a variable distance from the plane of the said coils.

Other features and advantages of the present invention will become more apparent from an examination of the following specification when read in conjunction with the appended drawings in which: Figure 1 is an illustration of typical prior art scope where the geometry, of the various coils of known magnetic stimulators, is clearly shown.

Figure 2 is a schematic drawing showing the inability of a single coil of figure 1 satisfactorily to focus its magnetic field.

Figure 3 is a schematic drawing of one coil array for a magnetic stimulator of the present invention.

Figure 4 is a schematic drawing showing a plurality of coils in a co-axial array to form a cup shaped magnetic stimulator of the present invention ideal for trans-cranial applications.

Figure 5A is a perspective sketch illustrating the cup shaped magnetic stimulator of Figure 4 in diagnostic use on a human skull - but held on a stand to keep the coils in a fixed postion.

Figure 5B is a perspective sketch showing the cup shaped magnetic stimulator of Figure 4 in the hand held exploratory mode to see the neurological effects on a patient for diagnosis mutatis mutandis.

Referring now to the figures of the drawings, the reference numeral 11 designates a prior art coil for a magnetic stimulator, the coil being of nine centimetres in diameter (3.543307 inches). Numerals 12 and 13 respectively designate angulated coils of five centimetres (1.9685 inches) and nine centimetres.

Numerals 14 and 15 respectively designate flat spiral coiils of fourteen centimetres (5.511811 inches) and 6.7 centimetres (2.637795276 inches) diameter. Numeral 16 designates twin oval coils arranged in a butterfly configuration; each coil is approximately four centimetres (1.5748 inches) in diameter and wound to give a current in each that has contraflow. A Cadwell MES-10 magnetic stimulator was used to generate the magrvRetitimuli delivered by coils 11, 12, 13 and 16. A Novametrix Magstim 200 was used to generate the stimuli delivered by coils 14 and 15. The said magnetic stimulators per se not being shown.

The coils 11 to 16 inclusive, tend to produce the magnetic field configuration of the magnetic coil 17 of Figure 2, whether or not it contains a magnetic core of iron. The magnetic field spreads out rapidly from the axis al a2 as it leaves the coil centre a3.

In contra-distinction, a reference to Figure 3 discloses an improved construction in which two concentric coils 18 and 19 having a common axis al a2 provide, when suitably energized with a changing current a focused magnetic field at A and B "outboard" of the plane of the coils, that is to say the surface plane P1 P2 directly under the focused magnetic field at B on the axis al a2. In one embodiment short bursts of alternating current were fed to the coils, such that the phase of the current in the inner coil 19 lagged on that of the phase of the current in the outer coil 18.

This produced a radially inward travelling magnetic field momentarily having a more intense flux density at B than that at A.

Without doubt it is advantageous to construct a magnetic stimulator in the shape of a cup as shown in Figure 4. The cup 23 contains three coils arranged co-axially (concentric if viewed along the axis al a2) on axis al a2. The coils 20, 21 and 22.

may be made to slide toward or away from each other within the cup 23, to give a focus to the magnetic field at 'f', on axis al a2. The coils 20, 21 and 22 are fed with a sophisticated electrical supply individually 'tailored' for each coil. The wave form typically having sinusoidal or overdamped recovery characteristics so that the magnetic field focuses at 'f', builds and collapses and lasts for but a fraction of a second. Clearly by movement of the coils combined, if necessary, with the electrical supply characteristics to the individual coils the focal distance 'd' of 'f' from the plane of the coils P1 P2 can be accurately varied and controlled in intensity. Again a ferromagnetic insert, not shown, may be incorporated in the coils of the device of Figure 4.

A cup is particularly advantageous for use on the human skull:- as an historical point en Dassant Benjamin Franklin in 1757 recorded in his Works, (1887) Vol. ll, p522, that "they cupped me on the back of the head" - not I hasten, to add, for magnetic stimulation.

In Figures 5A and 5B there is shown the cup magnetic stimulator 23 of Figure 4 in use medically. In figure 5A the magnetic stimulator 23 is held in a known support stand 24 giving rotations R1 R2 if needed in two planes at right angles from a support 25.

In Figure 5B the magnetic stimulator 23 is hand held by the neuro-surgeon, the hand being shown at 26.

Finally I should speak medically very briefly concerning the magnetic stimulator. A strong magnetic pulse from a plurality of coils near the skin induces a proportional current in tissue, this stimulates the nerves. One advantage of magnetic stimulation is its ability to penetrate bone without serious attenuation so it is ideal for noninvasive studies of the cranium and the inner cortex and other parts of the brain.

Potential uses of the magnetic stimulator of the invention would be: inter alia: Brain and Spinal Cord (a) Study of the integrity of motor pathways in a more selective way than is possible with existing magnetic stimulators, by virtue of the more circumscribed region of stimulation.

(b) Sensory and other neural pathways might be studied.

(c) Stimulation of specific intra-cerebral nuclei. Variability of precise positions of nuclei among individuals would need to be taken into consideration, but stimulation of neuro-transmitter-producing nuclei might be relevant to the diagnosis andlor treatment of conditions such as Parkinsonism and Alzheimer's disease, in which neurotransmitters (amines and acetylcholine respectively) are depleted.

Peripheral Nerves Nerve conduction studies have many applications in the diagnosis of nerve lesions and peripheral neuropathies. At present electrical stimulation is used, but more effectively localized magnetic stimulation would be more acceptable to patients and would also allow targeted stimulation of deeply situated limb nerves which are now effectively inaccessible.

Defibrillation of the heart, now performed electrically, might also be achieved more safely by magnetic stimulation.

The magnetic stimulator could also aid in the early diagnosis of Parkinson's and Alzheimer's diseases.

It will be abundantly clear to a layman that the sine qua non of the success of the magnetic stimulator is the ability to focus accurately the distance (d) of the magnetic pulse (f) from the edge of the cup or the plane of the coils. To that end a probe (i.e.

a detector of magnetic focal point 'f') may be used, the magnetic stimulator being held at a point on a linear centimetre scale and the probe also; the probe being capable of being moved toward and away from the magnetic stimulator so that the focus distance can be read from the scale. The magnetic stimulator may then be used in combination with a stereotaxic apparatus such as that known as the Horsley-Clarke frame, said to be probably the most complex of all the mathematical instruments of physiology.

Claims

1. A magnetic stimulator useful to make non-invasive magnetic stimulations inter alia of the human brain and peripheral nerves, wherein the stimulator comprises a plurality of coils co-axially arranged and suitably supplied with electrical energy to bring to a focus a transient magnetic field at a distance "outboard" of the plane of the coils as hereinbefore defined and substantially on said axis of co-axiallity, said distance being variable and in the control of the user.

2. A magnetic stimulator as claimed in claim 1 wherein the plurality of coils is in the same plane and is concentric.

3. A magnetic stimulator as claimed in claim 1 wherein the plurality of coils is co-axial and is three or more wthin a cup.

4. A magnetic stimulator as claimed in claim 1, 2 or 3 wherein the coils are augmented by the inclusion of an iron or similar ferromagnetic element.

5. A magnetic stimulator as claimed in any preceding claim wherein the coils are supplied from independent electrical currents of different or similar wave form.

6. A magnetic stimulator as claimed in claim 5 wherein the voltage applied to or the current following in any coil is alternating and able to produce a transient magnetic pulse for a well defined stimulation time at a known focal point.

7. A magnetic stimulator as claimed in any preceding claim wherein one or more of the coils islare able to be moved along the axis of co-axiallity to assist in the definition of a focal pulse of transient magnetic energy.

8. A magnetic stimulator as claimed in any preceding claim wherein the stimulator is used in conjunction with a stereotaxic frame.

9. A magnetic stimulator substantially as hereinbefore described with particular reference to the several figures (3 to 5B) of the accompanying drawings.