NL1037451C2

NL1037451C2 - Device, system and method for magnetic stimulation.

Info

Publication number: NL1037451C2
Application number: NL1037451A
Authority: NL
Inventors: Johannes Wilhelmus Maria Bree
Original assignee: Vabrema B V
Priority date: 2009-11-05
Filing date: 2009-11-05
Publication date: 2011-05-10

Description

DEVICE, SYSTEM AND METHOD FOR MAGNETIC STIMULATION

The present invention relates to a device, system and method for magnetic stimulation of an object, more specifically a body part of a human body.

5 For instance, transcranial magnetic stimulation (TMS) is a noninvasive method for exciting neurons in the brain. The neurons are excited by applying a changing magnetic field to the brain tissue. Usually the magnetic field is generated by a circular coil or a figure-of-eight shaped coil, through which a pulse or time varying current is injected.

The changing magnetic field causes eddy currents in the brain that can be detected by a 10 suitable detector arrangement. More specifically, neurons excited by the changing magnetic fields emit an electromagnetic signal that may be detected and analysed by one or more radiation detectors arranged in the neighbourhood of the coils. In this way brain activity can be triggered with minimal discomfort, enabling the study of the functionality of the brain.

In single or paired pulse transcranial magnetic stimulation (TMS) the magnetic pulse or 15 pulses cause a population of neurons in brain to depolarise and discharge an action potential. If used in the primary motor cortex, it produces a motor-evoked potential (MEP) which can be recorded on electromyography (EMG). If used on the occipital cortex, 'phosphenes' (flashes of light) might be detected by the subject. In most other areas of the cortex, the participant does not consciously experience any effect. These effects generally do not outlast the period of 20 stimulation.

In repetitive transcranial magnetic stimulation (rTMS) effects are produced which last longer than the period of stimulation. It could be a treatment tool for various neurological conditions (e.g. migraine, stroke, Parkinson's disease, dystonia, tinnitus) and psychiatric conditions (e.g. major depression, auditory hallucinations).

25 Since neurons behave as leaky capacitors, with time constants of a few hundred microseconds, efficiency of transcranial magnetic stimulation stimulation is maximized when the stimulator is operating at the highest possible voltage and using pulses of the shortest possible duration.

Generally in practice the transcranial magnetic stimulators consist out of a circuit which 30 drives the treatment coil which in turn produces the magnetic stimulation pulse applied to the patient. The circuitry comprises a power source, a capacitor and a switching element. A high voltage power source is used to charge the capacitor, typically a few kilovolts (i.e.l-3kV).The value of the capacitor is typically a few hundred micro Farad (i.e.l00-500uF). The coil is typically a few tens of micro Henry (i.e.l0-30uH) with a diameter of several centimetres (i.e. 2 35 or more centimetres) and multiple windings (i.e. five or more windings). The switching element 1 03 7 4 51 2 is typically a thyristor which can handle a peak current of several tens of kiloamperes (i.e. 10000 Amperes) and a changing current in time (dl/dt) of a few hundred amperes per microsecond(i.e 150 A/us). The high magnetic field is then generated by discharging the capacitor though the switching element across the treatment coil. Typically the pulse duration is around a few 5 hundred microseconds (i.e. 280us) and the changing magnetic field in time (dB/dt) is several tens of thousand Tesla per second (i.e. 30kT/s).

The treatment coils have a very low characteristic impedance, which make them very susceptible to energy losses and overheating from the internal resistance of the treatment coil. The safety standards for medical equipment (IEC-601) require the surface temperature to stay 10 below 41‘ C. The thyristor used as switching element determines the maximum operating voltage and pulse duration and therefore the efficiency of the magnetic stimulator.

A drawback of the transcranial magnetic stimulator with thyristor as switching element and treatment coils with several windings is the large amount of heat generated inside the treatment coil. This heat generation limits the maximum duration of sustained operation.

15 Another drawback of these multiple winding coils is that the changing current flowing through these windings causes a contraction in the windings. This may produce a loud clicking sound, which is unpleasant for the person under investigation. These drawbacks limit the treatment time and. cause a discomfort for the person being treated.

A further drawback is that focussing the magnetic field at a relatively small user 20 defined area inside: the object is difficult as result of the large dimensions of the existing treatment coil used to generate the magnetic field. If, however, using the circuitry above, the treatment coils are reduced in size by half, the required power needed increases 25-fold and the required current increases 5-fold to generate the same magnetic field required to stimulate neurons. This would even generate more heat and limit treatment time. Another drawback of the 25 large dimensions of the existing treatment coil is that they have to be manually moved to stimulate a different region.

An object to the present invention is to provide a device and method for magnetic stimulating of an object, especially a part of a human body, wherein at least one of the above identified and/or other drawbacks of the prior art has been removed or at least reduced.

30 Furthermore, it is an object of embodiments of the invention to provide a method and device wherein transcranial magnetic stimulation occurs with a reduced heat generation.

It is also an object of embodiments of the invention to provide a method and device wherein transcranial magnetic stimulation occurs without the loud noise made by existing treatment coils.

3 A further object of the present invention is to provide an improved focussing of the area to be treated with smaller treatment coils. Furthermore it is an object of embodiments of the invention to provide a method and device wherein transcranial magnetic stimulation occurs with multiple smaller treatment coils for stimulation of a relatively small user defined area.

5 According to a first aspect of the invention at least one object is achieved in a device for magnetically stimulating an object, more specifically a part of the human body, the device comprising: - at least one coil element that is configured to be arranged at or close to the object to be stimulated; 10 - an electric generator connected to the coil element, the generator being configured to discharge into the at least one coil element so that the at least one coil element generates a short duration pulsed magnetic field, the magnetic field being suitable to stimulating neurons present in the object when in operation the coil element is arranged at or close to the object, wherein the coil element is a single winding coil element.

15 -the switching device used in the electric generator to be a spark gap switch.

The device generates short duration pulses which in combination with a single winding coil causes little or no heat generation or clicking sound generated during the stimulation of the object. In order to generate a short duration high strength magnetic field, the coil element is 20 formed by a coil having a single winding only. Using a single winding coil element and, preferably, a coaxially built electric circuit, the self-inductance of the coil element and circuit can be kept relatively low, which means that a high current and/or a short rise time of the current in the coil element may be realised. A self-inductance of several tens of nanoHenrys or less can be achieved easily in this embodiment.

25 In an embodiment of the invention the electrical generator comprises a capacitor arranged so as to generate a magnetic field strength inside the coil element of at least 0.5 Tesla when it is discharged into the coil element. Generally, this value of the magnetic field strength is sufficient to affect neurons. In other embodiment of the invention the electrical generator comprises a capacitor arranged so as to generate a change in magnetic field strength of at least 30 10 MTesla/s.

In a further embodiment the single winding coil element comprises a coil with the lead of one end of the winding returning through the centre of the coil to the other end, so that both terminals of the winding are positioned at the same end of the single winding coil element. In further embodiments the single winding coil element has a generally circular of oval shape.

4

In a further embodiment the electric generator comprises a high voltage power supply (i.e. 10kV-100kV), a capacitor and a switching element, the switching element being configured for charging the capacitor in a first stage and discharging the capacitor in a second stage so as to provide a short duration high current in the single winding coil element. The switching element 5 is to be formed by a spark gap switch, preferable a two-gap spark gap switch, configured to break down at a predefined discharge voltage. The circuit may be built coaxially so that the self inductance is kept to a minimum. The rate of change of the current per unit of time can be as large as 100A/ns (or even larger).

In order to reduce the size of the device, more specifically the mutual distance between 10 the elements (for example the multiple spark gap switch), the generator may at least partly be arranged in an enclosure. The enclosure is then pressurized to a predetermined high pressure value, for instance at least about 8 bar.

According to another aspect of the present invention a method of magnetically stimulating an object, more specifically a part of the human body, is provided, the method 15 comprising: - positioning at least one single winding coil element at or close to the object to be stimulated; - discharging a capacitor of an electric generator into the at least one coil element so as to generate a short duration pulsed magnetic field, the magnetic field being suitable to stimulate 20 neurons present in the object, wherein the coil element is a single winding coil element.

Further advantages and characteristics of the present invention will become apparent from the description of several preferred embodiments thereof. In the description reference is made to the annexed drawings, that show: 25 Figure 1 a part of a circuit diagram of a generator connected to a single winding coil for stimulating the brain of a patient;

Figure 2 a schematic top view of a patient whose brain is stimulated by an embodiment of the single winding coil element according to the present invention; and

Figure 3 is a graphical representation of the magnetic field.

30 Figure 4 is a graphical representation of a matrix of treatment coils around the head.

Figure 1 shows a circuit diagram of an electric generator for driving a single winding coil element for stimulation the brain of a patient. The generator 1 comprises a high power source 2 (for instance a high voltage source), a resistor 4, a switching element 5 and a single winding coil 6. Optionally one or more damping resistors 12 in series are present as well.

35 Parallel to the power source 2 a capacitor 3 is arranged.

5

In an embodiment the switching element 5 is comprised of a spark gap switch (preferable a multiple spark gap switch). A multiple spark gap switch generally comprises an arrangement of conducting electrodes separated by a plurality of gaps. When a suitable voltage supplied, the sparks may be formed, causing ionization of the medium between the electrodes (for instance 5 air) and reducing the electrical resistance of the switch. An electrical current then flows until the path of ionized gas is broken or the current is reduced below a minimum current value.

Figure 2 shows a top view of the single winding coil element placed above a human head. The single winding coil 6 comprises an essentially circular winding 10.

In operation, the voltage source 2 charges the capacitor 3 causing a rise of the voltage 10 until the spark gaps of the multiple spark gap switch 5 fire. The spark gap switch can be seen as a switch which closes by a specific voltage. This voltage is dependent on the mutual distances (gaps) of the electrodes and the pressure of medium between the electrodes. A greater distance and/or higher (air) pressure will cause a higher breakdown voltage. When the spark gap switch 5 fires, the capacitor 3 discharges into the single winding coil 6 generating therein a pulse-shaped 15 current signal. The current in the coil 6 generates a short and intense magnetic field. Figure 3 shows that the magnetic field generated by the single winding coil element 6, as represented in figure 3 by field lines 14.

The self-inductance (typically as low as about 50-150nH) of the circuit arrangement of the generator is relevant for the amplitude of the current through the circuit and therefore for the 20 strength of the magnetic field created inside the coil. The inductance of the single winding coil itself is considerably lower, for instance in the order of magnitude l-5nH. The current though the single winding coil 6 generates the magnetic field. The strength of the magnetic field largely depends on the amplitude of the current. Fast rise times of at least 100A/ns can be easily achieved. With a peak current of at least lkA.

25 Figure 4 shows multiple treatment coil elements 26 arranged within a matrix 25 around the head 22 and connected to the same generator. The coil elements 26 are incorporated in a helmet (not shown) or similar device to maintain the coil at a fixed position relative to the head of the patient during treatment. In an embodiment groups of coil elements 26 or, in other embodiments, each individual coil element 26, can be driven independently by the electric 30 generator. This matrix of independent controllable treatment coil elements will improve the focussing of the magnetic field 24 on a relatively small user defined area 23. Furthermore, by appropriate switching of the coil elements it is possible to change the stimulation site or stimulation area without any physical displacement of the treatment coils with respect to the head. For instance, in figure 4 is shown the situation wherein four coil elements in the upper part 35 of the head are activated simultaneously, while the other coil elements remain inoperable.

6

Activating these four coil elements stimulates a corresponding portion (or area) in the upper part of the brain of the patient. If this stimulation leads to a specific anticipated reaction of the patient, for instance the movement of a finger, then the operator knows that the right portion of the brain has been stimulated. If stimulation does not lead to the desired reaction, another 5 portion of the brain can be stimulated, simply by activate one or more other coil elements and deactivate one or more of the previously activated coil elements. Displacement of the coil elements relative to the head and/or displacement of the head relative to the coil elements is not needed in these embodiments. This may give the patient a comfortable feeling and may at the same time reduce the duration of the treatment since the system may easily and efficiently find 10 the right portion of the brain to be stimulated to reach the desired effect. The winding of the coil element may be made of aluminium with a diameter of several millimetres.

Although the invention has been described with reference to specific embodiments thereof, it will be appreciated that invention is not limited to these embodiments and that changes and modification to the device and method described herein may be made without 15 departing from the invention. The rights applied for are defined by the following claims.

20 1037451

Claims

A device for magnetic stimulation of an object, in particular a part of the human body. The device comprises: 5. at least one coil element placed on or close to the object to be stimulated; - an electric generator connected to the coil element, the generator is configured to discharge in at least one coil element so that at least one coil element generates a briefly pulsed high magnetic field, the magnetic field is suitable for stimulating neurons present in the object when the coil element is placed on or close to the object, the coil element is a single wind coil.

The device of claim 1, wherein the single-wind coil element is generally circular or oval-shaped.

Apparatus as claimed in claim 1 or 2, wherein the electric generator comprises a capacity configured to generate a magnetic field of at least 0.5 Tesla in the single-wind coil element.

Apparatus as claimed in any one of the preceding claims, wherein the electric generator 20 comprises a power source, a capacitance and a switching element, the switching element is configured to charge the capacitance in the first phase and discharge the capacitance in the second phase to short-term, high current through the single-wind coil element.

5. Device as claimed in claim 4, wherein the switching element consists of a spark gap switch, preferably a two spark gap switch, configured to break through at a discharging voltage of, for example, 10 kV-100 kV.

6. Device as claimed in any of the foregoing claims, wherein the generator is at least partially configured in a casing and pressurized, at a high pressure of preferably 8 bar

7. Device as claimed in any of the foregoing claims, wherein the rise time of the current is at least 100 A / ns, the amplitude of the current is 1KA and / or the self-induction of the circuit consisting of, inter alia, the single-wind coil element, a few dozen nanoHenrys is. 1 03 74 51

The device of any one of the preceding claims, wherein it is a transcranial magnetic stimulation device that includes at least one coil element placed on or close to the head for locally influencing neurons in the head. 5

Apparatus according to any one of the preceding claims, comprising a detector for measuring an effect of stimulation of the neurons in the object to be stimulated.

10. System for magnetically stimulating an object, in particular a portion 10 of the human body, the system comprises a plurality of devices according to one of the preceding claims, wherein the devices are placed in a two-dimensional matrice so that different ones can be stimulated areas of the object are covered.

11. System as claimed in claim 10, comprising a controller attached to a plurality of devices, wherein the controller is configured to control a plurality of devices so that different areas to be stimulated are stimulated simultaneously.

12. System according to claim 10, comprising a controller attached to a plurality of devices, wherein the controller is configured to control the devices so that different stimulation areas are stimulated one after the other.

13. Method for magnetic stimulation of an object, in particular a part of the human body, the method comprises: - positioning of at least one single windy coil element placed on or close to the object to be stimulated. discharging the capacity of an electric generator in at least one coil element so that a short, high magnetic field is generated, the magnetic field stimulates the neurons present in the object, in which the coil element is a single wind coil element. 30

The method of claim 13, comprising a single wind coil coil element in which a magnetic field strength of at least 0.5 Tesla is generated.

15. Method as claimed in claim 13 or 14, wherein the electric generator comprises a power supply, a capacitance and a switching element, the method in which the switching element loads the capacitance in the first phase and discharges the capacitance in the second phase so as to achieve a short and high send current through the single-wind coil.

Method as claimed in claims 13-15, comprising charging the capacity 5 in the first phase and discharging the same capacity in the second phase to send a short-term and high current through the single-wind coil element.

17. Method as claimed in claims 13-16, comprises at least one coil element attached to the head or at least close to it, wherein the at least one coil element is positioned such that neurons are stimulated locally in the head. 1 03 74 51