US0077.44523B2
`
`(12) United States Patent
`Epstein
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,744,523 B2
`Jun. 29, 2010
`
`(54) DRIVE CIRCUIT FOR MAGNETIC
`STMULATION
`
`(75) Inventor: Charles M. Epstein, Atlanta, GA (US)
`(73) Assignee: Emory University, Atlanta, GA (US)
`
`(*) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 391 days.
`
`(21) Appl. No.: 11/759,537
`
`(22) Filed:
`
`Jun. 7, 2007
`
`(65)
`
`Prior Publication Data
`
`US 2008/0306326A1
`
`Dec. 11, 2008
`
`(51) Int. Cl.
`(2006.01)
`A6IN 2/00
`(52) U.S. Cl. .......................................................... 6OO/9
`(58) Field of Classification Search ................ 6OO/9 15
`See application file for complete search history.
`References Cited
`
`(56)
`
`U.S. PATENT DOCUMENTS
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`2005/0261542 A1* 11/2005 Riehl .......................... 600, 14
`
`FOREIGN PATENT DOCUMENTS
`WOO2,32504 A2
`4/2002
`WO
`WO O2/O72194 A2
`9, 2002
`WO
`WO O2/O85449 A2 10, 2002
`WO
`WO O2/O85454 A1 10, 2002
`WO
`WO O2/O94997 A2 11/2002
`WO
`WO WO 2005/OOO401 A1
`1, 2005
`* cited b
`c1ted by examiner
`Primary Examiner Samuel G. Gilbert
`(74) Attorney, Agent, or Firm Woodcock Washburn LLP
`
`ABSTRACT
`(57)
`The inventive technique includes devices and methods for
`generating a magnetic field. One such device may include an
`inductor for generating a magnetic field and a power Source
`for providing power. Such a device may also include a semi
`conductor Switching device that operatively couples the
`inductor and power source, wherein the semiconductor
`switching device directs power from the power source to the
`inductor to generate the magnetic field.
`
`44 Claims, 4 Drawing Sheets
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`Capacitor
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`Controller
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`Providing Power
`Using a Power
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`03
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`Charging an
`Energy Storage
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`05
`Using Switch to
`Discharge Energy
`Storage Device in
`Inductor
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`40? a--
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`Generating a
`Magnetic Field
`Using the
`Inductor
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`US 7,744,523 B2
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`1.
`DRIVE CIRCUIT FOR MAGNETIC
`STMULATION
`
`BACKGROUND
`
`2
`For example, to cause a stimulation coil to generate trains of
`rapid rTMS pulses, thousands of watts (W) of power are
`typically delivered to the coil. This amount of power leads to
`rapid coil heating. The amount of coil heating is so great that
`the coil often is heated to the point at which it would be
`uncomfortable or unsafe to use the coil on a patient. Thus,
`attempts have been made to cool stimulation coils using
`water, air or oil. Unfortunately, these cooling mechanisms are
`cumbersome, add complexity to the magnetic stimulation
`system, are expensive and are sometimes adversely affect the
`performance of the stimulator. A more advantageous
`approach would be to reduce the amount of power required by
`the magnetic stimulation device to generate atherapeutically
`equivalent magnetic pulse.
`
`SUMMARY
`
`In view of the foregoing drawbacks and shortcomings,
`devices and methods for generating a magnetic field are pro
`vided. One such magnetic stimulation device may include an
`inductor for generating a magnetic field and a power Source
`for providing power. Such a device may also include a semi
`conductor Switching device that operatively couples the
`inductor and power source, wherein the semiconductor
`switching device directs power from the power source to the
`inductor to generate the magnetic field.
`One such method may include providing power using a
`power Source and operatively coupling the power Source to an
`inductor using a semiconductor Switching device. The
`method may also include directing power from the power
`Source to the inductor using the semiconductor Switching
`device and generating the magnetic field using the inductor.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a diagram illustrating an example magnetic device
`according to an embodiment;
`FIG. 2 is a circuit diagram illustrating an example magnetic
`device drive circuit according to an embodiment;
`FIG. 3 is a screen shot illustrating an example plot of
`Voltage across an inductor in accordance with an embodi
`ment; and
`FIG. 4 is a flowchart illustrating an example method of
`producing a magnetic field in accordance with an embodi
`ment.
`
`DETAILED DESCRIPTION
`
`The subject matter of the disclosed embodiments is
`described with specificity to meet statutory requirements.
`However, the description itself is not intended to limit the
`Scope of this patent. Rather, the inventors have contemplated
`that the claimed subject matter might also be embodied in
`other ways, to include different steps or elements similar to
`the ones described in this document, in conjunction with other
`present or future technologies. Moreover, although the term
`“step” may be used herein to connote different aspects of
`methods employed, the term should not be interpreted as
`implying any particular order among or between various steps
`herein disclosed unless and except when the order of indi
`vidual steps is explicitly described.
`Overview
`According to an embodiment, an improved drive circuit is
`provided. The drive circuit may reduce the power required by
`a stimulation coil to generate atherapeutic magnetic pulse. As
`a result of the reduced power, the size and amount of heat
`generated by a magnetic stimulation coil may be reduced,
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`A number of medical ailments are treated and/or diagnosed
`through the application of a magnetic field to an afflicted
`portion of a patient’s body. Neurons and muscle cells are a
`form of biological circuitry that carry electrical signals and
`respond to electromagnetic stimuli. When an ordinary con
`ductive wire loop is passed through a magnetic field or is in
`the presence of a changing magnetic field, an electric current
`is induced in the wire.
`The same principle holds true for conductive biological
`tissue. When a changing magnetic field is applied to a portion 15
`of the body, neurons may be depolarized and stimulated.
`Muscles associated with the stimulated neurons can contract
`as though the neurons were firing by normal causes.
`A nerve cell or neuron can be stimulated in a number of
`ways, including indirectly via transcranial magnetic stimula
`tion (TMS), for example. TMS uses a rapidly changing mag
`netic field to induce a current in a nerve cell, without having
`to cut or penetrate the skin. The nerve is said to “fire' when a
`membrane potential within the nerve rises with respect to its
`normal negative ambient level of approximately -90 mV.
`depending on the type of nerve and local ionic conditions of
`the Surrounding tissue.
`The use of magnetic stimulation is very effective in reha
`bilitating injured or paralyzed muscle groups and may prove
`useful in other therapies involving peripheral nerve stimula
`tion including, but not limited to, pain mitigation, stimulation
`of neovascularization, wound healing and bone growth.
`Magnetic stimulation also has proven effective in stimulat
`ing regions of the brain, which is composed predominantly of
`neurological tissue. One area of particularinterest is the treat
`ment of depression. It is believed that more than 28 million
`people in the United States alone suffer from some type of
`neuropsychiatric disorder. These include conditions such as
`depression, Schizophrenia, mania, obsessive-compulsive dis
`order, panic disorders, and others. Depression is the "com
`40
`mon cold of psychiatric disorders, believed to affect 19
`million people in the United States and possibly 340 million
`people worldwide.
`Modern medicine offers depression patients a number of
`treatment options, including several classes of anti-depres- 45
`sant medications (e.g., SSRIs, MAOIs and tricyclics),
`lithium, and electroconvulsive therapy (ECT). Yet many
`patients remain without satisfactory relief from the symptoms
`of depression. To date, ECT remains an effective therapy for
`resistant depression; however, many patients will not undergo 50
`the procedure because of its severe side effects.
`Recently, repetitive transcranial magnetic stimulation
`(rTMS) has been shown to have significant anti-depressant
`effects for patients that do not respond to the traditional
`methods. The principle behind rTMS is to apply a subconvul- 55
`sive stimulation to the prefrontal cortex in a repetitive man
`ner, causing a depolarization of cortical neuron membranes.
`The membranes are depolarized by the induction of small
`electric fields in excess of 1 V/cm that are the result of a
`rapidly changing magnetic field applied non-invasively.
`To generate a magnetic pulse that is capable of providing a
`therapeutic effect on a patient, TMS, rTMS and Magnetic
`Seizure Therapy (MST) treatments all require a great deal of
`electrical power, typically in the range of several hundred
`joules (J) per pulse. Various attempts to optimize the design of 65
`the coil used in such treatments have not been able to sub
`stantially mitigate the need for a great deal of electrical power.
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`US 7,744,523 B2
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`which in turn reduces cooling requirements associated with
`the stimulation coil. Because of these reductions, the entire
`magnetic stimulation device may be made less complex,
`smaller and less expensive. As will be discussed below,
`embodiments may reduce the power requirement of TMS and
`rTMS by approximately 50%, which may produce again in
`efficiency of approximately 800% as compared to conven
`tional drive circuits and stimulation coils.
`Magnetic Device Overview
`For purposes of explanation and context, an overview of
`the operation and applications of a magnetic device in which
`aspects of the various embodiments may be implemented is
`now discussed. As is well knownto those skilled in the art, the
`magnitude of an electric field induced on a conductor is
`proportional to the rate of change of magnetic flux density
`across the conductor. When an electric field is induced in a
`conductor, the electric field creates a corresponding current
`flow in the conductor. The current flow is in the same direction
`of the electric field vector at a given point. The peak electric
`field occurs when the time rate of change of the magnetic flux
`density is the greatestand diminishes at other times. During a
`magnetic pulse, the current flows in a direction that tends to
`preserve the magnetic field (i.e., Lenz’s Law).
`As may be appreciated, various devices may take advan
`tage of the above principles to induce an electric field, and
`Such devices may be used in a variety of applications. For
`example, magnetic devices may be used for electrical stimu
`lation of the anatomy, and the like. While the discussion
`herein focuses on magnetic devices that are used in connec
`tion with magnetic stimulation of anatomical tissue, it will be
`appreciated that such discussion is so limited solely for pur
`poses of explanation and clarity. Thus, it will be understood
`that an embodiment is equally applicable to any application of
`a magnetic device in any field of endeavor. Thus, the present
`discussion of magnetic devices should not be construed as
`limiting embodiments of the invention to medical or other
`applications.
`Therefore, and turning now to the context of electrical
`stimulation of the anatomy, certain parts of the anatomy (e.g.,
`nerves, tissue, muscle, brain) act as a conductor and carry
`electric current when an electric field is applied. The electric
`field may be applied to these parts of the anatomy transcuta
`neously by applying a time varying (e.g., pulsed) magnetic
`field to the portion of the body. For example, in the context of
`45
`TMS, a time-varying magnetic field may be applied across
`the skull to create an electric field in the brain tissue, which
`produces a current. If the induced current is of sufficient
`density, neuron action potential may be reduced to the extent
`that the membrane Sodium channels open and an action
`potential response is created. An impulse of current is then
`propagated along the axon membrane that transmits informa
`tion to other neurons via modulation of neurotransmitters.
`Such magnetic stimulation has been shown to acutely affect
`glucose metabolism and local blood flow in cortical tissue. In
`the case of major depressive disorder, neurotransmitter dys
`regulation and abnormal glucose metabolism in the prefrontal
`cortex and the connected limbic structures may be a likely
`pathophysiology. Repeated application of magnetic stimula
`tion to the prefrontal cortex may produce chronic changes in
`neurotransmitter concentrations and metabolism so that the
`symptoms of depression are reduced or alleviated. While the
`discussion herein focuses on transcutaneous stimulation, it
`should be appreciated by one skilled in the art that the tech
`niques and devices discussed herein may, in Some embodi
`ments, be applied to stimulation involving a coil that may be
`placed anywhere relative to a patient. In one Such embodi
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`4
`ment, for example, the coil may be placed inside or proximate
`to any portion of a patients anatomy.
`In a similar fashion, non-cortical neurons (e.g., cranial
`nerves, peripheral nerves, sensory nerves) may also be stimu
`lated by an induced electric field. Techniques have been
`developed to intentionally stimulate peripheral nerves to
`diagnose neuropathologies by observing response times and
`conduction velocities in response to a pulsed magnetic field
`induced stimulus.
`As noted above, it should be appreciated that transcutane
`ous magnetic stimulation is not limited to treatment of
`depression. In addition to depression, the transcutaneous
`magnetic stimulation methods and apparatus of the invention
`may be used to treat a patient such as a human Suffering from
`epilepsy, Schizophrenia, Parkinson's disease, Tourette's Syn
`drome, amyotrophic lateral Sclerosis (ALS), multiple Sclero
`sis (MS), Alzheimer's disease, attention deficit/hyperactivity
`disorder, obesity, bipolar disorder/mania, anxiety disorders
`(e.g., panic disorder with and without agoraphobia, Social
`phobia also known as Social anxiety disorder, acute stress
`disorder and generalized anxiety disorder), post-traumatic
`stress disorder (one of the anxiety disorders in DSM), obses
`sive compulsive disorder (also one of the anxiety disorders in
`DSM), pain (such as, for example, migraine and trigeminal
`neuralgia, as well as chronic pain disorders, including neuro
`pathic pain, e.g., pain due to diabetic neuropathy, post-her
`petic neuralgia, and idiopathic pain disorders, e.g., fibromy
`algia, regional myofascial pain syndromes), rehabilitation
`following stroke (neuro plasticity induction), tinnitus, stimu
`lation of implanted neurons to facilitate integration, Sub
`stance-related disorders (e.g., dependence, abuse and with
`drawal diagnoses for alcohol, cocaine, amphetamine,
`caffeine, nicotine, cannabis and the like), spinal cord injury
`and regeneration/rehabilitation, stroke, head injury, sleep
`deprivation reversal, primary sleep disorders (primary insom
`nia, primary hypersomnia, circadian rhythm sleep disorder),
`cognitive enhancements, dementias, premenstrual dysphoric
`disorder (PMS), drug delivery systems (changing the cell
`membrane permeability to a drug), induction of protein Syn
`thesis (induction of transcription and translation), Stuttering,
`aphasia, dysphagia, essential tremor, and/or eating disorders
`(such as bulimia, anorexia and binge eating).
`Example Magnetic Stimulation Device
`A ferromagnetic core may be used in connection with a
`magnetic device to produce a magnetic field. In some
`embodiments, such a magnetic field may be for purposes of
`carrying out transcutaneous magnetic stimulation Such as, for
`example, Transcranial Magnetic Stimulation (TMS), Repeti
`tive TMS (rTMS), Magnetic Seizure Therapy (MST), diag
`nosis of nerve conduction disorders, reduction of peripheral
`nerve discomfort and so forth. Again, although some of the
`examples that follow may be discussed in connection with
`TMS and rTMS embodiments for the purposes of explanation
`and clarity, any type of transcutaneous magnetic stimulation,
`including all of those listed above, may be performed accord
`ing to an embodiment of the invention. In addition, and as
`noted above, embodiments are not limited to transcutaneous
`magnetic stimulation, as an embodiment may be used in
`connection with magnetic devices that generate a magnetic
`field for any purpose.
`Furthermore, the embodiments presented herein are not
`limited to the use of ferromagnetic core magnetic stimulation
`devices, as other core materials may be used such as, for
`example, air. Such air core configurations may include, but
`are not limited to, windings in a “figure eight circular, coni
`cal or double conical shape, or the like. The discussion herein
`therefore describes a ferromagnetic core magnetic stimula
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`tion device solely for purposes of explanation and clarity. In
`an embodiment, a ferromagnetic core may be substantially
`“C” shaped, and in another embodiment the ferromagnetic
`core may include a highly saturable magnetic material having
`a magnetic saturation of at least 0.5 Tesla. In some embodi- 5
`ments, a ferromagnetic core may be shaped to optimize the
`magnetic field distribution in the treatment area. Treatment
`areas for other forms of treatment (e.g., reduction of discom
`fort in peripheral nerves, etc.) may be more or less deep than
`is the case for TMS.
`FIG. 1 is a diagram illustrating an example magnetic device
`100. In magnetic device 100, power supply 116, capacitor
`114, Switch 112 and controller 120 form an electric circuit
`that provides a power signal to inductor 110. The power signal
`may be any time-varying electric signal capable of generating 15
`an electric and/or magnetic field. The inductor 110 may be
`used to conduct TMS, rTMS and/or Magnetic Seizure
`Therapy (MST), for example.
`Power supply 116 may be any type of power source that
`provides sufficient energy for inductor 110 to generate a 20
`magnetic field for its intended purpose whether for TMS,
`rTMS, MST or any other type of application. For example,
`power supply 116 may be a conventional 120 or 240 VAC
`main power source. Inductor may be any type of induction
`device such as, for example, a treatment coil having an air or 25
`ferromagnetic core, as was discussed above. In an embodi
`ment, such a treatment coil may be fabricated from high
`saturation core materials. The treatment coil may also employ
`a thin core design to optimize the coil for TMS, for example.
`In an embodiment, a treatment coil that employs a thin core 30
`design may be constructed as a substantially C-shaped core
`that has been reduced in thickness, thus providing a smaller
`cross-sectional area, less Saturable material and therefore
`lowered power requirements, as well as less weight, while
`still having a field strength that penetrates to the same depth as 35
`a conventional core design. It will be appreciated that a treat
`ment core that employs Such a thin core design may generate
`a magnetic field that stimulates a reduced Volume of tissue in
`the patient.
`Capacitor 114 provides energy storage for pulsing inductor 40
`110. While capacitor 114 is described herein, it should be
`appreciated that capacitor 114 may, in one embodiment, be
`any type of energy storage device. Thus, the term “capacitor
`is used herein merely as a shorthand reference to any type of
`energy storage device, which in one embodiment may be a 45
`capacitor. For example, in another embodiment power Supply
`116 may itself serve the energy storage functions of capacitor
`114, thereby obviating the need for capacitor 114 itself.
`Capacitor 114 may be used, for example, in applications
`where a 120VAC power source or the like is available. A 50
`typical doctors office may only be equipped with a conven
`tional (e.g., 120VAC or the like) power supply rather than a
`higher-power 240 VAC or three-phase power supply. As a
`result, the use of capacitor 114 to store energy for use in
`pulsing inductor 110 may enable device 100 to operate using 55
`higher power levels than might otherwise be possible if sim
`ply using power Supply 116 alone.
`Power supply 116 also may be comprised of any number
`and type of power Supplies. For example, power Supply 116
`may be the output of a power supply that runs off of 120VAC 60
`and then converts the AC input power signal to a DC output
`power signal. Alternatively, power Supply 116 may be a bat
`tery, which may be useful in applications where magnetic
`stimulation device is to be portable. In yet another embodi
`ment, power Supply 116 may be a combination of a power 65
`Supply and a battery. It will be appreciated that Such a con
`figuration may be useful when the power required to generate
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`a pulse, or train of pulses, exceeds the capacity (or a signifi
`cant percentage of the capacity) of the power Supply alone.
`Thus, the combined power of the power supply and battery
`may be used to generate the pulse(s), with the battery helping
`to Sustain Voltage during period(s) of high demand associated
`with the generation of the pulse(s), and then the power Supply
`can recharge the battery in between pulses, for example. A
`device that incorporates such a configuration could therefore
`use, for example, a standard 120 VAC outlet to generate
`pulses that otherwise need more power than a 120VAC outlet
`could provide, and/or could provide adequate power line
`regulation. Such a device could therefore be used in a location
`that has standard 120VAC outlets such as, for example, a
`medical professionals office.
`Capacitor 114 may include any number and/or type of
`capacitor(s) (or other type of energy storage devices) that are
`appropriate for the power level, charging time and/or pulse
`type required by device 100. Switch 112 may be any type of
`electrical switching device that can operate inductor 110 by
`Switching power from capacitor 114 and/or power Supply 116
`on and off. For example, switch 112 may be operated to
`Switch power from power Supply 116 to charge capacitor 114.
`Switch may also be used to discharge capacitor 114 through
`inductor 110, thereby creating a magnetic field that can be
`used for TMS treatment, for example. TMS controller 120
`may be any type of hardware, Software, or combination
`thereof, that controls switch 112 and/or power supply 116.
`FIG. 2 is a circuit diagram illustrating an example magnetic
`device drive circuit 200 according to an embodiment. It will
`be appreciated that circuit 200 is a simplified representation
`of various components illustrated in FIG. 1, and that any
`number and type of components in addition to the compo
`nents illustrated in FIGS. 1 and 2 may be used in connection
`with an embodiment.
`It can be seen that in an embodiment circuit 200 may be
`comprised of power supply 116, capacitor 114, inductor 110
`(which may be a stimulation coil), and switch 112, which may
`beformed by IGBT 120 and commutating diode 122, which
`may be connected in parallel. IGBT 120 may be used in an
`embodiment to discharge capacitor 114 into inductor 110 to
`generate a magnetic field. In addition, IGBT 120 may be
`protected from high Voltage spikes by a commutating diode
`122, which Suppresses Voltage transients. In Such an embodi
`ment, commutating diode 122 may be what is commonly
`referred to as a “snubber. While referred to herein as IGBT
`120 for purposes of clarity, switch 112 may comprise any type
`of device in which a commutating circuit is employed.
`It will be appreciated that while the discussion herein
`focuses on an embodiment in which IGBT 120 is employed,
`other semiconductor Switching devices may be employed in
`connection with an embodiment. For example, in one such
`embodiment, an embodiment, an Integrated Gate Commu
`tated Thyristor (IGCT) may take the place of IGBT 120 and
`commutating diode 122 in drive circuit 200. Other semicon
`ductor Switching devices with similar power handling and
`Switching characteristics may be used in connection with an
`embodiment.
`Power supply 116 may be any type of electrical power
`source that is appropriate for the intended function of circuit
`200, or of a device in which circuit 200 is a part. For example,
`power Supply 116 may comprise a DC power signal that has
`been converted (i.e., rectified) from an AC input power signal,
`for example. Power supply 116 may also comprise a battery
`or other power source, as was discussed above in FIG. 1.
`As was the case above in FIG. 1, capacitor 114 may be any
`type of energy storage device that is capable of pulsing induc
`tor 110 to generate a magnetic field. Inductor 110 may be a
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`ferromagnetic (e.g., iron) or air core magnetic coil. In one
`embodiment, inductor 110 may be an iron core magnetic coil.
`It will be appreciated that a magnetic coil that employs an iron
`core may be able to be Switched faster than a magnetic coil
`having an air core. In some embodiments, therefore, an iron
`core may be selected in applications involving the generation
`of short pulse widths that may be used in connection with the
`stimulation of cortical neurons, as will be discussed below.
`Regardless of the type of coil used in inductor 110, in an
`embodiment the shape of the coils core and/or the number
`and configuration of windings may be selected to cause
`inductor 110 to generate a magnetic field having a desired
`waveform. In an embodiment, the waveform may be selected
`to have a desired effect on a patient, for example. For an
`embodiment with high Voltage, high inductance, and low
`capacitance, the resonant frequency specified in Equation 1.
`below, may be much greater than would be possible with a
`thyristor Switch, allowing more selective stimulation and
`greater energy efficiency.
`Thus, it should be appreciated that an embodiments com
`bination of an iron core and a higher speed semiconductor
`device such as IGBT 120, IGCT or the like, may enable an
`increased resonance frequency (i.e., shorter pulse length),
`which in turn may provide for higher stimulation frequencies
`and more efficient stimulation of cortical neurons or axons.
`In contrast to conventional magnetic stimulation devices
`that use a thyristor (i.e., a silicon-controlled rectifier) as a
`main Switching element for high currents at high Voltage, an
`embodiment employs IGBT 120 as the main switching ele
`ment in place of, or in addition to, a thyristor. Thyristors have
`a significant turn-off time, which increases with the rated
`voltage and inhibits simultaneous optimization of operating
`frequency and operating Voltage. In contrast, an embodiment
`provides a capacitor 114 inductor 110 resonant drive circuit
`200 that uses IGBT 120 as a switching element (i.e., switch
`112), and is controlled by an isolated DC pulse (provided by,
`for example, power source 116) and timed to turn off during
`the reverse phase of the stimulation pulse. IGBT 120 is
`capable of Switching at higher frequencies than a thyristor,
`thereby enabling inductor 110 to generate a greater frequency
`range of magnetic pulses as compared to a thyristor-con
`trolled, conventional magnetic stimulation device. As a result,
`a magnetic stimulation device that is switched by IGBT 120,
`according to an embodiment, is able to operate using lower
`input power than a conventional magnetic stimulation device.
`Such power savings may come from, for example, two
`Sources: (1) operation at higher Voltage and lower current
`reduces resistive losses in the circuit, leading to greater
`charge recovery from each pulse and also a higher second
`phase of the cosine pulse waveform, (where, for example, the
`second phase performs the work), and (2) the shorter pulse
`waveform is more efficient, because neuronal cell membranes
`are “leaky, and part of the charge transferred across the cell
`membrane at the beginning of the pulse is lost by the time the
`pulse has ended. Shorter pulses may result in less membrane
`loss.
`A magnetic stimulation device according to an embodi
`ment may generate, for example, rapid TMS pulses having a
`pulse width less than approximately 200 us in duration (when
`Such an embodiment is intended to stimulate cortical neurons,
`for example). In one embodiment, the TMS pulses may have
`a pulse width of approximately 100 us-150 us in duration. It
`will be appreciated that such pulse widths may be optimized
`for the intended target Such as, for example, cortical neuron
`stimulation. Some IGBTs may not be able to handle high
`current loads. Thus, a configuration of circuit 200 may enable
`the use of IGBT 120 as switch 112 by lowering the amount of
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`current while simultaneously producing an equivalent mag
`netic field using inductor 110. To generate a magnetic field for
`TMS, rTMS, MST or other stimulation applications at a given
`frequency in some embodiments, a balance may be struck
`between the inductance and capacitance values used in the
`drive circuit. The relationship between capacitance and
`inductance and their effect on a resonant circuits frequency is
`governed by the well-known equation:
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`1
`
`f = W
`
`(1)
`
`Where f is the resonant frequency, L is the inductance and
`C is the capacitance of the circuit. Some embodiments may
`achieve this balance by running inductor 110 with as low an
`inductance as possible (e.g., commonly on the order of 10-24
`LH in a typical TMS application). As a result, the capacitance
`(and therefore the current used in the circuit) may need to be
`very large (e.g., at least 50 uF) in order for the circuit to
`generate the desired magnetic field. When Such a circuit is
`designed to run in this manner, parasitic energy losses and
`Stray inductances may require additional components (such
`as cooling equipment) to compensate for the problem. In
`addition, the use of IGBT 120 as switch 112 may be precluded
`in Such a configuration because of the high current levels that
`are present.
`An embodiment may use high inductance (on the order of
`approximately 50-55 uH in the typical TMS application
`referred to above, for example) in inductor 110. In addition,
`the capacitance of capacitor 114 may be reduced to a value of
`approximately 7 uF. The current in circuit 200 is therefore
`reduced, and the Voltage is increased. For example, a device
`according to an embodiment may operate at approximately
`1,200 A. Likewise, such a device may operate at approxi
`mately 3,000 V, for example.
`It may be appreciated that such increased Voltage may
`result in some additional Voltage-dependent losses, but the
`types of losses that occur from high Voltage are typically
`easier to account for than losses that occur from high current.
`Thus, additional efficiencies may be achieved by an embodi
`ment.
`According to such an embodiment, such a drive circuit 100
`may be capable of operating at approximately twice the fre
`quency and approximately /2 to "/8 the peak current of con
`ventional systems. This very low current requirement may
`lead to lower resistive loss in inductor 130, even at the
`uniquely high inductance of 50-55 uH, for example. The
`result is reduced inductor 130 heating and bulk, higher charge
`rec