United States Patent (19)
`Chaney
`
`54) MAGNETIC NEURAL STIMULATOR FOR
`NEUROPHYSOLOGY
`75) Inventor: Richard A. Chaney, Nashua, N.H.
`03062
`73 Assignee: Corteks, Inc., Chappaqua, N.Y.
`(21) Appl. No.: 411,932
`22 Filed:
`Sep. 25, 1989
`5ll Int. Cl. ............................................... A61N 2/04
`(52)
`(58)
`
`361/156
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`3,986,493 10/1976 Hendren ................................ 600/12
`4,551,781 11/1985 Bykerk......
`... 361/143
`4,561,426 12/1985 Stewart ................................ 128/15
`4,574,809 3/1986 Talish et al. ..
`128/419 F
`4,607,311 8/1986 Brown et al. ....................... 361/155
`4,778,971 10/1988 Sakimoto et al. ................ 219/10.43
`FOREIGN PATENT DOCUMENTS
`ll 13156 11/1981 Canada .................................. 600/14
`OTHER PUBLICATIONS
`Maass et al., "Contactless Nerve Stimulation and Signal
`Detection by Inductive Transducer', IEEE Transac
`tions on Magnetics, vol. Mag-6, No. 2, pp. 322-326, (Jun.
`1970).
`Hallgren et al., "Contactless Nerve Stimulating Trans
`ducer', IEEE Transaction on Biomedical Engineering,
`pp. 316-317, (Jul. 1972).
`Oberg, "Magnetic Stimulation of Nerve Tissue', Medi
`cal and Biological Engineering, pp.55-64, (Jan. 1973).
`Hallgren, "Inductive Neural Stimulator", IEEE Trans
`actions on Biomedical Engineering, pp. 470-472, (Nov.
`1973).
`Ueno et al., "Capacitive Stimulatory Effect in Magnetic
`Stimulation of Nerve Tissue', IEEE Transactions on
`Magnetics, vol. Mag-14, No. 5, pp. 958-960, (Sep. 1978).
`Polson et al., "Stimulation of Nerve Trunks with
`Time-Varying Magnetic Fields", Medical and Biologi
`10
`- - - - - -
`2
`
`20
`
`Patent Number:
`11
`(45) Date of Patent:
`
`5,061,234
`Oct. 29, 1991
`
`cal Engineering and Computing, pp. 243-244, (Mar.
`1982).
`Freeston et al., "Nerve Stimulation Using Magnetic
`Fields", IEEE Frontiers of Engineering and Computing
`in Health Care, pp. 557-561, (1984).
`McRobbie, "Design and Instrumentation of Magnetic
`Nerve Stimulator', The Institute of Physics, pp. 74-78,
`(1985).
`Young et al., "Clinical Neurophysiology of Conduction
`in Central Motor Pathways', Annals of Neurology, vol.
`18, No. 5, pp. 606-609, (Nov. 1985).
`Hess et al., "Magnetic Brain Stimulation: Central Motor
`Conduction Studies in Multiple Sclerosis', Annals of
`Neurology, vol. 22, No. 6, pp. 744-752, (Dec. 1987).
`Mills et al., "Magnetic and Electrical Transcranical
`Brain Stimulation: Physiological Mechanisms and Clini
`cal Application', Neurosurgery, vol. 20, No. 1, pp.
`164-168, (1987).
`Barker et al., "Magnetic Stimulation of the Human
`Brain and Peripheral Nervous System: An Introduction
`(List continued on next page.)
`
`Primary Examiner-William E. Kann
`Assistant Examiner-Kevin Pontius
`Attorney, Agent, or Firm-Pennie & Edmonds
`57)
`ABSTRACT
`A magnetic neural stimulator is disclosed for the stimu
`lation of biological tissue. The stimulator includes an
`inductive stimulation coil, an energy storage capacitor,
`a firing device and a charging circuit. The energy stor
`age capacitor is charged by the charging circuit to a
`voltage level which is greater than the voltage level
`supplied to the charging circuit. The energy storage
`capacitor is partially discharged into the stimulation :
`coil thereby producing a magnetic pulse. The charging
`and discharging of the capacitor is continuously per
`formed so as to produce a plurality of high frequency
`magnetic pulses. The stimulation coil and the energy
`storage capacitor operate in a resonant manner under a
`control circuit which performs timing and gating func
`tions.
`
`8 Claims, 7 Drawing Sheets
`
`Frn G
`MEANS
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`
`
`CHARGING
`POWER
`SUPPY
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`LOW
`WOTAGE
`POWER
`SUPPLY
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`CONTROL
`CRCUT
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`LUMENIS EX1034
`Page 1
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`5,061,234
`Page 2
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`OTHER PUBLICATIONS
`
`and the Results of an Initial Clinical Evaluation', Neu
`rosurgery, vol. 20, No. 1, pp. 100-109, (1987).
`Macabee et al., "Intracranial Stimulation of Facial
`Nerve in Humans with the Magnetic Coil', Electroen
`cephalography and Clinical Neurophysiology, vol. 70, pp.
`350-354, (1988).
`Schriefer et al., "Evaluation of Proximal Facial Nerve
`Conduction by Transcranial Magnetic Stimulator',
`
`Journal of Neurology, Neurosurgery, and Psychiatry, vol.
`51, pp. 60-66, (1988).
`Tsuji et al., "Somatosensory Potentials Evoked by
`Magnetic Stimulation of Lumbar Roots, Cauda Equina
`and Leg Nerves', Annals of Neurology, vol. 24, No. 4,
`pp. 568-573, (1988).
`Claus et al., "Central Motor Conduction in Degenera
`tive Ataxic Disorders: A Magnetic Stimulation Study',
`Journal of Neurology, Neurosurgery and Psychiatry, 51,
`pp. 790-795, (1988).
`
`LUMENIS EX1034
`Page 2
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`U.S. Patent
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`Oct. 29, 1991
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`U.S. Patent
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`Oct. 29, 1991
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`U.S. Patent
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`Oct. 29, 1991
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`U.S. Patent
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`Oct. 29, 1991
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`Oct. 29, 1991
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`U.S. Patent
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`Oct. 29, 1991
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`1
`
`MAGNETIC NEURAL STIMULATOR FOR
`NEUROPHYSOLOGY
`
`5
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`O
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`15
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`30
`
`BACKGROUND OF THE INVENTION
`The present invention relates generally to magnetic
`stimulation and more particularly to high-speed mag
`netic stimulation of biological tissue.
`For some time, electrical or magnetic stimulation
`devices have been used to stimulate biological tissue.
`Electric stimulation of biological tissue is wellknown
`and generally involves the use of a plurality of elec
`trodes strategically placed on the tissue of a subject
`(human or animal). Electric pulses are subsequently
`generated and applied to the electrodes. The tissue in an
`area separating the electrodes is thus stimulated by the
`passage of current therethrough.
`Magnetic stimulation of biological tissue is a much
`more recent development. Magnetic stimulation in
`20
`volves the application of a magnetic field to biological
`tissue. A magnetic transducer such as a wire wound coil
`is employed to generate the magnetic field. The coil
`may be contactless and operate a predetermined dis
`tance away from the subject. Alternatively, the coil
`25
`may contact or may be implanted in the subject.
`Magnetic stimulation offers many significant advan
`tages over electrical stimulation. Magnetic stimulation
`eliminates the need for direct physical electrical contact
`to the subject and thus removes problems associated
`with high skin electrical resistance, damage to tissue
`due to high current flow and the like. Additionally, the
`subject is exposed to less risk since hazards arising from
`electrical shock due to malfunctioning equipment are
`reduced, if not eliminated.
`35
`Potential applications of magnetic stimulation also
`include the stimulation of both the central and periph
`eral nervous systems. Specifically, magnetic stimulation
`followed by recordal of the corresponding motor
`evoked potentials (MEPs) is believed to be a promising
`application of magnetic neural stimulation. MEPs have
`been recorded over spinal cord, peripheral nerve and
`muscle following transcranial electrical stimulation.
`These MEPs have been demonstrated to accurately
`reflect the functional integrity of efferent spinal motor
`45
`pathways of rats and cats subjected to mechanical
`trauma, compression and ischemia, and also have been
`well correlated with the degree of clinical recovery.
`These results suggest major clinical applications for
`MEPs in evaluation of efferent motor pathways as well
`50
`as in intraoperative monitoring. Conventional neuro
`physiological techniques do not permit such examina
`tion of central motor pathways.
`With respect to diagnostic evaluation of efferent
`motor pathways, MEP abnormalities have been demon
`55
`strated in patients with multiple sclerosis, compressive
`and radiation induced myelopathies, and hereditary
`spastic paraparesis. In some cases subclinical dysfunc
`tion was detected.
`Further, intraoperative monitoring of efferent motor
`pathways is likely to achieve major importance supple
`menting somatosensory evoked potential (SEP) moni
`toring during neurosurgery. SEPs, which monitor prin
`cipally dorsal cord function, do not adequately predict
`the integrity of the descending motor pathways. Suc
`65
`cessful use of MEP monitoring has been reported in a
`large series of neurosurgical procedures, where moni
`toring results correlated well with clinical outcome.
`
`5,061,234
`2
`Although at present transpinal cord stimulation is not
`known to have been reported, this may also be possible.
`Magnetic neural stimulation is also likely to be a use
`ful research tool for brain physiology. Using electrical
`stimulation, MEP to cerebellar stimulation has been
`demonstrated and used to study interactions of pyrami
`dal and extra-pyramidal motor systems. MEPs have
`also been used to study the function of callosal path
`ways. It is likely that many further research applications
`for the study of brain physiology will emerge. Magnetic
`stimulation of the brain provides instrumental, noninva
`sive access to brain function, with the possibility of
`modification of that function.
`Magnetic neural stimulation offers several potential
`advantages over conventional electrical stimulation.
`Magnetic stimulation is painless. Transcutaneous elec
`trical stimulation produces the greatest current density
`in the most superficial skin layers, the skin layers most
`sensitive to pain. In contrast, lines of flux produced by
`magnetic stimulation penetrate the skin essentially unal
`tered, making it possible to stimulate nerves without
`exciting overlying cutaneous pain fibers, which have a
`higher threshold for stimulation.
`The painless nature of magnetic stimulation will
`likely prove a major practical advantage in performing
`peripheral nerve and SEP studies on children. More
`over, the electrophysiological examination of deeper
`lying and proximal peripheral nerve, which is inaccessi
`ble to study by conventional electrical stimulation be
`cause of the painful and potentially damaging currents
`required to excite deep lying tissue, appears to be feasi
`ble using magnetic stimulation.
`Rapid electrical stimulation of peripheral nerve is
`sufficiently painful so that this technique cannot be
`employed on conscious patients. Fortunately, magnetic
`stimulation at similarly high rates of stimulation is pain
`less.
`Magnetic stimulation produces substantially less stim
`ulus artifact than electrical stimulation. This will facili
`tate SEP recordings in settings where the time interval
`between stimulus and response is very short, such as
`recordings in infants and small children, trigeminal
`nerve SEPs, and blink reflex recordings.
`Magnetic stimulation may make transcutaneous clini
`cal evaluation of small, unmyelinated nerve pathways
`possible. Although potentially useful techniques for
`small and selective fiber stimulation have been pro
`posed, these techniques have not been successfully used
`transcutaneously in a clinical setting, most likely due to
`the high currents that would be required, as well as the
`effect of skin and nerve upon the pulse shape. Since
`magnetic fields are not substantially altered by the skin
`and nerve sheath, it may be possible to use a magnetic
`stimulator capable of delivering a shaped magnetic
`pulse to selectively stimulate smaller myelinated and
`unmyelinated fibers.
`Unfortunately, bractical use of magnetic neural stim
`ulation has been limited by the inability of currently
`available magnetic neural stimulators to selectively
`stimulate a limited area of neural tissue, especially cere
`bral, spinal or peripheral neural tissue. Presently avail
`able stimulators employ large stimulating coils which
`are incapable of focusing and limiting the lateral spread
`of the magnetic field. Additionally, the ability to selec
`tively stimulate more discrete areas of tissue such as the
`brain will be important in gaining an understanding of
`the physiology of magnetic nerve stimulation and may
`also enhance its ultimate clinical utility. Furthermore,
`
`LUMENIS EX1034
`Page 10
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`5,061,234
`3
`4
`many peripheral neurophysiological tests involve deter
`ply and at another end to the stimulation probe and the
`mination of nerve conduction velocity, and hence re
`energy storage means. In this preferred embodiment,
`quire accurate knowledge of precisely where nerve
`the inductor and the diode supply the current path to
`stimulation occurred.
`charge the energy storage means.
`Present devices also cannot stimulate at rates over
`The control circuit means includes a transformer
`about 1 Hz. As a result, clinical MEP testing and moni
`isolated gate driver and operates in conjunction with a
`toring is presently restricted for practical purposes to
`firing means such as a silicon-controlled rectifier (SCR),
`recording of electromyogram (EMG) activity only.
`linear amplifier or plasma switch. The gate driver is
`Although the sensitivity of clinical studies would likely
`powered by a low voltage power supply. The firing
`be enhanced by recording neural responses over spinal
`means is serially connected with the stimulation probe
`cord and peripheral nerve, such studies would require
`across the energy storage means and serves as a high
`signal averaging, which is impractical at low stimula
`current switch to fire the inductor of the stimulation
`tion rates.
`probe under control of the gate driver. Depending on
`Present magnetic neural stimulators are also unable to
`the configuration and firing scheme of the control cir
`operate at the speed necessary for SEP testing since
`15
`cuit, as well as the type of firing means, unipolar mag
`averaging of response to a prohibitively large number of
`netic pulses as well as magnetic pulses of alternating
`stimuli is required. Thus, magnetic peripheral nerve
`polarity may be produced. The control circuit is respon
`stimulation, although pain-free, is also not practical for
`sive to the charge in the energy storage means and, like
`SEP testing using presently available devices. Present
`the resonant charging circuit, is instrumental in restor
`devices are also unable to selectively stimulate a limited
`ing lost energy by extracting the amount of energy lost
`area of neural tissue.
`from the power supply and supplying it to the energy
`SUMMARY OF THE INVENTION
`storage means.
`Advantageously, use of the resonant charging circuit
`We have devised a high speed, compact, cost effec
`in conjunction with the inductor of the hand held stimu
`tive magnetic neural stimulator capable of selectively
`25
`lation probe eliminates the need for a high voltage
`stimulating small regions of tissue. Such stimulation can
`power supply. As will become apparent, the peak volt
`result in the sustained contraction of biological tissue.
`age across the capacitor bank during normal operation
`Broadly, the stimulator comprises a charging power
`supply, stimulation means, energy storage means, firing
`of the neural stimulator is significantly greater than the
`peak voltage of the charging power supply. As will be
`means, a resonant charging circuit and control circuit
`appreciated, each firing of the inductor in the stimula
`C2S.
`The charging power supply supplies the energy to
`tion probe will not entirely discharge the voltage across
`initially produce the magnetic pulses as well as the
`the capacitor bank, thus enabling rapid firing of the
`energy to compensate for the energy lost as heat during
`probe and sustained generation of high frequency mag
`steady-state operation. The output voltage of this sup
`netic pulses.
`35
`ply may be adjusted to give different stimulus intensi
`The neural stimulator of the present invention per
`tleS.
`mits restriction of stimulation to limited regions of tis
`The stimulation means is preferably an inductor in the
`sue. Additionally, the neural stimulator permits stimula
`form of a wire wound coil and is located within a hand
`tion at rates up to 400 Hz and beyond depending on the
`held stimulation probe. Magnetic pulse shaping material
`available power supply and coil cooling.
`is provided in close proximity to the inductor. The
`The stimulator may be used in a wide variety of appli
`inductor is connected to the system through cables and
`cations. For example, clinical applications include diag
`high current connectors. The inductor and the magnetic
`nostic and intraoperative real-time evaluation of effer
`pulse shaping material are configured to enable mag
`ent pathways, as well as allowing pain-free peripheral
`netic pulse shaping which facilitates stimulation of se
`45
`nerve stimulation for standard afferent pathway and
`lective regions of tissue, especially nerve fibers.
`peripheral nerve testing. Research applications include
`The energy storage means is powered by the charg
`studies of the physiological effects of focused cortical,
`ing power supply and stores the energy employed by
`cerebellar, and spinal cord stimulation, as well as in vivo
`the stimulation coil to produce magnetic pulses. Illustra
`selective fiber stimulation.
`tively, the energy storage means is implemented as a
`50
`Advantageously, magnetic neural stimulation renders
`group or bank of capacitors connected in parallel.
`the peripheral proximal nervous system, spinal cord and
`The resonant charging circuit operates in conjunction
`central motor pathways accessible to non-invasive, cost
`with the stimulation coil and energy storage means to
`effective neurophysiological testing, and permits moni
`generate high frequency magnetic pulses by transfer
`toring in critical care settings.
`ring energy from the energy storage means into a mag
`55
`Accordingly, it is a principal object of the present
`netic field and back into the energy storage means in a
`invention to provide new and improved magnetic stin
`resonant fashion. The resonant charging circuit, in com
`ulation of biological tissue.
`bination with the control circuit and firing means, re
`It is also an object of the invention to provide a mag
`stores energy lost through the stimulation coil and other
`netic stimulator including a resonant stimulator circuit
`components, wires, etc. by extracting the amount of lost
`to enable generation of magnetic pulses of increased
`energy from the power supply and supplying it to the
`frequency.
`energy storage means. Viewed another way, the reso
`It is a further object of the invention to provide a
`nant charging circuit, in combination with the control
`magnetic stimulator configured to selectively stimulate
`circuit and firing means, causes the energy storage
`small regions of biological tissue.
`means to only partially discharge through the inductor
`65
`of the stimulation coil. The resonant charging circuit
`It is another object of the present invention to elimi
`nate the need for a high voltage power supply in a
`may be in the form of a serially connected inductor and
`diode connected at one end to the charging power sup
`magnetic neural stimulator.
`
`30
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`Page 11
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`BRIEF DESCRIPTION OF THE DRAWINGS
`These and other objects, features and advantages of
`the invention will be more readily apparent from the
`following description of the invention in which:
`FIG. 1 is a simplified schematic of the present inven
`tion;
`FIG. 2 is a schematic of an alternate embodiment of
`the present invention;
`FIG. 3 is a schematic of the stimulation coil depicted
`in FIGS. 1 and 2;
`FIG. 4 is a graph depicting voltage and current char
`acteristics versus time of the energy storage capacitor
`C1 of FIG. 2 for a single current pulse;
`FIG. 5 is a graph depicting voltage characteristics
`versus the number of stimulations for the energy storage
`capacitor C1 of FIG. 2;
`FIG. 6 is a graph depicting voltage and current char
`acteristics versus time of the stimulation coil for a single
`current pulse during steady-state operation;
`FIG. 7 depicts a stimulation coil suitable for use with
`the present invention;
`FIG. 8 is a graph depicting the strength of the mag
`netic field versus applied current for the stimulation coil
`of FIG. 7; and
`FIG. 9 is a graph depicting the magnetic field
`strength versus penetration depth of a disk-shaped coil
`and a toroidal coil.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`FIG. 1 depicts, in simplified form, a magnetic neural
`stimulator 10 comprising a charging power supply 20, a
`stimulation coil 30, a charging circuit 40, energy storage
`means 42, a control circuit 50, firing means 55 and a low
`35
`voltage power supply 60.
`Charging power supply 20 comprises a power supply
`to power charging circuit 40 and charge energy storage
`means 42. Power supply 20 maintains energy storage
`means 42 at least partially charged during sustained
`operation of the magnetic neural stimulator. Power
`supply 20 preferably is an adjustable power supply and
`may be powered by a conventional 120VAC, 60 Hz, 15
`amp household wall outlet supply. Alternatively, a 220
`VAC supply may be employed. Desired operating char
`45
`acteristics such as magnetic pulse amplitude, duration
`and frequency generally dictate the power requirements
`and the output voltage setting for power supply 20.
`Stimulation coil 30 comprises means such as inductor
`L1, for producing a magnetic field upon the application
`of current. Inductor L1 may be housed in a hand held
`probe (not shown).
`Charging circuit 40 comprises serially connected
`inductor L2 and diode D1. The serial combination of
`inductor L2, diode D1 and energy storage means 42
`55
`which forms a resonant charging circuit is connected
`across charging power supply 20. Inductor L2 illustra
`tively is a 5 mH high current inductor. Energy storage
`means 42 illustratively is implemented as a group or
`bank of capacitors connected in parallel and is schemati
`cally depicted as capacitor C1. For example, energy
`storage means 42 may comprise sixteen 20 F, 1000
`volts polypropylene capacitors wired in parallel. Diode
`D1 blocks the discharge of energy storage means 42
`back into charging power supply 20.
`65
`Control circuit 50 and firing means 55 control the
`firing of stimulation coil 30 which forms a resonant
`discharging circuit with energy storage means 42. Con
`
`5,061,234
`6
`trol circuit 50 is provided with a capacitor voltage level
`input on line 52. Control circuit 50 preferably comprises
`a timing pulse generator and a transformer isolated
`driver or an optically isolated driver. Firing means 55 is
`preferably a silicon-controlled rectifier (SCR). Firing
`means 55 is controlled by a signal output by control
`circuit 50 on output line 54. The SCR permits current to
`flow in a forward direction under control of a trigger
`input; the SCR depicted does not permit reverse current
`flow, although reverse current flow may be permitted
`in other embodiments such as those which provide
`bipolar magnetic pulses, Low voltage power supply 60
`provides power to control circuit 50 on lines 56, 58.
`Magnetic neural stimulator 10 operates as follows.
`Initially, power for the stimulator is off and capacitor
`C1 is completely discharged. When the system power is
`turned on, charging power supply 20 supplies power to
`the stimulator. Current begins to flow through inductor
`L2 thereby charging capacitor C1. After some time
`(about 1 millisecond) the voltage on capacitor C1
`reaches the output voltage of charging power supply
`20, illustratively 100 volts. At this point, there is still
`substantial current through inductor L2. As a result,
`although the voltage across capacitor C1 is the same as
`that output by charging power supply 20, the current
`through inductor L2 continues to flow in the same
`direction thereby raising the voltage across capacitor
`C1 beyond that output by the power supply. More
`particularly, the energy stored in inductor L2 as current
`is discharged into capacitor C1 (which has previously
`been charged to 100 volts) until the voltage across ca
`pacitor C1 reaches about twice the output voltage of
`charging supply 20. The amount by which the voltage
`across C1 is increased depends, in part, on the amount
`of energy lost as heat during the charging of capacitor
`C1. Diode D1 blocks the discharge of capacitor C1
`back into the charging power supply.
`After the initial charging cycle is complete, the SCR
`can be triggered. When the SCR is triggered by control
`circuit 50, current flows from capacitor C1 into induc
`tor L1 until all of the energy stored in capacitor C1 as
`voltage is transferred into energy stored in inductor L1
`as current. At this point, the voltage across capacitor
`C1 is zero.
`Again, the current in the inductor-in this case induc
`tor L1-continues to flow in the same direction; and the
`energy stored as current in inductor L1 is transferred
`back into energy stored as a negative voltage across
`capacitor C1. Eventually, the current through inductor
`L1 reaches zero, at which point the voltage across ca
`pacitor C1 is about minus 180 volts. At this point, the
`SCR commutates and prevents the discharge of capaci
`tor C1 back through inductor L1.
`Meanwhile, starting at the time that the voltage
`across capacitor C1 fell below the output voltage of the
`charging power supply 20, current has resumed flowing
`through inductor L2. Now, however, since the voltage
`across capacitor C1 is about minus 180 volts, and since
`charging supply 20 is supplying 100 volts, there is about
`280 volts across the combination of inductor L2 and
`diode D1. As will be appreciated, the increased voltage
`across the combination of inductor L2 and diode D1
`produces a current through inductor L2 that charges
`capacitor C1 to a higher voltage, which can be shown
`to be approximately plus 380 volts.
`This process is repeated every time the SCR is fired
`by the control circuit; and the voltage across capacitor
`
`25
`
`50
`
`60
`
`LUMENIS EX1034
`Page 12
`
`

`

`20
`
`5,061,234
`7
`8
`C1 rises steadily toward a maximum value of approxi
`stimulation coil 130 includes a 3.7 uH inductor. Firing
`mately 600 volts.
`means 155 comprises diode means 156 and SCR means
`The transfer of energy stored as charge in capacitor
`157 and is adapted to provide magnetic pulses of alter
`C1 into current through inductor L1 and back as charge
`nating polarity. A stimulation trigger input 158 is also
`in capacitor C1 generates the pulsed unipolar magnetic
`included to initiate firing of the stimulation coil.
`field that stimulates the tissue. Advantageously, the
`Magnetic neural stimulator 100 operates as follows.
`resonant charging circuit eliminates the need for a high
`Initially, power for the stimulator is off, capacitor C1 is
`voltage power supply and facilitates generation of mag
`completely discharged, and charging power supply 120
`netic pulses.
`is illustratively set to generate an output of 100 volts.
`When the system power is turned on, charging power
`In an alternative embodiment, a diode is connected in
`10
`parallel with SCR 55 of FIG. 1 to permit reverse cur
`supply 120 provides 100 volts across charging circuit
`rent flow. The diode is connected with a polarity oppo
`140. Current begins to flow through inductor L2,
`site to that of the SCR. Such a connection permits cur
`thereby charging capacitor C1. After about 1 millisec
`rent to flow in a direction opposite from the current
`ond, capacitor C1 is charged to about 100 volts; and
`flow through SCR 55. As will be appreciated, this con
`additional energy is stored in inductor L2 due to the
`15
`figuration permits a negative voltage across capacitor
`current passing through it. The current in inductor L2
`C1 to be discharged through the diode and inductor L1
`continues to flow in the same direction so that the en
`thus producing a magnetic pulse of opposite polarity
`ergy stored in inductor L2 charges capacitor C1 to
`from that produced due to a discharge of a positive
`about 200 volts. Diode D1 prevents capacitor C1 from
`voltage across the capacitor into inductor L1. In this
`discharging back into charging power supply 120 by
`embodiment, successive magnetic pulses of alternating
`blocking the reverse current flow from capacitor C1.
`polarity are produced.
`Since the voltage across capacitor C1 is now about
`The inductance of inductor L2 is large, preferably at
`twice that supplied by supply 120, this circuit acts as a
`voltage doubler.
`least on the order of ten times the inductance of induc
`tor L1 and advantageously on the order of 50 times the
`At this point the system is ready to generate its first
`25
`inductance of inductor L1. The embodiment of FIG. 1
`pulse of current. When the gate of SCR 157 is triggered
`produces magnetic pulses of duration approximately
`by gate driver 150, it starts to conduct allowing capaci
`120 usec.
`tor C1 to discharge through the stimulation coil L1
`The circuit of the present invention will operate suit
`creating a high current pulse. As in the case of the
`ably for a variety of values for inductors L1, L2 and
`circuit of FIG. 1, capacitor C1 and inductor L1 form a
`capacitor C1. As will be appreciated, decreasing the
`resonant discharging circuit.
`inductance of inductor L2 will cause capacitor C1 to
`Current through inductor L1 continues to flow, es
`charge faster; decreasing the inductance of inductor L1
`tablishing a negative voltage across capacitor C1; and
`will cause the magnetic stimulation pulse to decrease in
`eventually the current through inductor L1 reaches
`amplitude; and decreasing the capacitance of capacitor
`zero. After the energy transfer from inductor L1 back
`C1 will reduce the width of the stimulation pulse.
`to capacitor C1 is complete, the voltage on capacitor C1
`The rate of generation of stimulation pulses is limited
`is about minus 180 volts. As a result there is about 280
`by the energy lost to heat and the amount of power that
`volts present across inductor L2.
`can be obtained from the power supply to which the
`At this point, the voltage across capacitor C1 is a
`device is connected. Typical household power outlets
`local maximum negative voltage and current starts
`supply 120 volts at a maximum current of 15 amps,
`flowing through inductor L1 in a reverse direction by
`providing 1800 watts of power. A stimulation rate of
`way of diode 156. This reverse current through induc
`tor L1 produces magnetic pulses of opposite polarity
`about 40 stimulations per second has been achieved
`with the embodiment of FIG. 1 and a 120 volt, 15 amp
`from that produced by forward current through induc
`power supply.
`tor L1. The reverse current through inductor L1 flows
`45
`Referring now to FIG. 2, there is depicted a magnetic
`until the voltage across capacitor C1 is zero, at which
`neural stimulator 100 capable of producing magnetic
`point one cycle is complete, i.e., a positive and a nega
`pulses of alternating polarity. Magnetic stimulator 100
`tive magnetic pulse have been produced.
`comprises a charging power supply 120, a stimulation
`Meanwhile, starting at the time that the voltage
`coil 130, a charging circuit 140, energy storage means
`across capacitor C1 fell below the output voltage of the
`50
`142, a gate driver board 150, firing means 155 and a low
`charging power supply 120, current has resumed flow
`voltage power supply 160.
`ing through inductor L2. This current continues flow
`Charging power supply 120, stimulation coil 130,
`ing as the Voltage drop across inductor L2 increases to
`energy storage means 142, gate driver board 150 and
`about 280 volts. As in the case of the circuit of FIG. 1,
`low voltage power supply 160 are structurally and func
`this voltage drop produces a current through