All publications, including patents and patent applications, mentioned in this specification are herein incorporated by reference in their entirety to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference.
The present invention relates to systems and methods for providing electrical power to transcranial magnetic stimulation (TMS) systems.
Multi-coil repetitive Transcranial Magnetic Stimulation (rTMS) presents significant power demands that are not easily met by conventional building wiring. A single Magstim Rapid®, for example, may require two 20 Amp circuits to power a single coil. Because each stimulator draws a large surge of power after each discharge, instantaneous power demand on the AC line spikes well above the average demand, resulting in the need to dedicate one or more AC circuits to each stimulator.
A conventional magnetic stimulator consists of a stimulating coil, a discharge capacitor connected to the coil via an electronic switch, and a high voltage DC power supply connected to the capacitor, possibly with series inductors between the power supply and the capacitor, and between the capacitor and the electronic switch. There may also be snubbers or other components designed to protect the capacitor and switches from transient voltages that occur when coil current is switched off. There may also be provision for recovering some of the energy from the coil after discharge and pumping it back into a storage capacitor. There may be additional capacitors for the purpose of storing energy to allow fast recharging of the discharge capacitor.
In use, the DC power supply charges the discharge capacitor rapidly to a predetermined voltage. When triggered, the electronic switch dumps the capacitor's charge into the coil and any series inductors, generating a powerful transient magnetic field. The series combination of coils and capacitors may form a resonant circuit makes it possible for the field to oscillate for several cycles if the switch is closed long enough to allow it. Many stimulators provide options for several different pulse shapes and amplitudes, specifically: biphasic, in which the field does one complete oscillation cycle and then stops, monophasic, in which the field does a half cycle, and polyphasic, in which the field does several oscillations. More exotic waveforms have been described as well, including “theta bursts” comprising pulses of 50 Hz oscillations spaced at 100mS intervals. The period of each magnetic pulse is largely determined by the total inductance of the stimulating coil and any inductors in series with it, and by the capacitance of the discharge capacitor, with resonant frequency Ft given (assuming negligible resistance in the circuit) by:
In a typical TMS unit, the coil has an inductance (L) of about 15-25 uH, and the capacitor has capacitance (C) of 5-50 uF, resulting in a resonant frequency of 10-20 KHz. It is well-known in the art to use other values to give other resonances as needed.
In most cases, the circuit involves one or more reservoir capacitors and switches that controllably dump charge into the resonant circuit . Once charge is dumped and resonance ceases (or the switch is opened), the capacitor presents a large load to the high voltage supply. The high voltage supply in turn presents a large instantaneous load to the AC line, and because of the nonlinear behavior of the rectifiers in line with the capacitor, the power factor can be 0.6 or less. TMS devices typically fire at rates of from 1 Hz for down-regulation, to 10 Hz or more for up-regulation, and each coil typically dissipates about 40 W of average power at a 1 Hz pulse rate. Because of the pulsed nature of the discharge, it is possible for the stimulator to present an instantaneous load that demands the full capacity of the AC circuit for hundreds of milliseconds as the capacitor begins to charge.
In U.S. Application 2008/0306326, Epstein describes combining an AC power supply and battery to allow a stimulator to generate pulses that otherwise need more power than a standard AC circuit can provide. This solution is limited in that it does little to regulate the power factor or the peak load demanded by the stimulator. Batteries have relatively high effective series resistance as compared to capacitors, and thus are not ideally suited for providing rapid bursts of very high current required by a TMS coil.
In U.S. Application 2007/0293916, Peterchev describes [0054] a programmable charger that allows an operator to set two independent target voltages for a pair of discharge capacitors. The invention is primarily concerned with inducing rectangular electric field pulses into a body organ, and does not address AC line power management.
It would be therefore be desirable to have a magnetic stimulator that presents less supply ripple so that its instantaneous power demand is much closer to its average power demand. This would make it practical to run several stimulators and coils off the same AC line.
The systems and methods described herein are directed to power supplies and power management for transcranial magnetic stimulation systems.
In one aspect, we provide a TMS system with a programmable high voltage power supply that charges the capacitor at a constant current until it reaches a predefined voltage or total stored energy:
where V is the voltage across the capacitor of capacitance C, and Q is the stored charge in the capacitor at voltage V.
Thus to compute the stored energy one must measure voltage V and either capacitance C of the capacitor or current during the charge cycle.
In another aspect, a multi-coil stimulator consists of one high-voltage supply with several discharge capacitors multiplexed to it via electronic switches and or diodes. There may be passive filtering elements to help manage load spikes between the capacitors and the supply, and snubbers to protect the switches from voltage spikes that arise from rapid switching of inductive loads.
The supply may be configured to charge all capacitors at once using a constant current charging profile until they reach a predefined threshold voltage. The programmed current and threshold voltage may be selected in accordance with a desired firing rate of the stimulator. For example, if the stimulator is to fire at 1 Hz, the supply may be programmed so that each of the n discharge capacitors is charged to the threshold voltage in 1/n second. For a capacitance C and voltage threshold V, the amount of charge Q (in Coulombs) required to charge the capacitor to the threshold voltage is given by:
Q=CV
Thus the average current required to charge the capacitor to threshold in 1 second is Q Amperes, (since an Ampere is defined as one Coulomb per second) and the average current required to charge it in 1/n second is nQ Amperes.
The input switches may be configured to switch the High Voltage (hereinafter abbreviated as HV) supply output rapidly from one capacitor to the next until all capacitors are charged just prior to the firing event.
It would be desirable to interpose a passive filter between the HV supply and the load switches.
The power supply may be a switching type with programmable output voltage, or it may use a multitap transformer in which the output voltage is programmably stepped from a relatively low voltage at the beginning of the charge cycle in steps to the threshold voltage at the end of the charge cycle. Alternatively a single HV supply may charge an array of capacitors through a distributor and a filter inductor. By switching among the capacitors at a rapid rate, the supply ripple is reduced. By providing many more capacitors than the number of coils, and switching banks of capacitors in parallel (possibly with diode logic), it is possible to further reduce supply ripple.
The power supply may have, an input that controls the average charge rate of the load, and another input that controls the threshold voltage.
There may be one HV supply per coil, each designed to draw an average load based on a programmed charging rate as noted above, or a single supply may be multiplexed via switches, diodes, or passive filters to several discharge capacitors.
Multiple discharge capacitors may be connected via switches to a single coil to allow bursts of rapid pulses, and still provide a reduced ripple load profile to the AC line.
Power factor compensation may be added to the HV supply in order to further reduce load harmonics on the AC line. Power factor is the ratio of apparent power (the voltage-current product) to the work done by the load, and is a measure of the degree to which the load appears to the supply as a pure resistive load. AC to DC converters, particularly switching ones, have nonlinear current characteristics that present low power factor loads to the AC line. Power factor may be compensated by analog filtering with high current inductors, or in a switching supply by adding a boost converter in series that is designed to maintain a constant output voltage while drawing a load that is matched to the input waveform as closely as possible.
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HV supply 302 may enable each charge gate in turn until its respective capacitor is fully charged, or may switch among all capacitors in rapid bursts, resulting in a roughly uniform rate of charging of all capacitors. Controller 303 is shown with at least three outputs to high voltage supply 302. Rate output 320 determines charge current as described above; Peak Voltage output 321 controls the maximum voltage delivered to the capacitors—this limits the maximum amount of energy delivered to a coil, and is typically set by the clinician with reference to the power level that elicits a motor response when a single coil is positioned over the motor cortex; one or more Trigger signals 321 function as described above to control the release of energy into a stimulus coil. Trigger signals 321 may consist for example of one signal line for each coil, one signal line for each discharge capacitor, an address bus that selects a specific capacitor or coil for discharge and a separate signal to fire it, a single line that fires all coils at once, or any other method well-known in the art for addressing and triggering multiplexed elements under program control.
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As for additional details pertinent to the present invention, materials and manufacturing techniques may be employed as within the level of those with skill in the relevant art. The same may hold true with respect to method-based aspects of the invention in terms of additional acts commonly or logically employed. Also, it is contemplated that any optional feature of the inventive variations described may be set forth and claimed independently, or in combination with any one or more of the features described herein. Likewise, reference to a singular item includes the possibility that there are plural of the same items present. More specifically, as used herein and in the appended claims, the singular forms “a,” “and,” “said,” and “the” include plural referents unless the context clearly dictates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation. Unless defined otherwise herein, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The breadth of the present invention is not to be limited by the subject specification, but rather only by the plain meaning of the claim terms employed.
Number | Date | Country | |
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61303188 | Feb 2010 | US |
Number | Date | Country | |
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Parent | 13512496 | Sep 2012 | US |
Child | 15249278 | US |