Abstract:
Efficient use of multi-coil arrays for magnetic nerve stimulation depends upon coordinating the coil polarity, the pulse phase and the pulse timing. Monophasic magnetic nerve stimulators produce more precise and predictable results in the stimulation of nerves than biphasic and polyphasic machines, but are less electrically efficient, and consequently limited in terms of pulse train speed. The present invention concerns the coordination of pulse polarity, phase, timing, and strength between multiple magnetic stimulation coils. The goal is to optimize the manner in which multiple coils may be used synergistically to control the activity of underlying neural tissue.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 60/954,018, filed on Aug. 5, 2007, titled “MONOPHASIC MULTI-COIL ARRAYS FOR TRANCRANIAL MAGNETIC STIMULATION.” 
     
    
     INCORPORATION BY REFERENCE 
       [0002]    All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. 
       FIELD OF THE INVENTION 
       [0003]    The devices and methods described herein relate generally to the use of electromagnets to stimulate the brain treatment of hypertension. 
       BACKGROUND OF THE INVENTION 
       [0004]    Concurrent use of more than one magnetic stimulation coil can be used to improve depth of stimulation within a brain, and to help control the location of a deep area of stimulation, as described in Schneider M B et al. 2004 U.S. Ser. No. 10/821,807, and Mishelevich et al. 2006 U.S. Ser. No. 11/429,504. However, presently available rTMS pulse generator units are limited in their ability to provide the optimal signal to such a coil array. 
         [0005]    Most magnetic nerve stimulators in use today are biphasic or polyphasic, for example the Magstim Rapid2 (Magstim Ltd., Wales, UK). Electrically efficient, they are well suited to sustained, rapid pulse trains needed for producing enduring brain modulation, for example depression treatments. However, the complexity of the polyphasic waveform, meeting the complexity of the nervous system frequently yields inconsistent results. In the case of multiple coil stimulation, there is an increased chance that one waveform phase from one coil could diminish or cancel a concurrent phase from another coil 
         [0006]    Monophasic magnetic stimulators provide more predictable neurostimulation effects, and they are well known in the art. For example, the Magstim 200 2  (Magstim Ltd, Wales, UK), and the MagPro X100 with MagOption by the Dantec division of Medtronic (Copenhagen, Denmark), which generates pulse shapes including biphasic, monophasic, and half-sine. Electrically inefficient, these machines are not capable of sustained, rapid pulse trains. The output of these devices is always to a single output to a coil in which all loop portions receive the same electrical waveform. 
         [0007]    The prior art does not provide any means for coordinating pulse phase, timing polarity or strength between more than one coil. 
       SUMMARY OF THE INVENTION 
       [0008]    Described herein are methods and devices for stimulating neural structures within the brain using multi-coil arrays. 
         [0009]    A device is described in which a biphasic or polyphasic electrical discharge from a magnetic nerve stimulation or rTMS machine is split into two separate monophasic pulses. These separate pulses are then sent to two separate coils. The method provides for the coordination of pulse polarity, phase, timing, and strength between multiple magnetic stimulation coils. The goal is to optimize the manner in which multiple coils may be used synergistically to control the activity of underlying neural tissue. 
         [0010]    In one embodiment a biphasic electrical pulse is passed through a high-power bridge rectifier, with the two outputs of which power the positive pole of one double coil and the negative pole of the other double coil, while the remaining pole of each of two double coils are both held to ground. The electrical pulses through each of the coils is thereby monophasic, and the induced magnetic field, is at least substantially monophasic. 
         [0011]    In an alternative embodiment, the two poles of a biphasic electrical pulse generator are passed through high-power diodes, the positive pole of one double coil and the negative pole of the other double coil, while the remaining pole of each of two double coils are both held to ground. The electrical pulses through each of the coils is thereby monophasic, and the induced magnetic field, is at least substantially monophasic. 
         [0012]    In another alternative embodiment, a biphasic electrical pulse source has its poles connected with opposite sides of a high-power bridge rectifier. The outputs from the remaining sides of the bridge rectifier are passed to the positive and negative poles of one double coil. The electrical pulses through each of the coils is thereby monophasic, and the induced magnetic field, is at least substantially monophasic. 
         [0013]    Pulse generation devices that produce such pulses are commercially available such as the Magstim Rapid stimulator by Magstim LTD (Wales, UKCoils that are useful within the context of the present invention are commercially available, for example the 70 mm double coil (Magstim LTD (Wales, UK). 
         [0014]    In one embodiment, coils may be situated substantially next to one another such that their magnetic fields are directed in a substantially similar direction if the coils are arranged in the same polarity, and a substantially opposite direction if the coils are arranged with opposite polarities. In another embodiment, the coils may be situated substantially opposite one another, such that their magnetic fields are directed in a substantially opposite direction if the coils are situated with the same polarity, and substantially the same if the coils are situated in opposite polarity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0015]      FIG. 1  shows a circuit diagram of a full-wave rectifier circuit applied to a two-coil array. 
           [0016]      FIG. 2  shows a circuit diagram of a half-wave rectifier circuit applied to a two-coil array. 
           [0017]      FIG. 3  shows a full-wave bridge rectifier powering one double coil from an array. Note the intentional absence of a smoothing capacitor. 
           [0018]      FIG. 4A  illustrates the approximate waveforms of the electrical current input and outputs, respectively associated with the circuit shown in  FIG. 1   
           [0019]      FIG. 4B  illustrates the approximate waveforms of the electrical current input and outputs, respectively associated with the circuit shown in  FIG. 2 . 
           [0020]      FIG. 4C  illustrates the approximate waveforms of the electrical current input and output, respectively associated with the circuit shown in  FIG. 2 . 
           [0021]      FIG. 5A  illustrates the placement of coils next to one another so as provide a similar orientation. 
           [0022]      FIG. 5B  illustrates the placement of coils on opposing sides of a patient&#39;s head so as to provide roughly opposite orientation. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]      FIG. 1  shows a circuit diagram of a full-wave rectifier circuit applied to a two-coil array. In this particular embodiment, the two coils in the array, coil  110  and coil  120  are double coils, for example 70 mm double coil manufactured by Magstim Ltd. (Wales, UK). In such a double coil, two separate concentric windings are wrapped in opposite directions with a crossover between the two portions, placed such that the positive and the negative going leads to the two coil portions run electrical current in the same direction where the two portions are adjacent to one another, creating the greatest magnetic field induction under the center. Positive electrical pole  130  and negative electrical pole  140  have a pulsatile polyphasic alternating current  135  between them, and represent the black and red output wire pins on a standard repetitive transcranial magnetic stimulation device. The positive going current from pole  130  and negative going current from pole  140  enter bridge rectifier  150 , which is composed of diodes  151 ,  152 ,  153  and  154 . Positive portions of the waveform are then sent to positive pole  112  of TMS coil  110 , while negative-going portions of the negative pole  124  of TMS coil  120 . Ground pole  122  of TMS coil  120  and ground pole  114  of TMS coil  110  both go to ground  165  by leads  162 , and  160 , respectively. The output of this circuit will be illustrated in  FIG. 4A , and positioning of these coils will be discussed with respect to  FIGS. 5A and 5B . The electrical pulses through each of the coils is thereby monophasic, and the induced magnetic field, is at least substantially monophasic. 
         [0024]      FIG. 2  shows a circuit diagram of a (half-wave) rectifier diodes applied to a two-coil array. Positive electrical pole  230  and negative electrical pole  240  have a pulsatile polyphasic alternating current  235  between them, and represent the black and red output wire pins on a standard repetitive transcranial magnetic stimulation device. The positive going current from pole  230  enters diode  251  and is passed to positive pole  212  of TMS coil  210 . Meanwhile, the negative-going current from pole  240  enters diode  250  and is passed to negative pole  224  of TMS coil  220 . Ground pole  222  of TMS coil  220  and ground pole  214  of TMS coil  210  both go to ground  265 . The output of this circuit will be illustrated in  FIG. 4B , and positioning of these coils will be discussed with respect to  FIGS. 5A and 5B . The electrical pulses through each of the coils is thereby monophasic, and the induced magnetic field, is at least substantially monophasic. 
         [0025]      FIG. 3  illustrates the use of a full wave bridge rectifier circuit as applied to a single coil. Positive electrical pole  330  and negative electrical pole  340  have a pulsatile polyphasic alternating current  335  between them, and represent the black and red output wire pins on a standard repetitive transcranial magnetic stimulation device. The positive going current from pole  330  and negative going current from pole  340  enter bridge rectifier  350 , which is composed of diodes  351 ,  352 ,  353  and  354 . Positive portions the waveform are then sent to positive pole  312  of TMS coil  310 , while negative-going portions of the negative pole  324  of TMS coil  310 . The output of this circuit will be illustrated in  FIG. 4C . The electrical pulses through the coils is thereby monophasic, and the induced magnetic field, is at least substantially monophasic. 
         [0026]      FIG. 4A  illustrates the input and outputs of the circuit shown in  FIG. 1 . Positive going pulses  410  and negative-going pulses  411  are input into the  FIG. 1  circuit, and emerge as positive-going pulses  412  on one TMS coils, and negative-going pulses  413  on the other TMS coil. When using this configuration, the two coils may be arranged on opposite sides of the head so as to maximize summation and minimize energy cannibalization, provided that the coils are properly flipped so as to summate rather than cancel, as will be described with respect to  FIGS. 5A and 5B . 
         [0027]      FIG. 4B  illustrates the input and outputs of the circuit shown in  FIG. 2 . Positive-going pulses  420  and negative-going pulses  421  are input into the  FIG. 1  circuit, and emerge as positive-going pulses  422  on one TMS coils, and negative-going pulses  423  on the other TMS coil. When using this configuration, the two coils may also be arranged on opposite sides of the head so as to maximize summation and minimize energy cannibalization, provided that the coils are properly flipped so as to summate rather than cancel, as will be described with respect to  FIGS. 5A and 5B . 
         [0028]      FIG. 4C  illustrates the input and output of the circuit shown in  FIG. 3 . Positive-going pulses  430  and negative-going pulses  431  are input into the  FIG. 3  circuit, and emerge as rectified positive-going pulses  432  the TMS coil. In typical rectifier circuits, a capacitor is generally used to smooth the output. However, in the present invention, no capacitor is used, as unevenness, with its associated DB/DT is a desirable quality for inducing neuronal depolarization. 
         [0029]      FIG. 5  describes two forms of spatial relationship that two or more coils in an array can have in boost the summation of their fields.  FIG. 5A  shows two TMS coils; coil  530  and coil  525  placed along side one another over head  500 . While their positions are obviously not identical due to space that each takes up on the scalp surface, it will be appreciated that their trajectories are substantially similar. In practice this means that a significant percentage of their magnetic field output of the same phase will sum rather than cancel.  FIG. 5B  shows two coils; coil  575  and coil  580  facing each other from opposite sides of the patients&#39; head  550 . In practice, this position will generally cause pulses from the two coils to summate only if they are in opposite phase. 
       Discussion 
       [0030]    Using the means provided herein, biphasic electrical pulse source are divided such that the current passing through each of the coils is monophasic, and the induced magnetic field, is at least substantially monophasic. Because magnetic field is induced as a function of change in electrical current per unit time, even a perfectly monophasic electrical pulse does not create a perfectly monophasic magnetic field pulse: the pulse of electrical current cannot continue to rise indefinitely (in practice the current pulses are typically of approximately 0.1 ms in duration), and then falls to baseline. This means that there will be some reversal of induced magnetic field direction. However, in accordance with the means provided, this opposite-phase component will be of substantially less magnitude than the principal component of the magnetic field pulse. 
         [0031]    Summation from two sources from standpoint of physics (as opposed to temporal and spatial forms of physiological summation) can be summarized as follows:
   Matched phase and opposite direction yields cancellation.   Opposite phase and similar direction yields cancellation.   Matched phase and similar direction yields summation.   Opposite phase and opposite direction yields summation.   
 
         [0036]    In practice under general conditions, however, side-by-side coils of same polarity and phase may either enhance or diminish the neuronal response, as the sharp DB/DT at the margin between these coils may produce a potent depolarizing effect. Likewise coils on the opposite side of the head of opposite polarity are subject to the same uncertainty. Therefore controllability of this phenomenon depends upon precise control of magnetic field direction and as well as precise controls of the coils with respect to specific anatomical geometry. 
         [0037]    Note that phase can be changed electronically, as described herein, reversing polarity of the coil, or by simply flipping the coil face with respect to the target. In accordance with the present invention, coil polarity and direction of aim are planned in order to maximize the extent to which vectors that are desired are summated, and vectors which are not desired are cancelled. 
         [0038]    The ability of those two coils to act synergistically to depolarize neurons, rather than to diminish each other&#39;s effects, however, depends upon depends upon coordinating the coil polarity, the pulse phase and the pulse timing, as well as the power of the sources. For example, two coils on the opposite sides of a patient&#39;s head, located 180 degrees apart, but with like coil faces (same polarity with respect to the head) may serve to diminish the stimulating effect in the area between the coils. By contrast, two side-by side magnetic coils oriented in essentially the same polarity, and pulsed simultaneously from identical sources, will summate to produce additive effects. Likewise, two coils on the opposite sides of a patient&#39;s head, located 180 degrees apart, but with one coil face flipped to provide opposite polarity, will also result in an additive effect in the space between the coils. 
       REFERENCES AND PRIOR ART 
       [0039]    Dantec magnetic stimulation product information on MagPro X100 with MagOption. http ://www.danica.nl/neuro/neuro-magnetische-stimulatoren.htm. 
         [0040]    Magnetic Stimulation in Clinical Neurophysiology. Second Edition. Hallet M, Chokroverty S, Ed. Elsevier Inc., Philadelphia, Pa., 2005. Chapter by Ruohonen, et al. Transcranial Magnetic Stimulation: A Neurochronometrics of Mind. Walsh V, Pascual-Leone A. MIT Press. Cambridge, Mass. 2003. 
         [0041]    Davey K, Riehl M. Deigning Designing Transcranial Magnetic Stimulation Systems. .IEEE Transactions on Magnetics. Vol. 41, No. 3, March 2005. 1142-1148. 
         [0042]    Barker A T. An Introduction to the Basic Principles of Magnetic Nerve Stimulation. Journal of Clinical Neurophysiology. Vol 8, No. 1, 1991: 26-37. 
         [0043]    “Robotic device for stereotactic transcranial magnetic stimulation.” Schneider M B and Mishelevich D J U.S. Ser. No. 10/821,807. 
         [0044]    “Trajectory-Based Transcranial Magnetic Stimulation,” Mishelevich D J and Schneider M B, Pending U.S. patent application Ser. No. 11/429,504.