Abstract:
A magnetic stimulator for neuro-muscular tissue uses a stimulating coil and generates a succession of electrical discharge pulses from the coil to produce magnetic pulses which induce electrical signals in the tissue. The coil has at least one set of generally circular turns, and is disposed within a casing connected to a conduit through which extend cables for the supply of electrical power to the coil. The conduit and casing allow for the flow of gaseous coolant around and over exposed surfaces of the coil, though the casing and along the conduit.

Description:
FIELD OF THE INVENTION 
     This invention relates to coil assemblies for magnetic stimulators intended for the electro-magnetic stimulation of neuro-muscular tissue. The invention particularly relates to an improved construction which provides for the supply of electrical power and the flow of coolant by way of the same conduit. 
     BACKGROUND TO THE INVENTION 
     It is known, for example from British patent 2298370 to provide a rapidly changing magnetic field, of the order of 20 kiloteslas per second, in the vicinity of tissue in order to induce electric current in the tissue and thereby stimulate the tissue. 
     Known magnetic stimulators comprise generally a charging circuit, a capacitor, a discharge control such as a control rectifier, and a coil which is of a size and power rating appropriate for the generation of the very large magnetic fields which magnetic stimulators require. Typically, the coil may be of a size adapted to fit partly over the cranium of a human patient. Others forms of coil are, as exemplified herein, in the shape of a large figure of eight. 
     In the aforementioned patent and also in U.S. Pat. Nos. 5,718,662 and 5766124 there are described stimulators which are capable of providing a rapid sequence of discharge pulses through the stimulating coil. Such a rapid sequence of pulses is useful in, for example, transcranial magnetic stimulation. 
     However, owing to the very large energy contained in the pulses, such a stimulation technique is very liable to produce overheating of a stimulating coil. It is known to provide in a stimulating system of this general nature, some means of monitoring the temperature in the vicinity of a stimulating coil and to provide some inhibiting function, such as disabling the stimulating system if a coil should overheat. It is customary to change coils in those circumstances. 
     The present invention has the general object of avoiding the need to change coils unnecessarily and is based on a coil design which allows cooling by the flow of a coolant gas around substantially all the exposed surfaces of the stimulating coil and preferably also a supply cable to it. 
     Other features and advantages of the invention will be apparent from the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a known circuit for providing high power pulses to a stimulating coil for neuro-muscular tissue stimulation; 
     FIG. 2 is an ‘exploded’ view of a coil assembly according to the invention, 
     FIG. 3 is a sectional view of part of the assembly shown in FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates one channel of a stimulating system corresponding to that described in our British patent number 2298370. 
     A high voltage power supply  1  is provided for charging a reservoir capacitor  2 . Discharge of the reservoir capacitor is controlled by a controllable series switch  3  which may be a controlled rectifier such as a thyristor. The switch  3  is connected to a series inductor  4 , which is connected to the upper plate of a transfer capacitor  5  of which the lower plate is connected to the lower plate of capacitor  2 . A reverse diode  6  is connected across the capacitor  5 , the diode blocking current in the direction of normal current flow through the switch  3  and the inductor  4 . 
     The upper plate of the transfer capacitor  5  is connected by way of an inductor  7  to the anodes of two thyristor switches  8   a  and  8   d  of which the cathodes are connected to opposite plates of a third, discharge, capacitor  9 . The inductor  7 , like inductor  4 , is a current limiter which is capable of transient energy storage. The lower plate of the transfer capacitor  5  is connected to the cathodes of thyristors  8   b  and  8   c , of which the anodes are connected to the upper and lower plates respectively of the discharge capacitor  9 . The upper plate of the capacitor  9  is connected to the anode of the thyristor  10   a  and the cathode of a thyristor  10   d , the cathode of thyristor  10   a  and the anode thyristor  10   d  being connected to upper and lower terminals respectively of a stimulating coil  11 . Similarly, the lower plate of the capacitor  9  is connected to the anode of thyristor  10   b  and to the cathode of thyristor  10   c , the cathode of thyristor  10   b  and the anode of thyristor  10   c  being connected to the upper and lower terminals of the stimulating coil  11 . The thyristors  10   a  to  10   d  constitute a bridge  10  which determines unidirectional flow of current through the coil  11  irrespective of the polarity of the voltage across the capacitor  5 . In this embodiment the lower terminal of the stimulating coil  11  is provided with a ground connection  12 . 
     As described in the aforementioned patent, the system described in FIG. 1 may be used for the repeated transfer of charge from the reservoir capacitor  2  to the transfer capacitor  5  and thence to the discharge capacitor  9  during appropriate time intervals. Modulation of the amplitude and frequency of the magnetic stimulating pulse fields provided by the coil  11  may be achieved. 
     The system shown in FIG. 1 may be multiplexed or combined with a plurality of other similar stimulating channels, as described in for example U.S. Pat. No. 5,718,662, so as to achieve a high pulse repetition rate. 
     As indicated previously, systems of this nature are subject to overheating of the coil  11  when rapid high power pulses are produced. Accordingly, the present invention provides for the cooling of the coil by means of a gaseous coolant. 
     FIGS. 2 and 3 illustrate one embodiment of an assembly for the coil allowing the coil to operate for relatively prolonged periods without overheating. 
     The description which follows is of a coil assembly which, broadly, has a casing in which the coil is disposed in a manner which allows the flow of coolant gas over the exposed surfaces of the coil. The assembly includes a hollow shaft which connects the casing for the coil to a further conduit and a T-piece having a branch for electrical cables that extends through the conduit and shaft to the coil and a branch for the flow of gas. In the preferred example, gas may be pumped by means of a vacuum suction pump through the assembly but an equivalent, which would require some modification to the embodiment, would be to allow for injection of compressed gas coolant into the assembly, for example at an inlet provided in the coil casing so that the gas exhausts at the other end of the conduit. 
     The assembly shown in the drawings includes the coil  11  which is disposed adjacent an insulating screen  12 . The coil and screen are disposed within a casing comprising a top cover  13  in a generally figure of eight form having a side extension  14  terminating in a semi-circular pipe section  15 . The bottom cover  16  is generally similar; it has a peripheral flange  17  and inner circular flanges  18  for the retention and location of the coil and the insulating screen  12 . Through the holes in the figure of eight assembly extend two screws  19 . These screws extend to filter assemblies comprising a respective filter well  20 , a filter  21  fitting within the respective filter well, and a filter cap  22 . 
     The semi-circular pipe sections  15  and  15   a  mutually abut and fit within a shaft  23  terminating in an adapter  24  for fitment over and securing to a nylon conduit  25 . A clip  26  fits into a recess  27  in the adapter  24  in order to secure the conduit  25  in adapter  24 . The other end of the conduit  25  fits into an adapter  28  of a T-piece assembly  29 . A clip  30  fits into recess  31  to secure the conduit  25  in adapter  28 . Branch  32  of T-piece  29  is intended for coupling to a vacuum suction pump (not shown). Depending from the body of the T-piece  31  is a short tube  33  to which is fitted a connector shell  34  within the lower end of which fits a ring spacer  35  within which fits a cable connector  36 . 
     As is shown in FIG. 3, a ground wire  37  is connected to the insulating screen  12  and this screen is adhered into the bottom cover  16 . A printed circuit board  38  is disposed in the bottom cover as are two heat sensors  39  and  40 . 
     Cables  41  extend from the coil to the printed circuit board and the printed circuit board  38  is coupled by way of connectors  42  to a cable assembly  43  in a rigid tube  44  which extends along the inside of the shaft  23 . 
     The cables are held rigidly as shown at  44  so that there is space for the passing of gaseous coolant through the assembly. 
     In the particular example shown in the drawings, air will be drawn in through the filters and after passing over the coil will be drawn out through the branch  32  of T-piece  29  after passing along the tube  23  and conduit  25 . 
     Modifications are feasible. For example, compressed gas could be applied to an inlet in the casing and pass back along the cable conduit  25  to an exhaust at the T-piece. 
     Further modifications may be made by those skilled in the art, and it is intended only to illustrate and not limit the invention by the foregoing example.