SPECIFICATIONTissue stimulatorThis invention relates to tissue stimulators.
The use of electrical tissue stimulators for stimulating human or animal tissue by the passage of electrical currents through the tissue for both diagnostic and therapeutic purposes is well known. Such electrical stimulators suffer the disadvantages however that careful placement of the surface electrodes of the stimulator on a patient is required, there is inevitably a high density of current flowing in the tissues of the patient between the electrodes which may cause pain in the patient and it is difficult to electrically stimulate deep lying tissues within the patient. Recently a number of magnetic tissue stimulators have been developed which eliminate some of the problems inherent in electrical tissue stimilators.Examples of such magnetic tissue stimulators are shown in respective articles in TheLancet, pages 1 106~1 107 published on May 11th, 1985 and in Journal of Physics E, volume 18, pages 74-78 published in 1985.
These articles each describe a surface coil which is placed on a surface of a patient so as to apply a varying magnetic field to the part of the patient adjacent to the coil. This then causes an induced current within the tissues of the part to be generated, thereby stimulating the tissues. The use of such magnetic stimulators does not require placement of surface electrodes on the patient, and is pain free to the patient.
Such known magnetic tissue stimulators suffer the disadvantage however that the stimulation is spatially non-selective being strongest within the surface tissues of the patient and decreasing as a function of distance away from the surface of the patient.
It is an object of the present invention to provide a magnetic tissue stimulator wherein this problem. is at least alleviated.
According to the present invention a magnetic tissue stimulator comprises a plurality of magnetic field coil systems, each system being energisable to produce an extended region of magnetic field maximum, the systems being arranged such that the extended regions intersect in a predetermined region, and means for sequentially energising the coil systems such that in use of the stimulator tissue located within said predeteremined region is selectively stimulated.
Two tissue stimulators in accordance with the invention will now be described by way of example only, with reference to the accompanying drawings in which:.
Figure 1 is a schematic diagram of a magnetic field coil system incorporated in the first tissue stimulator, together with the magnetic field profile produced by the coil system as a function of the distance along a direction perpendicular to the axis of the coil system, andFigure 2 is a schematic diagram of the magnetic field coil system incorporated in the second tissue stimulator, together with the magnetic field profile produced by the coil system as a function. of the distance along a direction perpendicular to the axis of the coil system.
Referring firstly to Figure 1, the first tissue stimulator to be described includes two magnetic field coil systems of the form indicated in the figure. Each system comprises three annular magnetic field coils 1, 3, 5 arranged in a parallel spaced configuration along a common axis 7. The outermost coils, 1 5 are connected in the same sense to a power supply (not shown), whilst the central coil 3 is connected to the supply in the opposite sense to that of the coils 1, 5. Each system is designed such that when it is energised by the supply an axial magnetic field is produced by the system whose magnetic flux density B varies with distance r from the axis 7 in the manner indicated in Figure 1, the flux density falling rapidly in the direction away from the axis 7. This field profile is maintained for a distance along the axis 7 such that there exists a linearly extended field maximum.The two coil systems within the stimulator are arranged such that their regions of field maxima intersect to define a localised region.
In use of the tissue stimulator, a patient is positioned such that the tissue of interest within the patient lies within the region of intersection of the regions of field maxima of the two coil systems. The magnetic coil systems are sequentially energised by the power supply such that two magnetic field pulses are produced in rapid succession, one pulse being produced by each system. The strength and duration of each of the pulses are chosen such that the product of the strength and the duration of each pulse on its own is less than the threshold value for stimulation of the tissue. Since the effect of successive pulses is cumulative a second pulse will cause the threshold value to be reached but only within the region of intersection of the regions of field maxima for the two coil systems.
Thus selective stimulation of the tissue of interest will occur.
The threshold value for stimulation will vary widely between tissue types, but generally magnetic field pulses with a peak value in the range 1 to 10 tesla, the duration of the two pulses together being in the range of 50 microseconds to 100 milliseconds will be adequate to induce sufficient current within the tissue to produce stimulation of the tissue.
Such magnetic field pulses would correspond to current densities of between 1 to 100 amps per square metre within the tissue. It will be appreciated that if application of the second pulse after the first pulse is delayed the effect of the first pulse will be attenuated with time. Generally the time allowed to elapse between the application of the two pulses will be in the range 10 microseconds to 10 milliseconds.
The tissue stimulator described above also finds particular application in the selective excitation of tissues which are especially responsive to respective excitation at a particular frequency, for example in the stimulation of the tissue of the cortex or deeper structures, such stimulation being required during neurosurgical operations. The stimulation of such tissue may be achieved by repetitively pulsing each of the two coil systems at the same frequency, usually within the range 1 to 100 Hertz, the pulse widths each being in the range 50 to 100 milliseconds, The relative timing of the pulses produced by each of the two coil systems is chosen such that the pulses are approximately evenly interleaved the overall pulse frequency being chosen to correspond to the particular frequency to which the tissue of interest is especially responsive.The tissue of interest will again be located within the region of intersection of the regions of field maxima of the two coil systems. Thus in this manner the tissue within the region of intersection of the regions of field maxima of the two coil systems will be selectively stimulated at the overall frequency of the pulses from the two coil systems. Tissue outside this region will experience smaller magnetic fields, and/or fields of the lower frequency represented by the pulses from only one of the coil systems.
Turning now to Figure 2, the second tissue stimulator to be described operates on the same general princicples at the first tissue stimulator described above, but incorporates two magnetic field coil systems of the alternative form indicated in this figure. Each coil system comprises two annular magnetic field coils 11, 13 arranged in a parallel spaced configuration along a common axis 15. The coils11, 13 are connected in opposite sense to a power supply (not shown). Each coil system is designed such that when it is energised by the supply, a radial magnetic field is produced in the plane 18 halfway between the coils 11,13, whose field maximum takes the form of an annulus surrounding the coils 11, 13 as indicated in the magnetic flux density B. distance r from the axis 15 plot shown in Figure 2.The two coil systems within the stimulator are again arranged such that the regions of their field maxima intersect, such that by sequential energisation of the two coil systems tissue placed within the region of intersection may be selectively stimulated.
It will be appreciated that whilst in the two magnetic tissue stimulators described herebefore by way of example, each stimulator incorporates two magnetic field coil systems each producing an extended region of magnetic field maximum a magnetic tissue stimulator in accordance with the invention may incorporate three or more such systems, these systems being arranged such that the regions of their field maxima intersect.
It will also be appreciated that whilst in the two magnetic tissues stimulators described herebefore by way of example, each coil system within the stimulator is of similar form, the coil systems may be of different form, as long as it is possible to arrange for their regions of field maxima to intersect to define a localised volume of space.
It will also be appreciate that many other coil systems may be used in a magnetic tissue stimulator in accordance with the invention. Such systems could include a number of coils which are not aligned along a common axis, or are tilted with respect to each other, to produce regions of maximum field extending along straight or curved paths.