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Research Papers

# A Novel Magnetic Stimulator Using Parallel Excited Coils and Capable of High Frequency Stimulation

[+] Author and Article Information
Syrpailyne Wankhar

e-mail: syrpailyne@cmcvellore.ac.in

Suresh Devasahayam

e-mail: surdev@cmcvellore.ac.in
Department of Bioengineering,
Christian Medical College,

Srinivasa Babu

Department of Neurological Sciences,
Christian Medical College,
e-mail: srinivas@cmcvellore.ac.in

Manuscript received March 28, 2013; final manuscript received August 30, 2013; published online December 6, 2013. Assoc. Editor: Carl A. Nelson.

J. Med. Devices 8(1), 011006 (Dec 06, 2013) (9 pages) Paper No: MED-13-1129; doi: 10.1115/1.4025422 History: Received March 28, 2013; Revised August 30, 2013

## Abstract

Magnetic stimulators are used for transcranial and peripheral stimulation of nerves for diagnostic, therapeutic, and research purposes. Stimulation is achieved by generating a rapidly changing magnetic field to induce a current at the nerve of interest. Effective nerve stimulation requires a current transient of about $108A$/s. This current is obtained by switching the current through a thyristor or an insulated gate bipolar transistor (IGBT). Insulated gate bipolar transistors have better turn off characteristics than thyristors. Due to the large currents, fast switching, and inductive load required in magnetic stimulators, spike voltages can occur and cause device damage. Therefore, they require elaborate protection circuitry. Contemporary magnetic stimulators are large, bulky, and give a current wave that is constrained by the device characteristics rather than decided by physiology. Recent instruments using IGBTs have addressed this question. However, the IGBTs require special considerations to protect them against damage. No magnetic stimulators reported so far can stimulate at rates greater than 60 Hz (Magstim Rapid2, two linked stimulators). A novel magnetic stimulator design is presented in this paper which uses a set of stacked coils driven by independent but synchronized electronic circuits to distribute the current so that only a fraction of the required current flows through any given circuit element. The coils can be arranged in several different geometries, depending on the location and shape of the nerves to be stimulated. While such paralleling of coils and control circuits is not so important for the thyristor circuit design, in the case of the IGBT design it allows the use of smaller IGBTs and better transient control. The design of the coils and independent excitation improves the current control and the magnetic field that is generated. The result is a portable instrument with well controlled rectangular pulse shapes. This stimulator is also capable of much higher frequencies (tested up to 100 Hz) than previously reported. Experimental tests have been compared with the biophysical analysis of stimulation with this instrument. Peripheral nerve stimulation and the elicited compound muscle action potential was used to validate the instrument. The instrument has been tested for the controlled recruitment of a compound nerve at up to 100 Hz. In this paper we present a portable magnetic stimulator capable of high frequency stimulation and rectangular stimulation pulse. These features should give fresh momentum to the use of magnetic stimulation in neurological investigations and interventions. In particular, we expect that it will find wide clinical use such as in pediatric neurology, psychiatry, and neuromodulation.

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## References

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## Figures

Fig. 1

Block schematic of a conventional RLC circuit based magnetic stimulator design

Fig. 2

(a) Schematic diagram of the new magnetic stimulator design using IGBTs (active switch) in parallel and multiple stacked coils, (b) a ramp shape current pulse and its time derivative rectangular pulse generated by the new magnetic stimulator circuit, and (c) the current waveform of one of the coils with different slopes when one, two, or three coils were activated at a time

Fig. 3

(a) Circular coil, and (b) multiple stacked coils

Fig. 4

Measured coil current from two coils showing the effect of mutual inductance (- - - indicates coil 1 current turned off, · · · indicates coil 2 current turned off)

Fig. 5

Experimental setup for recording the CMAP and force from the ADM muscle using magnetic stimulation: (a) a stimulator with the cover open, (b) a stimulator with the coil attached and positioned for stimulating the ulnar nerve at the elbow; a foot ruler is shown in the foreground stuck on the edge of the table

Fig. 6

Measured coil voltages when (a1) one coil, (a2) two coils, and (a3end)–(a3middle) three coils were activated. Calculated coil currents of each coil when (b1) one coil, (b2) two coils, and (b3end)–(b3middle) three coils were activated. (a3end) and (b3end) The end coils, and (a3middle) and (b3middle) the middle coil with three coils activated at different pulse widths (- - -, 90 μs; · · ·, 130 μs; and —, 160 μs).

Fig. 7

Tested pulse width and coil combination for different pulse strengths tabulated in Table 1 and plotted as circles on the hypothetical strength-duration map of a compound nerve. Each curve represent a twitch of a different size (1, 2, and 3 in the parentheses indicates the activation of one, two, or three coils, respectively, at different pulse widths).

Fig. 8

The left set of figures show the CMAP recorded with (a1) one, (a2) two, and (a3) three coils. The CMAP was recorded from the ADM muscle and the stimulation was delivered at the brachial plexus to show a long latency from the stimulation artifact (the latency is about 17.7 μs and the distance of the ADM from the brachial plexus is 70 cm). The right set of figures show the twitch force with (b1) one, (b2) two, and (b3) three coils; the force was recorded from the ADM muscle and the stimulation was delivered distal to the elbow to reduce movement of the arm.

Fig. 9

(a) and (d) CMAP obtained from the ADM muscle at different frequencies (1 Hz, 2 Hz, 5 Hz, 10 Hz, 20 Hz, 50 Hz, and 100 Hz) during magnetic stimulation of the ulnar nerve, and (b), (c), and (e) the force obtained from the ADM muscle at 1, 2, 5, 10, 20, 50, and 100 Hz, corresponding to a train of five stimulus pulses (the arrow indicates the stimulus pulse)

Fig. 10

Exponentially decaying current pulse and the measured coil current generated by a conventional magnetic stimulator built in the lab

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