Electronic circuitry for shaping current pulses for small scale transcranial magnetic stimulation coils

Abstract

Transcranial magnetic stimulation (TMS) uses electromagnetic induction to induce electric currents in brain tissue via a time-varying magnetic field from a coil, stimulating neuronal activity. While extensively studied in humans, small animal TMS coils are less explored, presenting challenges due to the high currents needed and heating issues from coil size and core hysteresis. This research focuses on the development of a new pulse generator tailored for TMS applications, with a specific emphasis on small-scale systems designed for animal studies. A supercapacitor-based design was implemented to serve as the power source, offering a compact, cost-effective solution for generating stable voltage pulses. The pulse generator demonstrated the ability to produce magnetic fields ranging 200 – 400 mT with an input of 1 V, making it a highly efficient tool for small animal TMS coils. In an effort to simplify pulse shaping without using complex circuitry, a mosfet was used as a dynamic element to control current. A triangular gate voltage waveform was introduced. This approach allowed for finer control over critical parameters such as rise time, fall time, and pulse width, which are key to TMS pulse optimization. In order to optimize pulse shape, a Matlab model to predict the behavior of coil current based on varying input gate voltage was developed. This model, which is based on a newly derived equation, align well with experimental data and enables the precise customization of pulse characteristics. It provides a valuable frame work for optimizing new and existing pulse shapes. This research demonstrates the supercapacitor-based pulse generator and its accompanying Matlab model for small-scale TMS applications. The flexibility in pulse generation and the ability to create custom pulse shapes provide a strong foundation for future exploration in TMS research, particularly for animal studies.