Controllable pulse parameter transcranial magnetic stimulator with enhanced circuit topology and pulse shaping

Abstract

Objective: This work aims at flexible and practical pulse parameter control in transcranial magnetic stimulation (TMS), which is currently very limited in commercial devices.

Approach: We present a third generation controllable pulse parameter device (cTMS3) that uses a novel circuit topology with two energy-storage capacitors. It incorporates several implementation and functionality advantages over conventional TMS devices and other devices with advanced pulse shape control. cTMS3 generates lower internal voltage differences and is implemented with transistors with a lower voltage rating than prior cTMS devices.

Main results: cTMS3 provides more flexible pulse shaping since the circuit topology allows four coil-voltage levels during a pulse, including approximately zero voltage. The near-zero coil voltage enables snubbing of the ringing at the end of the pulse without the need for a separate active snubber circuit. cTMS3 can generate powerful rapid pulse sequences (< 10 ms inter pulse interval) by increasing the width of each subsequent pulse and utilizing the large capacitor energy storage, allowing the implementation of paradigms such as paired-pulse and quadripulse TMS with a single pulse generation circuit. cTMS3 can also generate theta (50 Hz) burst stimulation with predominantly unidirectional electric field pulses. The cTMS3 device functionality and output strength are illustrated with electrical output measurements as well as a study of the effect of pulse width and polarity on the active motor threshold in ten healthy volunteers.

Significance: The cTMS3 features could extend the utility of TMS as a research, diagnostic, and therapeutic tool.

Figure 1

Diagram of key elements of the cTMS3 circuit. (top) Two half-bridges connected to energy storage capacitors (i = 1,2) drive differentially the stimulation coil L. (bottom) Each half-bridge, Q_i1/D_i1 − Q_i2/D_i2, is connected to an energy-storage capacitor C_i1 and charge control module I_i. Passive snubber circuits are shown in shaded blocks.

Figure 2

Measured active snubbing at the end of a positive monophasic TMS pulse with peak coil current of I_L = 3 kA. Ideally, I_L should decay to zero and remain there. However, ringing occurs between the coil and the switch capacitive snubbers. To dampen these oscillations, transistor Q₁₂ is switched with a period of 5 µs and various duty ratios d. (a) Coil current I_L and (b) corresponding induced electric field strength E are shown for a d range of 0.4–0.8 [d values printed over the traces in (a)]. For d > 0.625, the response transitions from underdamped to overdamped.

Figure 3

Motor threshold measurement in 10 human subjects as a function of pulse width and current direction. (a) Measured electric field waveform E of monophasic magnetic pulses with V_C21 = 0.2V_C11 and positive phase widths of 30, 60, and 120 µs. (b) Active motor thresholds for the three pulse widths with posterior–anterior and anterior–posterior direction of the initial phase of the induced current. Bars give mean values and whiskers give standard deviation. For comparison, the active motor threshold with a conventional monophasic TMS device (Magstim 200) is 1044±252 V [32].

Figure 4

Measured waveforms of (a) coil current I_L and (b) electric field E for a synthesized staircase pulse illustrating the four E levels that can be achieved within each pulse. Coil voltages corresponding to each E level are indicated in (b). Initial energy-storage capacitor voltages are V_C11 = 1430 V and V_C21 = 0.5V_C11.

Figure 5

(a) Measured electric field waveform E and (b) estimated neural membrane depolarization ΔV_m for a pulse approximating minimum energy pulses [34]. The initial capacitor voltages are V_C11 = 816 V and V_C21 = 1.1V_C11. The duration of the initial negative E phase is 100 µs and the width of the second, positive phase is either 40 µs (blue curves) or 54 µs (red curves).

Figure 6

Pair of biphasic pulses delivered with short interstimulus interval (3 ms). The plots show measurements of (a) coil current I_L and (b) electric field E, as well as (c) modeled neural membrane depolarization ΔV_m. The parameters of the first pulse are V_C11 = 1196 V, V_C21 = 0.22V_C11, and positive E phase width of 60 µs. In the second pulse, the positive E phase width is increased to 100 µs, resulting in larger membrane depolarization despite the reduced E amplitude.

Figure 7

Four pulses separated by inter stimulus interval of 2 ms forming a quadripulse burst [36]. The plots show (a) measured electric field waveform E (filtered with 3-point median filter to illustrate clearly pulse amplitude by removing switching spikes) and (b) estimated neural membrane depolarization ΔV_m. The initial capacitor voltages are V_C11 = 1230 V and V_C21 =0.22V_C11, and the duration of the first, negative phase of the electric field pulses is fixed at 200 µs. In order to compensate for the reduction of charge on the capacitors due to energy loss and the consequent reduction of pulse amplitude, the duration of the second, positive electric field phase is increased for each subsequent pulse, having values of 67, 69, 73, and 78 µs, respectively.