Commercially available transcranial magnetic stimulation (TMS) devices induce electric field (E-field) pulses with damped cosine shape and provide very limited control over the pulse parameters. Furthermore, conventional TMS devices consisting of a single power stage cannot produce rapid bursts (<10 ms inter pulse interval) with equal or increasing strength since the energy on the capacitor diminishes from pulse to pulse due to circuit losses, and the time between the pulses is too short for the capacitor charger to supply the lost charge. Consequently, conventional TMS topologies require combining the output of multiple power stages to generate paradigms such as paired-pulse and quadripulse stimulation. Addressing these limitations we have developed a third generation controllable pulse parameter TMS device (cTMS3) that allows extensive adjustment of the pulse parameters such as the pulse width, number and shape of phases, and directionality (ratio of positive to negative phase amplitude), as well as enables the generation of powerful pulse bursts.
To summarize the cTMS3 device capabilities and compare it to conventional TMS and other cTMS devices.
Materials and methods
The coil current and electric field were measured with a Rogowski current sensor and a search coil, respectively.
cTMS3 uses a circuit topology with two energy storage capacitors and four controllable switches. It can generate rapid-rate (⩽50 Hz) trains of mono/bi/polyphasic near-rectangular electric field pulses with adjustable amplitude (maximum capacitor voltages of 1 kV and 2.6 kV), pulse width (phase duration of 10 μs to over 200 μs), and directionality (positive/negative phase amplitude ratio range of 1–52). In contrast to earlier cTMS devices, in cTMS3 the pulse phases can assume 4 possible electric field levels, proportional to either of the two capacitor voltages, the capacitor voltage difference, or 0. Thus, cTMS3 can produce waveforms with a staircase shape that can approximate conventional sinusoidal current pulses ( Fig. 1 ). To counteract the loss of capacitor charge in rapid pulse bursts, the pulse width of the second and subsequent pulses can be increased, allowing their stimulation strength to be equal or even higher than the first pulse ( Fig. 2 ). In Fig. 2 , the voltage on the two energy storage capacitors is reduced by 2% and 17%, respectively, after the first pulse, but the second pulse is 67% longer than the first pulse. Consequently, the strength of the second pulse, as measured by modeled neural membrane depolarization (membrane time constant = 196 μs) is 45% higher than the first pulse. Thus, paired-pulse paradigms, where the second pulse can have higher strength than the first pulse, can be implemented. Similarly, increasing the pulse width in a rapid burst of pulses can enable quadripulse stimulation.
cTMS3 offers flexibility of pulse shape adjustment unparalleled in other available TMS devices. The combination of high energy efficiency of the near rectangular pulses, pulse energy recovery, and pulse width control allow the generation of rapid bursts of pulses with equal or even increasing strength that can implement paired-pulse and quadripulse stimulation. These features could extend the capabilities of TMS as a research and diagnostic tool, and could enable optimization of the stimulus in therapeutic interventions.