I wanted to check the timing of a TMS-protocol using an external trigger, and sometimes i want to demonstrate the current induction in class. MagVenture sells the MagProbe, to be plugged into BNC or DIN. This would allow you to measure the magnetic field induced by the TMS. They also offer a 3-D version, and obviously produce at high standards. Yet, i was cheap, and had several old, used BNC cables and a metal spring lying around. You can buy a BNC for around 2 EUR, and if you can not find an old spring, just drill a wire around a pen.

For construction, i first removed the insulation from the BNC and identified ground and lead of the BNC. The spring was then soldered at opposite ends to ground and lead of the BNC. That’s it. Electrical engineering work done, and it looks cool 😀

I tested it out using our MagVenture X100 and my 50 MHz  Rigol DS1052E.  I put the probe roughly in the middle of the coil, cranked the stimulator to around 30% MSO and triggered a TMS pulse. I set the scope to 200 mV and 100µs, and played with the scope trigger level until everything was fine – but your settings might vary depending on your probe. I tried out a biphasic pulse and a biphasic burst with 0.5 ms IPI. Pulse width looks fine (roughly the expected 280µs). IPI appears to be off slightly (around 550 µs instead of the expected 500µs), but this can most likely be explained because the induced current is the temporal derivative of the magnetic field, resulting in the peak to be shifted. Maybe if one adds an integrator circuit and send the output to a second BNC? Anyway, that’d be another project and results already look great as they are!

Put Shortly: For a very low cost, you can construct your own TMS probe, which will be sufficient for class demonstration and/or to check triggering and timing of your protocol.

P.S: I originally bought the Rigol as according to EEVBlog, it can apparently be hacked to a 100 MHz Oscilloscope, but i did not yet came around to try that. Regardless, 50 MHz is enough bandwidth to check the timing of your TMS, and afaik more than most electrophysiological amplifiers offer. If you do not want to afford a real oscilloscope, consider the DPscope or the Analog Discovery.