Transcranial magnetic stimulation

The skull is a poor conductor and an excellent obstacle. Current applied at the scalp spreads through it and mostly misses the target. A changing magnetic field, however, does not care about the skull at all — bone is essentially transparent to it, and the induced electric field appears directly in the tissue underneath.

What it is

A capacitor bank, some hundreds of microfarads charged to a couple of kilovolts, discharged through a coil held against the scalp by a thyristor or IGBT. Peak currents run to several kiloamps over a hundred microseconds or so. By Faraday’s law the collapsing field induces an electric field in the cortex, and where that field’s gradient along an axon is steep enough, the axon fires.

Why it matters

TMS made it possible to intervene in a specific piece of human cortex without surgery, which is a genuinely rare capability. Its clinical use in depression follows from that, as does a large fraction of what is known about human motor cortex, since a coil over the motor strip produces a twitch you can measure at the hand.

The interesting engineering constraint is that the pulse shape has, historically, not been a design choice. It is whatever the LC ringdown of the capacitor and coil produces — typically a damped cosine, because that is what the circuit does, not because anyone chose it for neural efficiency. The energy stored per pulse is

$$E = \tfrac{1}{2} C V^2$$

and nearly all of it becomes heat in the coil. Coil heating, not biology, is what limits how long a session can run.

That gap between the waveform the circuit produces and the waveform a neuron would prefer is where power electronics has something to offer, and it is the subject of my own work on flexible pulse shaping.