TMS: Biophysics

In order to understand how TMS works, it's important to know a little bit about the biophysics underlying the technology. The earth is surrounded by a magnetic field which shields us from harmful cosmic rays. The magnetic field strength of TMS is 30,000 times greater than the earth's magnetic field and is about the same strength as a standard MRI machine. But you could lie in one these all day and you wouldn't be very happy.


Magnetic resonance imaging uses a static or unchanging magnetic field. TMS uses what is called a time-varying magnetic field, first discovered by the British physicist, Sir Michael Faraday, in 1839. The question Faraday asked was, "If electricity can create magnetism, why can't magnetism create electricity?" The answer is: it can--as long as the magnetic field is moving. 


So we could move the magnet, but an easier and more effective way of generating this kind of magnetic field is to rapidly turn an electrical current on and off. This current flows through a thick copper wire tightly coiled around a piece of solid iron, which becomes a magnet every time the current flows through the coil. In the most common form of TMS used to treat depression, this happens 10 times every second. So what we're really talking about is rTMS--repetitive transcranial stimulation--as opposed to single pule or paired pulse TMS which are used in research.


rTMS can be fast or slow. In slow rTMS the magnetic pulses come at a rate of one every second. Anything more rapid is considered to be fast rTMS. The speed of the magnetic pulses determines how TMS affects the brain. Slow pulses decrease the excitability of brain cells while fast pulses increase the excitability of brain cells and can excite them so much that they continue to fire even long after the magnetic stimulation ceases. This is the basic underlying mechanism by which TMS acts to treat depression.