Brief definition of TMS

Transcranial magnetic stimulation (TMS) is a noninvasive, intermittent stimulation technique for exciting neurons in the brain, mainly the cortex to a depth of several centimeters, with potential clinical value for neuromodulation, i.e. controlled electrical stimulation of the nervous system. Weak electric currents are induced in nerve cells by a rapidly cycling magnetic field via a coil placed on the subject’s head. This technique has a variety of applications in neuroscience research for understanding the circuitry in the brain, as well as therapeutic and diagnostic applications. A pulse generator creates a pulse with an amplitude of 5,000 amperes or greater and a cycle period on the order of one millisecond. The current pulses are passed through a coil placed on the scalp. The rapidly changing current creates a strong focal magnetic field (one to two tesla, which is comparable to that of clinical MRI scanners, though more focused). The rapidly alternating magnetic field that results from the rapidly changing current induces electrical currents intracranially. By placing two TMS coils in a figure-eight manner, it is possible to achieve a focused electrical stimulation of the cortex underlying the center of the figure-eight coil. Figure 1 shows an MRI image of a subject with a figure-eight coil above the left hemisphere motor cortex and the resulting magnetic field (black lines) that is generated by the TMS device.

Figure 1

Greater clinical effect from TMS can be achieved by rapidly repeating the stimulations (up to 60Hz) for several seconds. This is called repetitive transcranial magnetic stimulation (rTMS), which is a technique in which effects last longer than the period of stimulation.