Transcranial Magnetic Stimulation: Introduction and Technical Aspects

When the transcranial magnetic stimulation (TMS) emerged in 1985, it represented a novel research tool for studying the functionality, morphology and connectivity of various cortical regions, especially the motor cortex. TMS is a technique for non-invasive stimulation of neuronal structures. Magnetic pulses are administered by passing a strong current through an electromagnetic coil placed upon the scalp that induces an electric field and, therefore, current in the underlying cortical tissue. The TMS stimulation circuit consists of a high-voltage power supply that charges a capacitor or a bank of capacitors, which are then rapidly discharged via a fast electronic switch into the TMS coil, to create the briefly changing magnetic field pulse. The TMS coil controls the distribution and strength of the stimulation field and therefore plays a crucial role in the characteristics of TMS. The most important trade-off in their design is the interplay between depth and focality.