Biomedical Engineering Reference
In-Depth Information
Chapter 1
Introduction
1.1 Transcranial Magnetic Stimulation
The idea of Transcranial Magnetic Stimulation (TMS) for non-invasive brain
stimulation is simple but brilliant: A strong, rapidly increasing current is driven
through a magnetic coil placed on the head of a subject. The generated magnetic
field passes through the human skull and—due to Electromagnetic Induction—
induces an electric current inside the cortex which can lead to local stimulation
[
17
]. However, it took almost a century, using huge magnets around the head in the
very beginning, until Anthony Barker successfully introduced TMS in 1985 [
5
]. At
this early stage, only muscle contractions could be clearly observed when stimu-
lating the motor cortex.
Nowadays, TMS has not only become an important tool in clinical routine, but
particularly repetitive Transcranial Magnetic Stimulation (rTMS) is a promising
tool for treatment of a variety of medication resistant neurological and psycho-
logical conditions.
Moreover, for (cognitive) neuroscience and brain research, TMS is a key
technique to study the brain's functionality and connectivity. In general, TMS is
applied for non-invasive and painless cortical brain stimulation.
1.1.1 Principle of TMS
During TMS, cortical neurons are activated by a current distribution that is
induced by a transient magnetic field. This time-varying magnetic field is gener-
ated by a short high-current pulse (4-20 kA) sent through a stimulation coil
located on the scalp of the individual. The magnetic stimulator itself generates
high voltages of 400-3,000 V. The resulting magnetic field lasts for a few milli-
seconds and can reach peak strengths of 1-10 T [
24
,
29
]. This magnetic field
passes easily through the human skull and induces a current density distribution
that is characterized by a direction and magnitude that both vary within the cortex
[
71
]. These quantities are determined by the coil position and geometry, and by the
