Introduction - Robotized Transcranial Magnetic Stimulation - page 5

Fig. 1.3 Electric fields of two typical TMS coils measured by a field sensor. The measures are
displayed in mV as induced in the sensor. a The field of the circular coil is rotational symmetric.
b The field of the figure-of-eight coil produces a focal area underneath the center. Figures from
[ 48 ], with friendly permission of the author and the copyright holder, 2008 John Wiley and
Sons
cortical region is affected by the stimulation. Figure-of-eight (figure-8) or butterfly
coils produce a more focal magnetic field. They consist of two circular coils that
are located in parallel. Below the intersection of the two circles, the induced
electrical field of both circles is added. Therefore, the induced electric field has a
focus beneath the center of the intersection which is typically the center of the coil.
Figure-of-eight coils are thus used for (more) focal brain stimulation. Figure 1.3
shows the spatial electric field distribution of a typical circular and a figure-of-
eight coil.
Main applications of figure-of-eight coils are in rTMS experimental treatment
and brain research. Circular coils are primarily used in clinical diagnosis and
routine as they have a broad focus. Producing stimulation effects with these coils is
thus easier than with figure-of-eight coils.
However, the magnetic field properties of both coil types are specified by the
number and the diameter of the coil windings. In general, a smaller coil diameter
results in a more focal electric field but has the drawback of faster coil heating. In
contrast, larger coil windings lead to a better depth penetration. By increasing the
number of windings also the magnetic field strength increases but at the cost of
faster coil heating.
However, due to the general behavior of magnetic fields, the strength of the
induced electric field decreases quasi-exponentially with depth [ 8 , 22 ].
Besides these two common coil types, specialized coil designs have been
introduced and investigated. The H-coil, for instance, produces a higher induced
electric field compared to circular coils [ 68 ]. Therefore, it is intended for stimu-
lation of deeper brain regions [ 69 ]. However, deeper regions cannot be targeted
without stimulating superficial brain structures [ 16 ].