Introduction - Robotized Transcranial Magnetic Stimulation - page 8

to calculate the MT. With the available computer program, the usage is very
convenient and it has become the standard method in TMS research [ 79 ]. How-
ever, as this method, and all other methods mentioned, do not take the strength of
the MEP into account, it might lead to a wrong MT estimate.
1.2 State-of-the-Art: Neuro-Navigated TMS
Different approaches exist to locate the stimulation target and to position the coil.
In its simplest way, localization is based on external anatomical landmarks, e.g.,
midline or ear-to-ear-line. The TMS coil is now placed in relation to these ana-
tomical landmarks.
Commonly, a hot-spot at the M1 as the optimal stimulation site is estimated
prior to actual stimulation. Therefore, different stimulation points are investigated
until the best stimulation outcome, e.g. maximal muscle twitching of a specific
muscle, is obtained. The hot-spot is usually estimated in the M1-HAND area.
Using a function-guided coil positioning procedure, the stimulation site is esti-
mated in relation to this hot-spot. Standard distances are for instance 5 cm anterior
for stimulation of the Dorsolateral prefrontal cortex (DLPFC) [ 26 ], 2-3 cm
anterior for stimulation of the Premotor Cortex (PMC) [ 23 ] or 3 cm posterior for
the Primary Somatosensory Cortex [ 37 ].
Another way is to take advantage of the 10-20 system of electrode placement
for EEG recordings [ 30 ]. The TMS coil can be placed relatively to these electrode
positions [ 27 ]. With use of electrode caps, the 10-20 System is very practical.
However, due to individual anatomical and functional variability, the coil posi-
tioning may lead to errors of a few centimeters—depending on the stimulation site
[ 54 ].
For the proper analysis of TMS effects, exact coil positioning is essential [ 80 ].
A current technique for coil positioning and target localization is therefore the
application of real-time frameless stereotaxic systems [ 43 ]. These neuro-naviga-
tion systems combine high resolution three-dimensional (3D) scans of the patient's
head with a real-time tracking system [ 73 ]. Commonly, Magnetic Resonance
Imaging (MRI) scans of the patient are used as navigation source. From these
scans, the the three-dimensional (3D) head and the brain's anatomy are recon-
structed. After registration and with tracking of TMS coil and head, the TMS coil
can be positioned based on the 3D head scan. Neuro-navigated TMS has become
the state-of-the-art tool for precise target localization in TMS research as it takes
the individual anatomy of the patient into account [ 78 ]. Beside precise target
localization, it also improves the repeatability of coil positioning within and
between TMS sessions [ 43 ]. Furthermore, it was shown that neuro-navigated TMS
also has its value for rTMS treatments of chronic tinnitus [ 40 ] and depression [ 75 ].
Currently, various commercial neuro-navigation TMS systems are available,
e.g. Visor2 TM (Advanced Neuro Technology B.V., Enschede, The Netherlands),
Brainsight TM 2 (Rogue Research Inc., Montreal Quebec, Canada) or NBS System