Biomedical Engineering Reference
In-Depth Information
Threshold MT as a function of orientation that are available in the literature are not
very precise also with steps at least as large as 45 for muscles in the face and the
hand [ 4 , 16 ]. Beside the MT, the MEP amplitude [ 6 ] or latency have been
investigated [ 23 , 33 ]. Additionally, brain mapping with different current directions
has been studied [ 18 , 20 ]. However, optimal directions have then been inferred
from fits of sinusoids to the results. In particular, Balslev et al., as the most recent
study, have reported a high inter-individual variability of 63 in optimal coil
orientation [ 4 ]. Therefore, the precise measurement of optimal coil orientation is
important to obtain reliable results.
Fox et al. have proposed a cortical column cosine (C 3 ) model that calculates the
effective electric field based on the cortical orientation in relation to the absolute
electric field [ 10 ]. In particular, the model supports that the effect of coil orien-
tation observed by Brasil-Neto et al. and Mills et al. is important for the interaction
of TMS with the cortex [ 6 , 16 ]. Simulations show that for identical coil currents,
the magnitude of the induced current in the brain critically depends on the ori-
entation of the coil relative to gyri and sulci, without any reference to the con-
figuration of any neuron [ 19 , 30 ]. Furthermore, it has been hypothesized that
neurons are stimulated only if their axons curve away from the current induced in
the tissue [ 10 ]. For the test of both models coil orientation is important.
In this section, we show that the current standard coil orientation for stimulation
of the foot is not optimal. Rather, the optimal coil orientation for stimulation is
almost equal to the standard coil orientation of the hand area. We thus conclude
that the orientation of the precentral gyrus is the key factor for the best coil
orientation. We use structural MRI images to support this conclusion. To this end,
we use the robotized TMS system to precisely rotate the TMS coil. As the MEP
amplitude is greatly variable [ 34 ], we measure the MT instead. Furthermore, we
ensure that the coil maintains tangential orientation to the scalp by using the
robotized system.
3.1.1 Experimental Realization Setup
We use an MC-B70 Butterfly coil with a slight bend and the MagPro X100
stimulator with MagOption (MagVenture A/S, Farum, Denmark) for focused
biphasic stimulation. To reach sufficiently high stimulation intensity the 'power
mode' of the device is used, which allows a 1 : 4 times higher stimulation power
compared to standard mode. Recording of MEPs are accomplished using a 2-
channel DanTec Keypoint Portable (Alpine Biomed Aps, Skovlunde, Denmark)
with surface electrodes. For placing and holding the coil precisely, we use the
robotized TMS system ( Sect. ). The MTs are estimated with a computer
program that provides stimulator outputs as a result to reactions to stimulations
with previous stimulator outputs [ 1 - 3 , 17 ]. Reactions are classified as successful if
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