The Importance of Robotized TMS: Stability of Induced Electric Fields - Robotized Transcranial Magnetic Stimulation - page 26

Chapter 2
The Importance of Robotized TMS:
Stability of Induced Electric Fields
From an engineering point of view, robotic Transcranial Magnetic Stimulation
(TMS) outperforms hand-held TMS in terms of accuracy, reproducibility and
repeatability. However, from a clinical/neuroscience point of view, stability and
comparability of the stimulation outcomes are more important. Due to the neuronal
effects and the dimensions of the magnetic field produced by the TMS coil, we
cannot conclude that improved coil positioning is directly linked to better stimu-
lation outcomes.
The reasons for a TMS treatment success are manifold and yet not fully
understood. Many different—partially unknown—parameters influence the success
of the treatment. A key factor is the stimulation accuracy throughout the treatment.
Due to head motion the focus of the TMS coil may move during treatment and
therefore the accuracy may decrease. However, measuring the accuracy of TMS
in vivo is more than difficult. Merely, motor cortex stimulation results in a
quantitative detectable activity. Using Electromyography (EMG), motor evoked
potentials can be measured for a target muscle. However, the variance in the Motor
Evoked Potential (MEP) amplitude is quite large [ 24 ]. Hence, the accuracy of
TMS in general cannot be derived from the MEP amplitude.
To study the actual impact of motion on TMS, and to evaluate the effectiveness
of different approaches to handle motion, we investigate different scenarios to
perform TMS and compare the time-dependent stability of induced electric fields.
First, we propose to assess the end-to-end effect of motion based on measurement
of the actual electric field. Second, we describe three different treatment scenarios
and our setup to measure head motion. Third, we study a number of recorded
motion traces and establish the actual effect on TMS. Finally, we discuss our
results, which indicate that active motion compensation using a robotized TMS
system provides superior accuracy with respect to magnitude and orientation of the
electric field.
Parts of this section have been already presented in [ 16 , 17 ].