Closing Remarks - Robotized Transcranial Magnetic Stimulation - page 164

10.2.4 Double-Coil Robotized TMS
To evaluate the functional connectivity of the human brain, TMS protocols using
two (focal) coils are well suited. Hence, the functional connectivity of Primary
Motor Cortex (M1) to other cortical brain areas can be studied with a high tem-
poral resolution. To this end, a conditioning TMS pulse is applied to a brain area
which is subsequently followed by a second pulse to M1. Now, the MEP in the
corresponding muscle can be recorded. In this way, the impact of other brain areas
on M1 can be studied by comparing the resulting MEP. See [ 4 ] for an introduction
to double-coil TMS.
As the inter pulse interval between conditioning and test pulse is typically in the
range of a few milliseconds, two coils must be used and placed in parallel on the
subject's head. For meaningful and comparable results, accurate coil placement on
both targets is essential. However, we know already that high positioning accuracy
with a single coil—even with neuro-navigation—is hard to achieve. Accurate
positioning of two coils simultaneously is even more challenging. Commonly,
these stimulations last for several minutes [ 8 ]. Therefore, head motion must also be
considered in these studies.
For precise simultaneous targeting of two TMS coils on a subject's head, an
advanced robotized TMS system might be very helpful. Clearly, the use of two
independent robotized TMS systems does not work because both robots will
interfere with one another. Therefore, an advanced control setup is required which
takes the position and size of both coils and robots, and—most importantly—of the
patient's head into account. Hence, trajectory planning and motion compensation
must be advanced to guarantee collision avoidance. However, from an engineering
or robotics point of view this is a challenging task which might lead to a better
understanding of the brain's connectivity and interaction, and the functionality of
TMS in the brain.
10.2.5 Robotized Interleaved TMS/fMRI
Functional Magnetic Resonance Imaging (fMRI) measures changes of the blood
oxygen level (called Blood oxygenation level dependent (BOLD) effect) inside the
brain with a relatively high spatial resolution. This change can be related to
neuronal activity. The duration of a whole brain image is in the range of a few
seconds, see [ 9 ] for an introduction to fMRI.
Almost a decade ago, it was shown that TMS can be applied inside an MR
scanner and that the BOLD activation evoked by the TMS pulse could be mea-
sured with fMRI [ 5 ]. As both, MRI and TMS, produce magnetic fields, application
of TMS during an MRI image acquisition leads to strong artifacts in the image.
Therefore, interleaved TMS/fMRI is used: TMS and fMRI are synchronized such
that a TMS pulse or train of pulses is only given when no fMRI image is recorded.