The Importance of Robotized TMS: Stability of Induced Electric Fields - Robotized Transcranial Magnetic Stimulation - page 40

robot-and-rest lead only to a very slight further improvement in stimulation
accuracy.
The recorded head motions in our study were taken from young, healthy and
cooperative volunteers. This may be a factor for the relative small movements. For
patients suffering from neurological or psychiatric diseases, however, we can
expect larger head movements. In this case, the influence on the induced electric
field and therefore on the stimulation outcome might be even worse for hold-and-
restrain and hold-and-rest. For the robotized TMS system, in contrast, the results
indicate that active motion compensation will also be capable for larger head
movements.
For TMS, as a focal brain stimulation technique, target identification is essential
[ 8 ]. In recent years a lot of effort was made to find optimal stimulation parameters
for best stimulation outcomes when using repetitive Transcranial Magnetic
Stimulation (rTMS) for treatment of different neurological or psychiatric condi-
tions. As an example, recent studies used Positron Emission Tomography (PET)
[ 13 , 14 ] or functional Magnetic Resonance Imaging (fMRI) [ 9 ] to detect the
stimulation target in the Primary Auditory Cortex (PAC) for the treatment of
chronic tinnitus. Furthermore, various studies were conducted using different
frequency or stimulation intensity settings. Also, the stimulation duration and the
number of total stimulation pulses differed [ 7 - 9 ].
For targeting the PAC, the cortical target region is relatively small. On average,
its cortical length and width are 9.8 and 6.0 mm, respectively, [ 1 ]. When using a
coil holder with motion avoidance or with a head rest for rTMS (move-and-
restrain and move-and-rest), after 10 min the total translational head movement
can be larger than 10 mm. Hence, the stimulation focus will move out of PAC and
the selected cortical target region will only be stimulated partially. Most of the
induced electric field will be delivered to a neighboring brain region. Therefore,
after 10 min of stimulation, the stimulation will not be as effective as in the
beginning. Note that the target region also has roughly the same size for most other
TMS applications.
However, any compensation of the loss in induced electric field strength by
increasing the stimulation intensity is prohibited as this can lead to undesired
effects and is potentially dangerous for the patient [ 18 ].
In contrast, the robotized TMS system with active motion compensation
maintains the stimulation strength and orientation in the target region as planned.
Hence, advanced navigated treatments based on image planning location and
direction can be performed.

2.5 Derived Requirements for Robotized TMS

As shown in this chapter, robotized TMS with motion compensation allows to
maintain accurate stimulation throughout the application. However, to compensate
for

a maximum of head movements, the robotic system must fulfill some