Biomedical Engineering Reference
In-Depth Information
2.4 Consequences
Even though the magnitude of head motion over time is relatively small, less than
20 mm and less than 6 after 30 min, the impact on the induced electric field is
very strong when using hold-and-restrain or hold-and-rest. After 10 min the
intensity of the induced electric field in the cortex is 14-18 % lower than the initial
value. After 30 min this is even worse when head motion results in a reduction of
the field magnitude of approximately 32 and 20 %, respectively. Besides a
decrease in the induced electric field strength, the orientation of the field changed
up to 8.6 and 10.6 , respectively.
Note that the change in electric field orientation is larger than the actual head
rotation. Due to the translational head motion, the target point moves from the
center of the coil. As the coil's electric field is composed from two ringlike electric
fields [ 21 ], a shift of the target point also results in a change of the electric field
direction. This relationship is illustrated in Fig. 2.10 . As there is an optimal
direction of induced currents in the brain [ 3 , 10 ], a stable orientation is of crucial
importance for comparable stimulation outcomes.
The robotized TMS system has a latency of approximately 200-300 m/s and
therefore cannot follow head movements instantaneously (cf. Sect. ) . For
continuous motion, this latency would be problematic [ 5 , 11 ]. In this case, we can
expect a constant error. For TMS, however, only spontaneous motion is likely as
supported by the recorded head motion. Our results for the robotized scenarios
indicate that the effect of the system latency can be neglected for TMS. On
average, the electric field intensity decreases \4 % due to mispositioning. Also,
the orientation of the induced electric field maintains constant (r\0 : 5 ). More
interestingly, this is also true when head motion is not restricted (move-freely).
Thus, use of a head rest or asking the patient not to move the head is not required
for the robotized system to maintain stimulation accuracy. Robot-and-restrain and
Fig. 2.10 a Schematic view of the induced electric field (grey arrows) produced by a standard
figure-of-eight coil (cf. [ 21 ]). Due to head motion the target point P (black circle) shifts to P 0
(dark grey circle). The black arrows denote the induced electric field for both points. b Zoomed
view: A translational shift t of the target point P to P 0
also results in a change of the electric field
direction with an angle a due to the coil's curvature
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