Biomedical Engineering Reference

In-Depth Information

For analysis of the orientation of the gyrus that is stimulated, we use a trans-

versal plane of the individual MRI images at that level where the foot area is

suggested in the precentral gyrus. We then project the hot-spot from the scalp

surface down to the cortex. At this point, we estimate the angle of the underlying

precentral gyrus to the interhemispheric cleft at the medial wall. This estimation is

done by visual inspection. Due to the fact that there is often no clear direction of

the precental gyrus, we use the bisecting line of the precentral gyrus as the ref-

erence for the angle approximation. The angles are estimated in a blinded fashion

by two examiners and the average gyrus angle estimate of each subject is used. For

further analysis, we compare this angle to the detected optimal coil orientation. We

calculate the correlation between gyrus orientation and optimal coil orientation

and estimate the significance of correlation coefficients r with a t-test on r

q

n
2

1
r
2

,

where n denotes the number of subjects.

3.1.2 Stimulation Outcomes

The stimulator's 'power mode' has been mostly well accepted by the subjects.

However, two subjects have felt inconvenience due to the strong muscle twitching

and the impact on the skin. Therefore, subjects 'Ti' and 'Pa' only participated in

one of the two sessions. For subject 'La' we have performed session 2 twice ('La1'

and 'La2'). As for both trials no minimum was found, subject 'La' was excluded

from further analysis.

The estimated hot-spots are located close to midline at the medial lip of the

precentral gyrus for all subjects and for both sessions.

Figure
3.2
illustrates the motor thresholds with respect to the coil orientation

for sessions 1 and 2 for all subjects. Note that all the curves are monotonic. In all

sessions the MT minimum is between 20

and 50

clockwise from the reference

coil orientations at 0
and 180
.

Averaging the MTs for all subjects at each orientation,the minimum for session 2

is at 210
with 53
:
8
17
:
7 % of MSO which is 30
clockwise to the standard right-

to-left coil orientation. The mean MT at the standard orientation is 57
:
6
16
:
0 %.

For the opposite coil orientation (coil handle towards the left hemisphere), the local

minimum is at 30
with 54
:
6
14
:
9 % of MSO. The average MT at 0
is 66

19
:
4 % of MSO. The optimal coil orientation shows significant differences to the

standard coil orientation (p
¼
0
:
0014 and p
¼
0
:
0011 for sessions 1 and 2,

respectively). Figure
3.3
shows the average MTs as a polar plot. In this figure the

two opposite minima are clearly demonstrated. Both minima are rotated 30

clockwise from the reference orientation. Table
3.1
summarizes the mean motor

thresholds with the standard deviations for all coil orientations for sessions 1 and 2.

The average coil orientation for the minimum threshold is 33
:
3
with Standard

Deviation (SD) of 12
:
1
for session 1, and 213
:
1
with SD of 18
:
3
for session 2.

The mean MT difference between optimal orientation and reference orientation is

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