TMS Intensity and Focality Correlation with Coil Orientation at Three Non-Motor Regions
Authors: Gomez-Feria Jose , Fernandez-Corazza Mariano , Martin Rodriguez Juan Francisco , Mir Pablo 
Published: August 2021

Objective: The aim of this study is
to define the best transcranial magnetic stimulation (TMS) coil orientations
for three clinically relevant brain areas (pre-supplementary motor area,
inferior frontal gyrus, and posterior parietal cortex) by means of simulations in
12 realistic head models of current density. Methods: We computed the current
densities generated by TMS in our three volumes of interest (VOI) that were
delineated based on published atlases. We then analyzed the maximum intensity
and spatial focality for the normal and absolute components of the current
density considering different percentile thresholds. Lastly, we correlated
these results with the different anatomical properties of our VOIs. Results:
Overall, the spatial focality of the current density for the three VOIs varied
depending on the coil’s orientation. There was major interindividual
variability, and there was therefore no specific orientation that produced the
best focality for all subjects. Further analysis showed that the differences in
individual brain anatomy were related to the amount of focality achieved. In
general, a larger percentage of sulci resulted in larger spatial focality.
Moreover, larger normal current density intensity was achieved when positioning
the coil axis perpendicular to the predominant orientations of the gyri of each
’s VOI. Conclusion: For a rough approximation, better coil orientations can be
based on the individual’s specific brain morphology at the VOI. Moreover, TMS
computational models should be employed to obtain better coil orientations in
non-motor regions of interest. Significance: Finding better coil orientations
in non-motor regions is a challenge in TMS and seeks to reduce interindividual
variability. Our individualized TMS simulation pipeline leads to lesser
interindividual variability in the focality, likely enhancing the efficacy of
the stimulation and reducing the risk of stimulating adjacent, nontargeted
areas.

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