Analysis of 16 Coils Over 50 MRI-Derived Head Models in Transcranial Magnetic Stimulation

Transcranial magnetic stimulation (TMS) is a non-invasive neuromodulation technique investigated for the treatment of various neuropsychiatric disorders such as major depressive disorder and obsessive-compulsive disorder. Coil design and analysis has been a popular topic in TMS studies, primarily as the coil geometry significantly affects the induced electric field (
$E$
-field) distribution. In this work, we compare the distribution of the induced
$E$
-field of 16 different TMS coils by finite element analysis. The coils are positioned at two head locations: the vertex and the dorsolateral prefrontal cortex (DLPFC). Due to the coil geometry, only nine of the 16 coils are positioned at the DLPFC. Notably, the authors use 50 heterogeneous head models derived from MRI scans of healthy patients in this analysis. As a result, the sensitivity of the
$E$
-field intensity and focality to anatomical variations is investigated. The maximum
$E$
-field intensity induced in the brain (
$E$
-Max brain), and the volume in the brain exposed to the
$E$
-field with at least half the
$E$
-Max (
$V$
-Half) were metrics used to assess the intensity and focality of the induced
$E$
-field. It was observed that some of the coils induced high
$E$
-field intensity and were highly sensitive to anatomical variations. Other coils exhibited high focality and lower sensitivity to anatomical variations at the same time. For the coils positioned at the DLPFC, less variability was observed when compared to the vertex location. This study provides an understanding of the effect of coil geometry and anatomical variation on the induced
$E$
-field during TMS.