Transcranial magnetic stimulation: comparison of 15 coils with 50 MRI derived head models

Transcranial Magnetic Stimulation (TMS) has been gaining popularity for the treatment of Major Depressive Disorder (MDD) due to its non-invasive, painless and out-patient nature[1]. It also under investigation as a therapeutic tool for treating other psychiatric and neurological disorders such migraines, Schizophrenia, Parkinson's disease, and Post Traumatic Stress Disorder (PTSD). TMS is based on the principal of Faraday's law in which a large amount of current is passed through coils to generate time varying magnetic fields, thus inducing electric fields in the brain. There are several coils commercially available for both clinical and research studies which provide stimulation profiles that are characteristically unique in both stimulation intensity and focality. Understanding how stimulation characteristics differ between coils is important for researchers and clinicians as they attempt to choose a coil whose stimulation profile is best for a specific use case. Differences in coils have been characterized in detail in both spherical models, and in a single heterogeneous head model, but never with a population-based modeling approach [2], [3]. It is well known within the TMS community that anatomical variability, including brain-scalp distance, cerebrospinal fluid, and gyral anatomy, exists between individuals and thus contributes to varying stimulation profiles[4–6]. Population-based modeling approaches that utilize many head models, can illustrate the effects of this anatomical variability in order to see group average stimulation characteristics. Here, we use population-based modeling with 50 unique, heterogeneous head models to calculate the induced electric field in TMS from 15 coil designs. The models used in this study were taken from the Population Head Model repository [5], and contain anatomical components for skin, skull, cerebrospinal fluid, grey matter, white matter, cerebellum, and ventricles. Simulations utilizing low-frequency magneto-quasi-static solvers were used in SEMCAD X to calculate the induced electric field from various coil designs. Parameters were calculated in post-processing describing stimulation intensity (both in brain and scalp), spread (both surface and volumetric), and “hot spot” location are used to illustrate similarities and differences between these coils. Figure 1 illustrates four coils positioned on the vertex of a MRI derived head model. In this study, simulations were ranboth over the vertex and dorsolateral prefrontal cortex for all subjects and all coil designs to give perspective from two different locations of stimulation. The results described in this study will serve as a guide for both researchers and clinicians wanting to understand the differences between coils stimulation profiles.