Investigational Effect of Brain-Scalp Distance on the Efficacy of Transcranial Magnetic Stimulation Treatment in Depression

Transcranial magnetic stimulation (TMS) is a tool for neuromodulation that uses a time-varying magnetic field to stimulate the brain via an induced electric field (E-Field). Clinical data from Premier Psychiatric Research Institute on repetitive TMS (rTMS) for the treatment of depression shows that patients older than 55 years respond to deep rTMS with better efficacy than younger patients. Brain volume shrinks with age in adults older than 40 years, increasing the distance between their brain and scalp. In this paper, we have investigated how the induced E-Field from TMS is affected by brain-scalp distance (BSD). We have developed heterogeneous head models from MRI data of 50 subjects and created simplified head models using concentric spheres. Using finite-element analysis, we simulated TMS on these models to test the role of BSD in determining the strength and distribution of the E-Field.