Mapping transcranial magnetic stimulation (TMS) fields in vivo with MRI

Transcranial magnetic stimulation (TMS) is a non-invasive technique for investigating brain function that uses pulsed magnetic fields created by special coils to induce localized neuronal depolarization. Despite the technique's expanding application, the exact magnetic field produced by TMS coils have never been directly measured in human subjects. Using a standard 1.5T MR scanner and TMS coils constructed from non magnetic materials, we have obtained 3D maps of the magnetic field created by TMS coils in human volunteers. Further, we mapped the combined field of two coils and demonstrated that combinations of coils might be used to focus the magnetic field to achieve improved stimulation patterns and, perhaps, reach areas out of reach of single coils.