Direct Measurement of the Electric Field Induced by a Transcranial Magnetic Stimulator

This work proposes a simple method to carry out in the laboratory a direct measurement of the primary solenoidal electric field that transcranial magnetic stimulation (TMS) coils generate in the brain. This method avoids indirect estimation of this electric field from measurements of magnetic fields and, at the same time, overcomes the difficulty posed by the presence of a dominant conservative electric field, also produced by the coil. This conservative electric field, which is removed in practice by human tissue, is eliminated in the proposed measurement method by using a simple setup that does not require the introduction of the measurement probe inside a conducting solution resembling the human tissue. The proposed measurement method allows for measuring the primary solenoidal electric field in front of the coil in the air. This method has been validated by comparing the results of electromagnetic simulation with the measurement of the magnetic field and the nonconservative electric field produced by a commercial TMS coil.