Comparison of Current Distributions in the Head in Electroconvulsive Therapy and Transcranial Magnetic Stimulation

INTRODUCTION Electroconvulsive therapy (ECT), in which electric currents are applied to the brain, improves severe mental illnesses such as depression [1]. Transcranial magnetic stimulation (TMS) is a method to stimulate neurons using eddy currents generated by pulsed magnetic fields [2,3]. Because TMS has a potential to give a comparable therapeutic effect to ECT with less invasiveness, TMS has been used for treatments of depression in numerous studies. However, these trials have not necessarily given beneficial results. Because the mechanisms of ECT largely remain to be understood, the optimum stimulus intensity and the desirable current distribution in ECT are still not clear. In many cases, however, ECT with a commonly used voltage (100 V) and an electrode position (a pair of electrodes attached to the tempora) has produced an improvement of depression. Thus, an appropriate approach for the initial attempt of TMS therapy is to find a stimulus condition which gives a similar current distribution in the brain to ECT. In a previous study, we calculated current distributions in ECT and TMS using the finite element method (FEM) [4]. As a result of this calculation, TMS could generate eddy currents of comparable intensity to ECT. Nadeem et al. performed an impedance method simulation for the same purpose, and found that the current densities and electric fields in the ECT case were stronger and deeper penetrating than the corresponding TMS quantities [5]. However, these studies obtained results for only one condition of TMS. In the present study, we investigated dependences of the current distribution in TMS on coil current intensity, coil diameter, and coil position. Current distributions in ECT and TMS were compared under various conditions to find optimum conditions of TMS as an alternative to ECT.