Noninvasive electric and magnetic stimulation of the brain: numerical modeling and dosimetrical assessment

The use of electromagnetic (EM) fields for medical applications has extensively grown over the past decades, to the point that the novel term "Electroceuticals" was coined to identify the potentialities of EM fields as an option over drug treatments ("Pharmaceuticals"). A particular branch of electroceuticals are the noninvasive brain stimulation (NIBS) techniques, examples of which are transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), that have been extensively studied for the treatment of several psychiatric disorders and neurological diseases. These techniques exert their therapeutic effect by means of an electric field, either induced by a pulsed magnetic field, or generated by injected currents, respectively. Nonetheless, the field of brain stimulation is constantly growing, and increasing efforts are applied to develop novel techniques or to ameliorate existing ones. A fundamental tool that supports the growth and expansion of noninvasive brain stimulation advances in the medical field is the use of numerical dosimetry, that provide an important insight to the mechanisms of action behind brain stimulation by correlating the physiological outcome with the induced EM quantities. Furthermore, it can help optimizing existing techniques to improve the treatment efficacy, as in the case of neuronavigated stimulation, or assessing innovative approaches. In this regard, an accurate modeling, that considers the stimulation device as well as the subject's head, becomes of paramount importance together with understanding all the parameters that can affect the estimation of the induced EM quantities. In this thesis numerical dosimetry of standard and innovative brain stimulation techniques is conducted. Concerning standard techniques, such as tDCS and TMS, the aim is to investigate the influence of tissue properties on the EM solution, moreover an alternative EM dosimetry custom code for neuronavigated TMS is proposed. A new methodology is also proposed to assess the therapeutic efficacy of a novel treatment that apply low frequency pulsed magnetic fields to treat patients with acute cerebral ischemic stroke. Finally, aiming to overcome TMS low spatial selectivity, a prototype of miniaturized coil was developed and characterized both experimentally and numerically to demonstrate its ability to elicit stimulation of neural tissues on healthy subjects.