Non-Invasive Characterization of Brain Tissue Electrical Properties with MRI

Abstract

The study of the electrical properties (EPs) of tissues, i.e. conductivity and permittivity, is of fundamental importance to understand the biophysical interactions and the effects of electromagnetic fields on our body, such as those produced by MRI scanners or by Transcranial Magnetic Stimulators (TMS). Knowledge on subject-specific tissue EPs is valuable for diagnostic purposes in oncology, and could allow more accurate specific-absorption-rate (SAR) estimations for RF patient safety. Additionally, tissue EPs measurements can provide better subject-specific guidance and dosimetry of TMS treatments. In this thesis, various methodologies to measure tissue EPs both at low frequencies (kHz) and high frequencies (radiofrequency RF range: hundreds MHz) were investigated, characterized and experimentally explored using MRI scanners. At low frequencies, fundamental and experimental problems hampering EPs reconstructions by means of MR compatible current induction methods were characterized. The first investigated methodology used the switching of MR gradients to induce electric currents in tissues. To increase the level of induced currents, in the second methodology electric currents were induced using a combined TMS-MRI setup. This setup included a 3T MRI system (Achieva, Philips) and a TMS stimulator (Rapid2, Magstim). To allow correct synchronization between devices, in-house hardware and a novel TMS-MRI measurement protocol were developed. Nevertheless, it has been shown that MRI measurements of the incident TMS magnetic field are feasible. These measurements are important to improve guidance of TMS dosimetry and to validate TMS coil models adopted in simulations to investigate TMS induced neuronal activation. Since EPs vary with the frequency, it has been also investigated the feasibility of accurately measuring EPs at RF frequencies by means of electromagnetic simulations and MR measurements (3T MRI scanners, Achieva and Ingenia, Philips). At RF frequencies, MR-Electrical Properties Tomography (MR-EPT) is widely employed to reconstruct tissue EPs. Although this technique is very appealing as it only requires an MR scanner, several issues affect the EPs reconstructions. In this thesis, it has been thoroughly analyzed the impact of numerical errors arising from the reconstruction process. It has been concluded that MR-EPT is not able to provide detailed EPs measurements on a voxel level for low field strengths MRI systems (1.5 T and 3 T). Still, MR-EPT can be used to investigate volumetric EPs changes, e.g. for better tumor characterization. Finally, a newly proposed MR-based technique was investigated: water-content EPT (wEPT). Although wEPT is built upon an empirical framework, it allows EPs reconstructions at RF frequencies for healthy brain tissues on a voxel level. In this thesis, the validity of the empirical framework employed by wEPT has been validated for conductivity reconstructions in the brain white matter brain. Future studies should focus on investigating the validity of wEPT in pathological situations. By correlating wEPT reconstructions with independent MR-EPT reconstructions and MRI sodium imaging, better understanding of the relationships between electrical conduction and tissue structure and composition may be obtained. Ultimately, correct understanding of the physiological background of tissue EPs will be fundamental before EPs measurements can be used as a new endogenous biomarker for diagnostic purposes.