Electrical and thermal behavior of non-ferrous noble metal electrodes exposed to MRI fields

MRI is a valuable tool in invasive epilepsy surgery evaluation. However, its use is limited due to concerns that under those conditions it may injure the brain. This study investigates the thermal and electrical behavior of electrodes exposed to MRI fields, using a phantom brain implanted with depth and grid electrodes. Voltages and temperatures were measured during various MRI scan sequences at electrodes whose numbers and orientations were varied in a pre-specified manner. Maximum SAR recorded during the scans was 1.5 W/kg. Temperature changes and DC voltages were negligible. Peak-to-peak voltage induction depended on the position, number of electrodes, MRI modality and sequence. Voltages exceeded 40 V at 63.6 MHz, only when electrode wires formed loops, reaching peak values of 72 V at RF. Estimated charge densities due to leakage currents were well within the safety limit for neural tissue, which is 30 microC/cm(2), even at the highest voltages. Although in the absence of loops, voltages and charge densities recorded in this study were, under most conditions within safe limits for brain tissue, their dependence on: a) the physical dimensions of electrode arrays; b) their position/orientation in reference to each other and to the plane of the magnetic field, and c) the MR sequence and slice thickness, may make these observations and conclusions not generalizable and thus not directly applicable to other conditions with regard to safety risks assessment.