Minimizing Induced Electric Fields in Human Head-Size MPI Systems

Abstract

Magnetic Particle Imaging (MPI) utilizes time-varying magnetic fields called drive fields to excite the magnetic nanoparticles for imaging. Time-varying magnetic fields induce electric fields within the human body, which can result in peripheral nerve stimulation (PNS) above a certain threshold. A conventional single-channel drive coil, while possessing a large homogeneous region, presents a challenge by exposing a large portion of the human head to the time-varying magnetic field, consequently leading to larger induced electric field. In this work, we propose using drive array coils to gain versatility by individualizing each winding with varying currents to enable safer imaging by minimizing the induced electric fields in MPI systems designed for human head imaging. To find the optimal current amplitudes for this array configuration, an optimization problem was formulated. Specifically, the objective of the optimization was defined as minimizing the peak induced electric field on the surface of a body model. This goal was accomplished by considering a set of constraints, setting a maximum current amplitude and an upper bound for the deviation from the targeted magnetic field within a specified region of interest. We utilized a realistic and simplified body model and performed simulations in COMSOL to compute the magnetic field sensitivity maps and the induced electric field maps on the body model. These essential data are then fed to the subsequent optimization algorithm to minimize the induced electric fields on the simplified body model using a drive array with optimized current amplitudes.