Eddy Current Characterization Using Efficient Finite Element Analysis for Z-Gradient Coils in a Low-Field MRI Scanner

Despite passive shielding, eddy current effects due to z-gradient coils in a biplanar permanent magnet low-field magnetic resonance imaging (MRI) system could still introduce artifacts in images. A finite element analysis was performed to characterize the secondary fields induced by eddy currents, in which an effective equivalent of the complex system model was introduced to reduce the computational complexity. A gradient sensor was also designed to inductively measure the dynamic gradient field and validate the accuracy of the simulation results. Furthermore, the gradient eddy-current impulse response function (GEIRF) was obtained through the finite element analysis and then used to guide the design of the preemphasis to compensate for eddy current and its effects. The proposed framework was applied to a 50 mT portable MRI system, and the effectiveness of the gradient prediction was demonstrated by comparing it to the measurement waveforms. Imaging experiments were also performed with a preemphasis given to the proposed and traditional methods. Moreover, the error caused by different typical conductivity values of conducting structures was investigated by simulations and experiments. Finally, the proposed finite element-based eddy current characterization is proven to effectively predict eddy current effects and remove image artifacts with a range of material conductivities, increasing its suitability for implementation in the process of gradient system design and optimization.