A theoretical framework of gradient coil designed to mitigate eddy currents for a permanent magnet MRI system

Abstract

BACKGROUND: High installation and operating cost have limited applications for many circumstances. In practice, primary and shielding coils cannot insert into the magnet pole simultaneously owing to deficient workspace for the planar permanent MRI systems OBJECTIVE: To minimize eddy currents induced in the resist-eddy current plates and pole piece when the gradient coil current switches on and off rapidly. METHODS: A theoretical framework that have minimum power dispassion and magnetic energy with eddy plate is proposed for a planar gradient coil. The mirror image of the magnetostatic model is substituted into the stream function for designing a minimum power dispassion planar gradient coil. A finite-difference is used to formulate the coil distribution that makes magnetic field similar to the required magnetic field for gradient coil design. RESULTS: A coil designed with actively shielded was simulated and compared with the designed gradient coils using mirror image theory and piece pole effect. According to the numerical evaluation of the x and z coils, the operating currents in the cases were reduced to 34.4% using magnetostatic mirror-image method to replay the active shielding. Moreover, there was a significant improvement on the shielding effect when added to resistive eddy current plate. CONCLUSIONS: Using the magnetostatic mirror image theory and mirror-image model, the current density function that could not only gives the minimum power dissipation and magnetic energy with the presence of the eddy plate and pole piece effect, but also provides excellent coil performance compared with active shielding solution.