Designing Gradient Coils Considering the Ferromagnetic Effect for a Movable Permanent MRI System

The gradient coils represent an indispensable constituent within magnetic resonance imaging systems. Their performance significantly impacts the quality of images, particularly the nonlinearity of the gradient magnetic field. Due to the presence of ferromagnetic materials surrounding the gradient coil in the permanent magnet system, the magnetic field of the gradient coil experiences influence. Consideration must be given to ferromagnetic materials during the design phase. The objective of this study is to design gradient coils that mitigates the impact of ferromagnetic materials on gradient field linearity. In this paper, the original coil structure is formulated utilizing the discrete trajectory method, while introducing mirrored current to elucidate the effects of ferromagnetic material. Through the integration of these two methods, gradient coil structures with excellent linearity are achieved. Ultimately, the optimal gradient coils are fabricated, and computational as well as experimental findings demonstrate concordance between measured nonlinear degree and efficiency of the gradient coils with theoretical calculations in the presence of ferromagnetic materials.