The Optimal Target Magnetic Field Method for Passive Shimming in MRI

In a magnetic resonance imaging system, the magnetic field homogeneity of the imaging area plays a decisive role on the image quality. The passive shimming technique uses ferromagnetic material to eliminate the field impurity in the imaging area. In existing passive shimming algorithms, the target magnetic field (TMF) was usually set as a fixed value, such as the DC component of the magnetic field, leading to a rigid constraint to the formed optimization model, and it may cause shimming inefficiency or even failure. In this paper, an optimal target magnetic field (OTMF) method was proposed, which set the TMF as an optimization variable built in a linear programming model. In the established optimization problem, no extra computational burden was involved. To demonstrate the effectiveness of the proposed method, the bare magnetic field of a 3.0-T superconducting magnet was shimmed. Compared with conventional linear programming methods, the proposed scheme could provide better shimming solution with higher field homogeneity and less consumption of ferromagnetic material.