Passive shimming method for permanent magnet MRI based on dynamic target magnetic field

To improve the imaging quality of permanent magnet MRI systems, passive shimming of the main magnetic field is necessary. However, traditional passive shimming methods face challenges such as overly stringent constraints and reliance on a single measurement volume. To address these issues, this paper proposes a dynamic target magnetic field (DTMF)-based shimming method for permanent magnet MRI systems. By dynamically adjusting the target magnetic field values, this method relaxes feasibility constraints and enhances the efficiency of obtaining viable solutions within the solution space. First, building on the fixed target magnetic field method (FTMF), we establish a shimming optimization model for the DTMF method and introduce an incremental revised simplex algorithm for solving the model. Next, a 0.2T permanent magnet MRI shimming experimental platform is constructed, involving the configuration of sampling points and shim pad placement. Finally, shimming experiments validate the effectiveness of the proposed method, and quantitative evaluations are conducted using assessment metrics for the shimming results. The results demonstrate that, compared to the fixed target magnetic field method, the DTMF method achieves a 29.17 % improvement in field homogeneity and a 24.24 % reduction in shim pad consumption. This method provides a more efficient, comprehensive, and flexible solution for enhancing the magnetic field homogeneity of permanent magnet MRI systems.