Automatic Magnetic Field Shimming by Spectrum Convergence Method: Evaluation at a 7-T Animal MRI System

The ultrahigh field magnetic resonance imaging (MRI) superconducting magnets require a high homogeneous magnetic field within the diameter of spherical volume (DSV) to generate high-quality images. Spherical harmonic-based B₀ shimming relies significantly on the fitting process, which can be computationally demanding, particularly when managing multiple shim coils. The study presents an automatic room-temperature shimming approach that iteratively adjusts shim coil currents to enhance magnetic field homogeneity, using the full width at half maximum (FWHM) of the spectrum as a metric. The method efficiently identifies the optimal shim coil currents, enhancing spatial field homogeneity by minimizing the FWHM. Applying the method to the automatic shimming function of the system software, experimental validation conducted on a 7-T MRI superconducting magnet confirmed the efficacy of the proposed approach. Specifically, employing gradient coils and four shim coils, the automatic room-temperature shimming approach effectively improved the field homogeneity of a 100-mm DSV from 5.478 to 2.989 ppm (p-p) for imaging rat brains.