Hybrid active and passive local shimming (HAPLS) for two-region MRI

Purpose: An MRI scanner is equipped with global shim systems for shimming one region of interest (ROI) only. However, it often fails to reach state-of-the-art when shimming two isolated regions of interest simultaneously, even though the two-area shimming can be essential in scan scenarios, such as bilateral breasts or dyadic brains. To address these challenges, a hybrid active and passive local shimming technique is proposed to simultaneously shim two isolated region-of-interest areas within the whole FOV. Methods: A local passive shimming system is constructed by optimized bilateral ferromagnetic chip arrays to compensate for the magnet's significant high-order B₀ inhomogeneities at the boundary of the manufacturer's specified homogeneous volume, thus locally improving the available FOV. The local active shimming consists of 40-channel DC loops powered by 64-channel current amplifiers. With the optimized current distribution, active shimming can correct the residual low-order B₀ inhomogeneities and subject-specific field inhomogeneities. In addition, active shimming is used to homogenize the center frequencies of the two regions. Results: With the implementation of the hybrid active and passive local shimming, the 95% peak-to-peak was reduced from 1.92 to 1.12 ppm by 41.7%, and RMS decreased from 0.473 to 0.255 ppm by 46.1% in a two-phantom experiment. The volume ratio containing MR voxels within a 0.5-ppm frequency span increased from 64.3% to 81.3% by 26.3%. Conclusion: The proposed hybrid active and passive local shimming technique uses both passive and active local shimming, and it can efficiently shim two areas simultaneously, which is an unmet need for a commercial MRI scanner.