Multiband RF Pulse Shimming for Uniform Slab Excitation in MRI

The patent divides the imaging volume into multiple sub-slabs along the slice-select direction. Each sub-slab is independently excited with its own optimized RF pulse, allowing the B1+ field inhomogeneity to be addressed locally rather than globally. This segmentation enables independent optimization of flip angles for each sub-slab, resolving the contradiction between maintaining high SNR at UHF and achieving uniform B1+ field distribution.

The patent applies local quality optimization by determining subject-specific RF shim settings for each transmit element and each sub-slab individually. The complex B1+ field sensitivity maps are used to calculate optimized magnitude and phase weights that are specific to each local region (sub-slab) rather than applying a uniform global setting. This local optimization approach allows the system to maintain high signal-to-noise ratio while achieving homogeneous flip angles within each sub-slab.

The patent employs dynamic parallel transmission where the magnitude and phase of RF pulses applied to each transmit element are varied in a time-dependent manner during the RF pulse sequence. This dynamic adjustment allows the system to achieve homogeneous B1+ field distribution across the imaging volume by continuously optimizing the contribution of each transmit element throughout the pulse duration, rather than using static fixed settings.

The patent changes multiple parameters simultaneously including the magnitude and phase of RF pulses for each transmit element, the timing of pulse application, and the slice-select gradient parameters. By optimizing these parameters subject-specifically using measured B1+ field sensitivity maps, the system achieves improved B1+ homogeneity without requiring overly complex hardware configurations, as the complexity is managed through sophisticated parameter optimization rather than hardware complexity.

The patent overcomes the limitations of static B1+ shimming by segmenting the imaging volume into multiple thin sub-slabs. Each sub-slab is excited with a dedicated RF pulse with independently optimized shim settings. This segmentation allows static shimming to be applied effectively within each narrow sub-slab where B1+ field variation is minimal, while the overall large slab achieves homogeneity through the combination of multiple uniformly excited sub-slabs.

The patent applies partial action by focusing the static B1+ shimming optimization on achieving uniform excitation within each individual sub-slab rather than attempting to optimize the entire thick slab at once. By applying shimming settings that are sufficient (excessive) for each thin sub-slab individually, the cumulative effect across all sub-slabs achieves homogeneous excitation across the complete imaging volume, overcoming the limitation of static shimming for large slabs.