10.5 T In Vivo Head Imaging With Universal RF Shimming

Brain MR imaging at 10.5 T ultra-high field offers significant improvements in signal-to-noise ratio (SNR), but faces challenges with B₁ ⁺ inhomogeneity. Parallel-transmission (pTx) can be used to achieve a more uniform RF field distribution, but necessitates the use of B₁ ⁺ calibration in the region of interest. This study explores a universal B₁ ⁺ shim solution on 10.5 T that could eliminate the need for time-consuming subject-specific B₁ ⁺ calibration.

B₁ ⁺ data from 7 participants (19 sessions) were used to develop the universal B₁ ⁺ shim, which was then validated against traditional subject-specific approaches using T₁-weighted MP2RAGE structural images in 5 participants (6 sessions). Statistical comparisons of tissue and subcortical segmentations were conducted using popular neuroimaging tools SPM and FreeSurfer, respectively.

The universal shim rapidly converged with a small training dataset, likely due to consistent positioning and the simplicity of B₁ ⁺ shimming used for head imaging. Whole-brain tissue segmentation showed no statistically significant differences between universal and subject-specific solutions, with only minor variations near the ventricles and inferior brain regions in the detailed subcortical segmentation. The proposed universal B₁ ⁺ shim reduces examination time by removing the need for separate data acquisition and optimization.

These findings suggest that the universal B₁ ⁺ shim is a viable substitute for subject-specific approaches, offering a more efficient solution for neuroimaging applications. Additionally, it confirms that 10.5 T MRI can produce reliable structural brain imaging data, paving the way for broader adoption of ultra-high field MRI in neuroimaging research.