A 32-Channel Multi-Coil Setup Optimized for Human Brain Shimming at 9.4 T
An optimized multi‐coil shim setup was designed and constructed for human whole‐brain shimming. Similar performance of the multi‐coils with many channels can be achieved with a fewer number of channels when the coils are optimally arranged around the target.
A, The arrangement of the coils on the cylinder surface before and after multi‐coil optimization. Optimization started from a symmetric arrangement of the 32 coils in 4 rows with a side length of 60 mm. B, Experimental realization of the optimized 32-channel multi-coil. C, Comparison of the simulated shimming performance between optimized multi‐coil, multi‐coils with a different number of coils, and spherical harmonics basis set. Shimming is carried out in global and dynamic slice‐wise fashion while the current is constrained and unconstrained.
Started with a symmetric arrangement of 32 independent coils. Three parameters per coil were optimized in parallel, including angular and axial positions on a cylinder surface and size of the coil, which were constrained by cylinder size, construction consideration, and amplifiers specifications. Therefore, the size and position of a set of square coils are optimized to improve the shim performance without increasing the number of local coils. Utilizing such a setup is especially beneficial at ultrahigh fields where B0 inhomogeneity in the human brain is more severe.
The resulting proof-of-concept 32-channel optimized MC shimming array has comparable global shimming performance of full 5th order spherical harmonics shimming or 65-channel symmetric MC shimming array. The optimized multi‐coil performs better by 14.7% based on standard deviation (SD) improvement with constrained global shimming in comparison to the symmetric positioning of the coils. Global shimming performance was comparable with a symmetric 65‐channel multi‐coil and full fifth‐order spherical harmonic shim coils. On average, an SD of 48.4 and 31.9 Hz was achieved for in vivo measurements after global and dynamic slice‐wise shimming, respectively.
Magn Reson Med. 2020 Feb;83(2):749-764.