Improving brain B0 shimming using an easy and accessible multi-coil shim array at ultra-high field

Abstract

Object: Improve shimming capabilities of ultra-high field systems, with addition of an accessible low-complexity B₀ shim array for head MRI at 7 T.

Materials and methods: An eight channel B₀ shim coil array was designed as a tradeoff between shimming improvement and construction complexity, to provide an easy to use shim array that can be employed with the standard 7 T head coil. The array was interfaced using an open-source eight-channel shim amplifier rack. Improvements in field homogeneity for whole-brain and slice-based shimming were compared to standard second-order shimming, and to more complex higher order dynamic shimming and shim arrays with 32 and 48 channels.

Results: The eight-channel shim array provided 12% improvement in whole brain static shimming and provided 33% improvement when using slice-based shimming. With this, the eight-channel array performed similar to third-order dynamic shimming (without the need for higher order eddy current compensation). More complex shim arrays with 32 and 48 channels performed better, but require a dedicated RF coil.

Discussion: The designed eight-channel shim array provides a low-complexity and low-cost approach for improving B₀ field shimming on an ultra-high field system. In both static and dynamic shimming, it provides improved B₀ homogeneity over standard shimming.

Keywords: B0 field; Magnetic resonance imaging; Multi-coil array; Shimming; Ultra-high field.

Fig. 1

a Geometry optimization was performed using a center coil with a ring of coils around it on the outside of the RF coil to minimize coupling. Input parameters to the optimization were the number of coils, the radius of the coil elements (r₁), the radius of the larger ring (r₂), the angle of the outer ring with respect to the main magnetic field direction (α) and the z-offset of the array. b Final result shown with the inner and outer surface of the RF coil. c Realized eight-channel shim array mounted on the RF coil

Fig. 2

B₁⁺ mapping a–c and receiver array noise correlation matrix d–f with either the eight-channel shim coil (MC8) in place (a,d) or with the coil removed (b,e). The difference in B₁⁺ c showed a reduction of 2% transmit efficiency with the shim array in place. The difference between the coupling values f shows a mean and maximum change of 1% and 5%, respectively

Fig. 3

Simulated shimming performance of different approaches over ten subjects. Static second-order shimming (S2) is used as the standard, and all other analysis was performed after removing up to second-order fields from the maps. On top of this, the static eight-channel multi-coil (MC8) array resulted in improve field homogeneity, but did not perform as good as the larger shim arrays with 32- or 48-channels (MC32 and MC48 resp). For dynamic (slice based) shimming, the eight-channel array in combination with dynamic linear shimming (D1) outperformed first and second-order dynamic shimming (D2) and performed not significantly different from dynamic third-order dynamic shimming (D3). Again, dynamic shimming with the larger arrays (MC32 and MC48) performed best

Fig. 4

Example of shimming performance on three orthogonal slices (left) in one subject. The field distribution before shim (a) of 71 Hz was improved to 33 Hz with second-order spherical harmonic shimming (b). Use of the eight-channel MC coil in combination with second-order shimming for whole brain (“static”) shimming improved the shim further to 29 Hz (c, simulated) and 30 Hz (d, measured). Slice-based (“dynamic”) MC8 shimming, in combination with static second-order shimming, resulted in 25 Hz (e, simulated) and 26 Hz (f, measured) standard deviation over the brain

Fig. 5

Undistorted 7 T gradient echo (GRE) images as reference (1st column), simulated B0 map with whole brain second-order shimming (2nd column), and B0 map using dynamic shimming with the eight-channel shim array (3rd column, both scaled from − 200 to 200 Hz). Single shot 2 mm multi-slice EPI imaging with standard whole brain second-order shimming (4th column), and single shot EPI imaging using dynamic shimming with the eight-channel shim array (5th column). The EPI image acquired using dynamic shimming shows reduced signal drop-out and reduced image distortions especially in lower brain areas (red arrows)