INTRODUCTION

Although ultrahigh field offers many advantages, static magnetic field inhomogeneity that scale with the field strength leads to serious challenges¹. Signal loss, geometric distortions, banding artifacts and wide spectral linewidth are some of the well-known consequences of the poor B₀ inhomogeneity. As an alternative to conventional B₀ shimming using scanner’s built-in setup based on spherical harmonic (SH) functions, multi-coil setup is proposed that works more effectively for local inhomogeneities², where an array of multiple small loops are placed around the volume of imaging. Principally, utilizing more number of loops gives higher degrees of freedom and therefore a better shimming performance³. However, the need for more dedicated amplifiers, challenges with maintenance and control make an increased number of loops highly challenging. As another possibility, optimizations of the coil’s geometry and the position has been proposed for a limited number of slices⁴. The objective of this work is keeping the number of the coils fixed, but optimize the position and the size of the individual coils that adapt best for shimming of the human brain while practical constraints are taken into account.

METHODS

The optimization was performed on a 32-channel multi-coil setup. As a starting point, the square-shaped coils with a side length of 60mm were symmetrically positioned in four rows on a cylinder with a diameter of 165 mm. A nonlinear constrained optimization based on sequential quadratic programming (SQP) algorithm was implemented in MATLAB (Natick, MA) aiming to minimize the B₀ uniformity for the whole brain and thus to achieve a better global shimming (Figure 1). Each coil has three degrees of freedom, the coil size, the coil position in axial z direction and angular coordinate (θ) on the cylinder. The maximum side length of the coil was limited to 100 mm, and the constraints for the spatial coil location was given by cylinder geometry. Whole brain B₀ map of 12 volunteers after shimming with the scanner’s 2^nd order SH setup was measured at 9.4T whole‐body Siemens scanner (Erlangen, Germany). The dataset was split into two groups of 8 and 4 maps for training and testing, respectively. Possible current amplitude was chosen during the optimization based on the coil size; smaller coils were permitted to be fed with a higher current (maximum 3.0 A) and vice versa (minimum 1.5 A). Similar to our previous setup⁵, the RF transceiver coil housed in the multi-coil cylinder, but the cylinder radius was chosen smaller in the current design to shorten the distance between the shim coils and the target. Performance of the optimized 32-channel multi-coil setup was evaluated with ∆B₀ mapping and bSSFP sequences after global shimming of the whole brain. Field map was measured with a 3D double echo GRE sequence and the following parameters: TE1/TE2/TR = 2.6/7.4/15 ms, FA = 12^o and 2 mm3 isotropic resolution. Images with a 3D balanced SSFP sequence were acquired using sign-alternated RF pulses with the following protocol: TE/TR = 5/10ms, FA = 14^o and 1mm³ isotropic resolution. Dummy scans were employed at the beginning to reach a steady state. Both sequences were configured to a low bandwidth readout to minimize eddy current effects.

RESULTS AND DISCUSSION

Figure 2 shows coils arrangement on a cylinder outer surface before and after optimization as well as the experimental realization of the optimized coil. Axes are according to the Siemens MR scanner’s gradient coordinate system. Among 32 coils, the size of 22 coils increased after optimization. Looking at the coils distribution, the highest density is above the frontal cortex, where the most severe field inhomogeneity in the human brain exists. Figure 3 compares B₀ inhomogeneity in simulation after shimming with scanner’s 2^nd order SH, symmetric 32-channel multi-coil and optimized 32-channel multi-coil for one of the test data. The optimized multi-coil improve the field inhomogeneities by 18.3% better (from 60.7 Hz to 49.6 Hz) compared to the symmetric multi-coil. Figure 4 shows the experimental result of the global shimming with a prototype of the optimized multi-coil in a healthy volunteer at 9.4T. B₀ perturbation is well suppressed in the pre-frontal cortex with the optimized multi-coil even though the shimming is restricted to global shimming (no slice-specific shimming is performed). Figure 5 shows the impact of the shimming with the optimized multi-coil for banding artifacts reduction in balanced SSFP images. Banding artifacts well express the spots with poor B₀ shim. Shimming with the optimized multi-coil setup successfully mitigated the banding artifacts in the frontal brain region close to the sinuses.

Acknowledgements

No acknowledgement found.