Dynamic B0 Shimming of the Human Brain at 9.4 T with a 16-Channel Multi-Coil Shim Setup

We have developed a multi-coil (MC) shimming array with 16 independent circular loops for dynamic shimming of brain at the 9.4T. In this work, we demonstrate the feasibility of a hybrid method for dynamic shimming by combining scanner’s spherical harmonics (SH) system with our MC shimming array.

A, Photograph of the 16-channelmulti-coil shimarray. The coils are arranged in 2 rows and spaced equally on the cylinder surface. B, Schematic of the used setup. The setup consists of 4 layers. Starting with the innermost layer, they were RF transceiver coil, RF coil holder,multi-coil shim array, and a chassis to fix themulti-coil setup on the patient table of the scanner. C, Photograph of the assembled setup. D, Field inhomogeneities mapped to the brain surface with 3 orthogonal cuts to demonstrate shimming performance in the depth of the brain. The SD is calculated for the whole brain.

A homogenous B0 field is a key prerequisite for a large set of MRI and MRS methods. Inhomogeneities arise primarily from varying magnetic susceptibility within the subject. In the human brain, the susceptibility difference between air and tissue, particularly in the temporal lobes and the prefrontal cortex, results in a complex distortion of the magnetic field.
A 16-channel multi-coil shimming setup was developed to mitigate severe B0 field perturbations at ultrahigh field and improve data quality for human brain imaging and spectroscopy. The shimming setup consisted of 16 circular B0 coils that were positioned symmetrically on a cylinder with a diameter of 370mm. The latter was large enough to house a shielded 18/32-channel RF transceiver array. The presence of the multi-coil shim did not influence the performance of the RF coil. The performance of the proposed setup was similar to a full third-order spherical harmonic shim system in the case of global static and dynamic slice-wise shimming. Compared with MC shimming array, this hybrid method further improves field homogeneity by 12%. In conclusion, the combination of a multi-coil setup and the linear shim channels of the scanner provides a straightforward solution for implementing dynamic slice-wise shimming without requiring an additional pre-emphasis setup.

1.
Aghaeifar A, Mirkes C, Bause J, Steffen T, Avdievitch N, Henning A, Scheffler K.:
Dynamic B0 shimming of the human brain at 9.4 T with a 16-channel multi-coil shim setup.
Magn Reson Med. 2018 Oct;80(4):1714-1725.
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