Design of a shim coil array matched to the human brain anatomy

Abstract

The purpose of this study is to introduce a novel design method of a shim coil array specifically optimized for whole brain shimming and to compare the performance of the resulting coils to conventional spherical harmonic shimming. Methods: The proposed design approach is based on the stream function method and singular value decomposition. Eighty-four field maps from 12 volunteers measured in seven different head positions were used during the design process. The cross validation technique was applied to find an optimal number of coil elements in the array. Additional 42 field maps from 6 further volunteers were used for an independent validation. A bootstrapping technique was used to estimate the required population size to achieve a stable coil design. Results: Shimming using 12 and 24 coil elements outperforms fourth-and fifthorder spherical harmonic shimming for all measured field maps, respectively. Coil elements show novel coil layouts compared to the conventional spherical harmonic coils and existing multi-coils. Both leave-one-out and independent validation demonstrate the generalization ability of the designed arrays. The bootstrapping analysis predicts that field maps from approximately 140 subjects need to be acquired to arrive at a stable design. Conclusions: The results demonstrate the validity of the proposed method to design a shim coil array matched to the human brain anatomy, which naturally satisfies the laws of electrodynamics. The design method may also be applied to develop new shim coil arrays matched to other human organs.

A fieldmap-driven few-channel shim coil design for MRI of the human brain

“…We remark that similar approaches have been presented recently (Jia et al 2020, Arango et al 2019, Can et al 2019. In Jia et al (2020), fieldmaps are shimmed by unconstrained SH of some order (5 th or 6 th ), then SO-SFs are computed under power dissipation constraints, tuned to achieve a residual field equivalent to that obtained by the SH shimming. Therefore, an indirect minimization of power dissipation is performed.…”

A 31‐channel integrated “AC/DC” B0 shim and radiofrequency receive array coil for improved 7T MRI

“…An alternative MC shimming approach for UHF MRI is to place dedicated multi‐turn shim coils just outside the transmit coil shield, as recently demonstrated at 9.4T 13,60 . Compared to AC/DC arrays, this approach provides (i) reduced interaction of DC shim components with the transmit coil(s), (ii) less complexity for the receive‐array design, and (iii) greater flexibility to use target field‐based shim coil designs optimized for the human head anatomy 55,58,61 . However, shim coil placement outside the shield could be geometrically constrained, for example if a head‐gradient insert coil is used.…”

“…A zoomed inset shows the improvements in outlier voxels achieved using MC shimming provides (i) reduced interaction of DC shim components with the transmit coil(s), (ii) less complexity for the receive-array design, and (iii) greater flexibility to use target field-based shim coil designs optimized for the human head anatomy. 55,58,61 However, shim coil placement outside the shield could be geometrically constrained, for example if a head-gradient insert coil is used. Also, the higher inductance may be an obstacle for S6) some emerging local field control applications 62,63 where waveforms with KHz-range frequencies (eg, 1-25 KHz) are played out on MC arrays to provide supplementary spatial encoding.…”

CoilGen: Open‐source MR coil layout generator

“…Further design objectives such as the reduction of mechanical force and torque have been introduced 11 . Since the power requirements of actively shielded cylindrical gradient coils tend to increase sharply with the bore radius of the MR scanner, 5 non‐cylindrical or segmented current‐carrying surfaces have been investigated to locally increase gradient efficiency by a better geometrically match to the human body 11‐15 …”