A true comparison of B0 shimming with a very high order spherical harmonic based setup and a multicoil shim array
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Shim (computing)
Harmonic
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High-quality magnetic resonance imaging and spectroscopic measurements require a highly homogeneous magnetic field. Different from global shimming, localized off-resonance can be corrected by using multi-coil shimming. Previously, integrated RF and shimming coils have been used to implement multi-coil shimming. Such coils share the same conductor for RF signal reception and shim field generation. Here we propose a new design of the integrated RF-shim coil at 3-tesla, where two independent shim current paths are allowed in each coil. This coil permits a higher degree of freedom in shim current distribution design. We use both phantom experiments and simulations to demonstrate the feasibility of this new design.
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Multislice
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Radiofrequency coil
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Purpose To describe and implement a strategy for dynamic slice‐by‐slice and multiband B 0 shimming using spherical harmonic shims in the human brain at 7T. Theory For thin axial slices, spherical harmonic shims can be divided into pairs of shims (z‐degenerate and non‐z‐degenerate) that are spatially degenerate, such that only ½ of the shims (non‐z‐degenerate) are required for single slice optimizations. However, when combined, the pairs of shims can be used to simultaneously generate the same in‐plane symmetries but with different amplitudes as a function of their z location. This enables multiband shimming equivalent to that achievable by single slice‐by‐slice optimization. Methods All data were acquired at 7T using a spherical harmonic shim insert enabling shimming up through 4th order with two additional 5th order shims (1st‐4th+). Dynamic shim updating was achieved using a 10A shim power supply with 2 ms ramps and constrained optimizations to minimize eddy currents. Results In groups of eight subjects, we demonstrated that: 1) dynamic updating using 1st‐4th+ order shims reduced the SD of the B 0 field over the whole brain from 32.4 ± 2.6 and 24.9 ± 2 Hz with 1st‐2nd and 1st‐4th+ static global shimming to 15.1 ± 1.7 Hz; 2) near equivalent performance was achieved when dynamically updating only the non‐z‐degenerate shims (14.3 ± 1.5 Hz), or when a using multiband shim factor of 2, MB s = 2, and all shims (14.4 ± 2.0 Hz). Conclusion High order spherical harmonics provide substantial improvements over static global shimming and enable dynamic multiband shimming with near equivalent performance to that of dynamic slice‐by‐slice shimming. This reduces distortion in echo planar imaging.
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Harmonic
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Purpose A 16‐channel multi‐coil shimming setup was developed to mitigate severe B 0 field perturbations at ultrahigh field and improve data quality for human brain imaging and spectroscopy. Methods The shimming setup consisted of 16 circular B 0 coils that were positioned symmetrically on a cylinder with a diameter of 370 mm. The latter was large enough to house a shielded 18/32‐channel RF transceiver array. The shim performance was assessed via simulations and phantom as well as in vivo measurements at 9.4 T. The global and dynamic shimming performance of the multi‐coil setup was compared with the built‐in scanner shim system for EPI and single voxel spectroscopy. Results 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. Dynamic slice‐wise shimming with the multi‐coil setup outperformed global static shimming with the scanner's second‐order spherical‐harmonic shim. The multi‐coil setup allowed mitigating geometric distortions for EPI. The combination of the multi‐coil shim setup with the zeroth and first‐order shim of the scanner further reduced the standard deviation of the B 0 field in the brain by 12% compared with the case in which multi‐coil was used exclusively. 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.
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Flip angle
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