A true comparison of B0 shimming with a very high order spherical harmonic based setup and a multicoil shim array
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Purpose 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 fifth‐order 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.
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Abstract Purpose: To develop a new concept for a hardware platform that enables integrated parallel reception, excitation, and shimming. Theory: This concept uses a single coil array rather than separate arrays for parallel excitation/reception and B 0 shimming. It relies on a novel design that allows a radiofrequency current (for excitation/reception) and a direct current (for B 0 shimming) to coexist independently in the same coil. Methods: Proof‐of‐concept B 0 shimming experiments were performed with a two‐coil array in a phantom, whereas B 0 shimming simulations were performed with a 48‐coil array in the human brain. Results: Our experiments show that individually optimized direct currents applied in each coil can reduce the B 0 root‐mean‐square error by 62–81% and minimize distortions in echo‐planar images. The simulations show that dynamic shimming with the 48‐coil integrated parallel reception, excitation, and shimming array can reduce the B 0 root‐mean‐square error in the prefrontal and temporal regions by 66–79% as compared with static second‐order spherical harmonic shimming and by 12–23% as compared with dynamic shimming with a 48‐coil conventional shim array. Conclusion: Our results demonstrate the feasibility of the integrated parallel reception, excitation, and shimming concept to perform parallel excitation/reception and B 0 shimming with a unified coil system as well as its promise for in vivo applications.
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Purpose We designed and implemented an orthogonal shim array consisting of shim coils with their planes perpendicular to the planes of neighboring RF coils. This shim coil improves the magnetic field homogeneity by minimizing the interference to RF coils. Methods Using realistic off‐resonance maps of the human brain, we first evaluated the performance of shim coils in different orientations. Based on simulations, we developed a 7‐channel orthogonal shim array, whose coil plan was perpendicular to neighboring RF coils, at the forehead. A programmable open‐source current driver supplied shim currents. Results The 7‐channel orthogonal shim array caused only marginal SNR loss to the integrated 32‐channel RF‐shim array. The 7‐channel orthogonal shim array itself improved the magnetic field homogeneity by 30% in slice‐optimized shimming, comparable to the baseline shimming offered by the scanner’s 2nd order spherical harmonic shimming. Conclusion Orthogonal shim coils can improve the field homogeneity while maintaining high image SNR.
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To describe the process of calibrating a B0 shim system using high-degree (or high order) spherical harmonic models of the measured shim fields, to provide a method that considers amplitude dependency of these models, and to show the advantage of very high-degree B0 shimming for whole-brain and single-slice applications at 9.4 Tesla (T).An insert shim with up to fourth and partial fifth/sixth degree (order) spherical harmonics was used with a Siemens 9.4T scanner. Each shim field was measured and modeled as input for the shimming algorithm. Optimal shim currents can therefore be calculated in a single iteration. A range of shim currents was used in the modeling to account for possible amplitude nonlinearities. The modeled shim fields were used to compare different degrees of whole-brain B0 shimming on healthy subjects.The ideal shim fields did not correctly shim the subject brains. However, using the modeled shim fields improved the B0 homogeneity from 55.1 (second degree) to 44.68 Hz (partial fifth/sixth degree) on the whole brains of 9 healthy volunteers, with a total applied current of 0.77 and 6.8 A, respectively.The necessity of calibrating the shim system was shown. Better B0 homogeneity drastically reduces signal dropout and distortions for echo-planar imaging, and significantly improves the linewidths of MR spectroscopy imaging. Magn Reson Med 79:529-540, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
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A multi-coil shim setup is designed and optimized for human brain shimming. Here, the size and position of a set of square coils are optimized to improve the shim performance without increasing the number of local coils. Utilizing such a setup is especially beneficial at ultrahigh fields where B0 inhomogeneity in the human brain is more severe.The optimization started with a symmetric arrangement of 32 independent coils. Three parameters per coil were optimized in parallel, including angular and axial positions on a cylinder surface and size of the coil, which were constrained by cylinder size, construction consideration, and amplifiers specifications. B0 maps were acquired at 9.4T in 8 healthy volunteers for use as training data. The global and dynamic shimming performance of the optimized multi-coil were compared in simulations and measurements to a symmetric design and to the scanner's second-order shim setup, respectively.The optimized multi-coil performs better by 14.7% based on standard deviation (SD) improvement with constrained global shimming in comparison to the symmetric positioning of the coils. Global shimming performance was comparable with a symmetric 65-channel multi-coil and full fifth-order spherical harmonic shim coils. On average, an SD of 48.4 and 31.9 Hz was achieved for in vivo measurements after global and dynamic slice-wise shimming, respectively.An optimized multi-coil shim setup was designed and constructed for human whole-brain shimming. Similar performance of the multi-coils with many channels can be achieved with a fewer number of channels when the coils are optimally arranged around the target.
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We add user-controllable direct currents (DC) to the individual elements of a 32-channel radio-frequency (RF) receive array to provide B0 shimming ability while preserving the array's reception sensitivity and parallel imaging performance.
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Purpose To test an integrated “AC/DC” array approach at 7T, where B 0 inhomogeneity poses an obstacle for functional imaging, diffusion‐weighted MRI, MR spectroscopy, and other applications. Methods A close‐fitting 7T 31‐channel (31‐ch) brain array was constructed and tested using combined Rx and ΔB 0 shim channels driven by a set of rapidly switchable current amplifiers. The coil was compared to a shape‐matched 31‐ch reference receive‐only array for RF safety, signal‐to‐noise ratio (SNR), and inter‐element noise correlation. We characterize the coil array’s ability to provide global and dynamic (slice‐optimized) shimming using ΔB 0 field maps and echo planar imaging (EPI) acquisitions. Results The SNR and average noise correlation were similar to the 31‐ch reference array. Global and slice‐optimized shimming provide 11% and 40% improvements respectively compared to baseline second‐order spherical harmonic shimming. Birdcage transmit coil efficiency was similar for the reference and AC/DC array setups. Conclusion Adding ΔB 0 shim capability to a 31‐ch 7T receive array can significantly boost 7T brain B 0 homogeneity without sacrificing the array’s rdiofrequency performance, potentially improving ultra‐high field neuroimaging applications that are vulnerable to off‐resonance effects.
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The performance of the multicoil shimming is much better than that of the spherical harmonic coils in many aspects. The diameters and locations of the multicoil are optimized for the B0 shimming of the third order spherical harmonic field. And then every independent coil is decomposed into two series coils to take full advantage of a limited number of shim current supplies. The performance is improved by 16.7% when quantified by the surface root mean square.
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