Development and implementation of an 84‐channel matrix gradient coil

Littin, Sebastian; Jia, Feng; Layton, Kelvin J.; Kroboth, Stefan; Yu, Huijun; Hennig, Jürgen; Zaitsev, Maxim

doi:10.1002/mrm.26700

Cited by 48 publications

(51 citation statements)

References 28 publications

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“…6e shows a diagram of the coil design and photograph of the completed insert coil. Simulations of 3 T brain shimming show that this coil outperforms 4th order SH shimming even when constrained to 10 amps per channel (Jia et al, 2016; Littin et al, 2017). Even better performance is achieved when the current limit is relaxed.…”

Section: Overview Of B0 Shim Field Generation Hardwarementioning

confidence: 99%

In vivo B 0 field shimming methods for MRI at 7 T

2018

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Functional MRI (fMRI) at 7 T and above provides improved Signal-to-Noise Ratio and Contrast-to-Noise Ratio compared to 3 T acquisitions. In addition to the beneficial effects on spin polarization and magnetization of deoxyhemoglobin, the increased applied field also further magnetizes air and tissue. While the magnets themselves typically provide a static B0 field with sufficient spatial homogeneity, the diamagnetism of tissue and the paramagnetism of air causes local field deviations inside the human head. These spatially-varying field offsets (ΔB0) cause image artifacts, especially in single shot EPI, including geometric distortion, signal dropout, and blurring. These effects are particularly strong near air-tissue interfaces such as the frontal sinus, and ear canals. Furthermore, if the field offsets are dynamically modulated through physiological processes such as respiration or motion, then the effect on the image time-series can be even more problematic. While post-processing methods have been developed to mitigate these effects, the ideal solution would be to reduce the ΔB0 variations at their source. Typically 7 T scanners contain 2nd and some 3rd order spherical harmonic shim coil terms to cancel static ΔB0 variations of low spatial order. In this article, we will motivate the need for improved, higher-order compensation for B0 inhomogeneity and potentially add dynamic control of these fields. We discuss and compare several promising hardware approaches for static and dynamic B0 shimming using either higher-order spherical harmonic shim coils or multi-coil shim arrays as well as passive shimming approaches, and active variants such and adaptive current networks.

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Section: Overview Of B0 Shim Field Generation Hardwarementioning

confidence: 99%

In vivo B 0 field shimming methods for MRI at 7 T

2018

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“…Without a small flip angle approximation, but with some BS shift approximations, excitation at each spatial location for time varying RF and gradients may be calculated. As gradient and traditional RF coils gain more channels and spatial–temporal flexibility, much more can be achieved using multiphoton MRI to encode spatial, spectral, and temporal information.…”

Section: Discussionmentioning

confidence: 99%

Multiphoton magnetic resonance in imaging: A classical description and implementation

Han

2020

Magnetic Resonance in Med

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Funding information UC BerkeleyPurpose: To develop a classical geometric interpretation of multiphoton excitation and apply it to MRI. To investigate ways in which multiphoton excitation can enable novel imaging techniques. Theory and Methods: We present a fully geometric view of multiphoton excitation by taking a particular rotating frame transformation. In this rotating frame, we find that multiphoton excitations appear just like single-photon excitations again, and therefore, we can readily generalize concepts already explored in standard singlephoton excitation. With a homebuilt low frequency coil, we execute a standard slice selective pulse sequence with all of its excitations replaced by their equivalent twophoton versions. In the case of no extra hardware, we use oscillating gradients as a source of extra photons for excitation. Finally, with the multiphoton interpretation of oscillating gradients, we present a novel way to transform a standard slice selective adiabatic inversion pulse into a multiband version without modifying the RF pulse itself. The addition of oscillating gradients creates multiphoton resonances at multiple spatial locations and allows for adiabatic inversions at each location. Results: With Bloch-Siegert shift corrections, analytical multiphoton excitation expressions match with Bloch equation simulations. Two-photon gradient-echo images of a lemon and a pork rib match with their single-photon counterparts.Frequency-offset RF combined with oscillating gradients generate excitation where the RF alone does not. Conclusion: The multiphoton interpretation presents new flexibilities for imaging.Excitation needs not be bound to the Larmor frequency, which opens doors to RF pulse design beyond the usual filter design and the potential for further imaging innovations. K E Y W O R D Sadiabatic, Bloch-Siegert shift, multiband, multiphoton, selective excitation, two-photon ORCID Victor Han https://orcid.org/0000-0003-3093-2549 Chunlei Liu https://orcid.org/0000-0001-8816-4832

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“…Eddy current effects are neglected in this study because of the relatively small coil radius compared to the magnet radius. For larger‐radius applications, either each element must be self‐shielded or a shield can be designed as an array of coils. The power requirements for a gradient coil increase dramatically with increasing radius ; therefore, more powerful amplifiers will be required.…”

Section: Discussionmentioning

confidence: 99%