Designing MR Shim Arrays With Irregular Coil Geometry: Theoretical Considerations

While, Peter T.; Korvink, Jan G.

doi:10.1109/tbme.2013.2293842

Cited by 18 publications

(6 citation statements)

References 24 publications

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“…In future designs, elements may be positioned with an eye toward trading off parallel imaging and shim performance, with priority accorded to one or the other metric depending on the application. An additional degree of freedom is the use of irregular element shapes, which show improved accuracy and efficiency for generating low‐order Δ B 0 fields in recent MC shim array simulations .…”

Section: Discussionmentioning

confidence: 99%

A 32‐channel combined RF and B₀ shim array for 3T brain imaging

Stockmann

Witzel

Keil

et al. 2015

Magnetic Resonance in Med

123

182

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Purpose 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. Methods Shim performance using constrained DC current (±2.5A) is simulated for brain arrays ranging from 8 to 128 elements. A 32-channel 3-tesla brain array is realized using inductive chokes to bridge the tuning capacitors on each RF loop. The RF and B0 shimming performance is assessed in bench and imaging measurements. Results The addition of DC currents to the 32-channel RF array is achieved with minimal disruption of the RF performance and/or negative side effects such as conductor heating or mechanical torques. The shimming results agree well with simulations and show performance superior to third-order spherical harmonic (SH) shimming. Imaging tests show the ability to reduce the standard frontal lobe susceptibility-induced fields and improve echo planar imaging geometric distortion. The simulation of 64- and 128-channel brain arrays suggest that even further shimming improvement is possible (equivalent to up to 6th-order SH shim coils). Conclusion Including user-controlled shim currents on the loops of a conventional highly parallel brain array coil is feasible with modest current levels and produces improved B0 shimming performance over standard second-order SH shimming.

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Section: Discussionmentioning

confidence: 99%

A 32‐channel combined RF and B₀ shim array for 3T brain imaging

Stockmann

Witzel

Keil

et al. 2015

Magnetic Resonance in Med

123

182

View full text Add to dashboard Cite

show abstract

“…Simulations of shim performance in Juchem et al (2015a) show that the 48ch MC shim system can in principle outperform global 1st–5th order and dynamic 1st–4th order SH shimming. Other researchers have simulated the added benefit of using irregular optimized loop shapes (Zivkovic et al, 2016; While and Korvink, 2014). …”

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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“…Thus advances in B 0 shimming hardware and software are required to exploit the full potential of spinal cord 1 H MRS at 7 T. Potentially useful shim methodology includes constrained image based B 0 shimming considering real B 0 shim field distributions, ensuring convergence of the optimization routine in the presence of hardware constraints and non‐orthogonal fields and offering the possibility to trade the shim quality between different regions of interest . Higher order shim systems , multi‐coil B 0 shimming or numerically optimized shim coils might further enhance the B 0 homogeneity and thus the spectral quality in the spinal cord.…”

Section: Discussionmentioning

confidence: 99%

¹H MRS in the human spinal cord at 7 T using a dielectric waveguide transmitter, RF shimming and a high density receive array

et al. 2016

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Multimodal MRI is the state of the art method for clinical diagnostics and therapy monitoring of the spinal cord, with MRS being an emerging modality that has the potential to detect relevant changes of the spinal cord tissue at an earlier stage and to enhance specificity. Methodological challenges related to the small dimensions and deep location of the human spinal cord inside the human body, field fluctuations due to respiratory motion, susceptibility differences to adjacent tissue such as vertebras and pulsatile flow of the cerebrospinal fluid hinder the clinical application of (1) H MRS to the human spinal cord. Complementary to previous studies that partly addressed these problems, this work aims at enhancing the signal-to-noise ratio (SNR) of (1) H MRS in the human spinal cord. To this end a flexible tight fit high density receiver array and ultra-high field strength (7 T) were combined. A dielectric waveguide and dipole antenna transmission coil allowed for dual channel RF shimming, focusing the RF field in the spinal cord, and an inner-volume saturated semi-LASER sequence was used for robust localization in the presence of B1 (+) inhomogeneity. Herein we report the first 7 T spinal cord (1) H MR spectra, which were obtained in seven independent measurements of 128 averages each in three healthy volunteers. The spectra exhibit high quality (full width at half maximum 0.09 ppm, SNR 7.6) and absence of artifacts and allow for reliable quantification of N-acetyl aspartate (NAA) (NAA/Cr (creatine) 1.31 ± 0.20; Cramér-Rao lower bound (CRLB) 5), total choline containing compounds (Cho) (Cho/Cr 0.32 ± 0.07; CRLB 7), Cr (CRLB 5) and myo-inositol (mI) (mI/Cr 1.08 ± 0.22; CRLB 6) in 7.5 min in the human cervical spinal cord. Thus metabolic information from the spinal cord can be obtained in clinically feasible scan times at 7 T, and its benefit for clinical decision making in spinal cord disorders will be investigated in the future using the presented methodology. Copyright © 2016 John Wiley & Sons, Ltd.

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Designing MR Shim Arrays With Irregular Coil Geometry: Theoretical Considerations

Cited by 18 publications

References 24 publications

A 32‐channel combined RF and B₀ shim array for 3T brain imaging

A 32‐channel combined RF and B₀ shim array for 3T brain imaging

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

¹H MRS in the human spinal cord at 7 T using a dielectric waveguide transmitter, RF shimming and a high density receive array

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Designing MR Shim Arrays With Irregular Coil Geometry: Theoretical Considerations

Cited by 18 publications

References 24 publications

A 32‐channel combined RF and B0 shim array for 3T brain imaging

A 32‐channel combined RF and B0 shim array for 3T brain imaging

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

1H MRS in the human spinal cord at 7 T using a dielectric waveguide transmitter, RF shimming and a high density receive array

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Product

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A 32‐channel combined RF and B₀ shim array for 3T brain imaging

A 32‐channel combined RF and B₀ shim array for 3T brain imaging

¹H MRS in the human spinal cord at 7 T using a dielectric waveguide transmitter, RF shimming and a high density receive array