Combined passive and active shimming for in vivo MR spectroscopy at high magnetic fields

Juchem, Christoph; Müller‐Bierl, Bernd; Schick, Fritz; Logothetis, Nikos K.; Pfeuffer, Josef

doi:10.1016/j.jmr.2006.09.002

Cited by 51 publications

(30 citation statements)

References 27 publications

(33 reference statements)

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“…The quality of spectra from voxels at the border areas of the brain and the associated metabolite maps could be enhanced by improved B 0 correction of these areas, e.g. by additional passive shimming (41). This is of special importance as signal loss due to T 2 Ã relaxation has to be considered when using FIDLOVS with long acquisition delays.…”

Section: Discussionmentioning

confidence: 99%

Slice‐selective FID acquisition, localized by outer volume suppression (FIDLOVS) for ¹H‐MRSI of the human brain at 7 T with minimal signal loss

et al. 2009

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In comparison to 1.5 and 3 T, MR spectroscopic imaging at 7 T benefits from signal-to-noise ratio (SNR) gain and increased spectral resolution and should enable mapping of a large number of metabolites at high spatial resolutions. However, to take full advantage of the ultra-high field strength, severe technical challenges, e.g. related to very short T(2) relaxation times and strict limitations on the maximum achievable B(1) field strength, have to be resolved. The latter results in a considerable decrease in bandwidth for conventional amplitude modulated radio frequency pulses (RF-pulses) and thus to an undesirably large chemical-shift displacement artefact. Frequency-modulated RF-pulses can overcome this problem; but to achieve a sufficient bandwidth, long pulse durations are required that lead to undesirably long echo-times in the presence of short T(2) relaxation times. In this work, a new magnetic resonance spectroscopic imaging (MRSI) localization scheme (free induction decay acquisition localized by outer volume suppression, FIDLOVS) is introduced that enables MRSI data acquisition with minimal SNR loss due to T(2) relaxation and thus for the first time mapping of an extended neurochemical profile in the human brain at 7 T. To overcome the contradictory problems of short T(2) relaxation times and long pulse durations, the free induction decay (FID) is directly acquired after slice-selective excitation. Localization in the second and third dimension and skull lipid suppression are based on a T(1)- and B(1)-insensitive outer volume suppression (OVS) sequence. Broadband frequency-modulated excitation and saturation pulses enable a minimization of the chemical-shift displacement artefact in the presence of strict limits on the maximum B(1) field strength. The variable power RF pulses with optimized relaxation delays (VAPOR) water suppression scheme, which is interleaved with OVS pulses, eliminates modulation side bands and strong baseline distortions. Third order shimming is based on the accelerated projection-based automatic shimming routine (FASTERMAP) algorithm. The striking SNR and spectral resolution enable unambiguous quantification and mapping of 12 metabolites including glutamate (Glu), glutamine (Gln), N-acetyl-aspartatyl-glutamate (NAAG), gamma-aminobutyric acid (GABA) and glutathione (GSH). The high SNR is also the basis for highly spatially resolved metabolite mapping.

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Section: Discussionmentioning

confidence: 99%

Slice‐selective FID acquisition, localized by outer volume suppression (FIDLOVS) for ¹H‐MRSI of the human brain at 7 T with minimal signal loss

et al. 2009

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“…The Oxford group showed some success with this approach using diamagnetic graphite (Wilson and Jezzard, 2003; Wilson et al, 2002). This approach is typically used in conjunction with the scanner spherical harmonic shims (Juchem et al, 2006). …”

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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“…Because the AC44 system was designed for a 3 T human head scanner, the second-order shim capabilities were far too small for shimming the monkey brain in our 7 T MR system. For the experiments conducted with the AC44 gradient and shim system, a combined passive and active shimming method was developed to overcome the severe hardware limitations for second-order shimming [22]. The optimization of our methodology and the current results reported in this study were obtained using both gradient and shim inserts, the BGA38 and subsequently installed AC44 systems.…”

Section: Mr System and Experimental Setupmentioning

confidence: 99%

“…Depending on the head anatomy and rotation as well as the positioning of the monkey, the second-order field distortions ranged up to 75 Hz/cm 2 (for the Z2 shim term) in the monkey visual cortex at 7 T [22]. The third-order and fourth-order terms were dominated by the tesseral shim terms Z3 and Z4, with values below F20 Hz/cm 3 and F10 Hz/cm 4 , respectively, for a 15Â15Â15-mm 3 cube volume.…”

Section: Shim Strategy For Rapid Removal Of the Field Inhomogeneitiesmentioning

confidence: 99%

1H-MRS of the macaque monkey primary visual cortex at 7 T: strategies and pitfalls of shimming at the brain surface

Juchem

Logothetis

Pfeuffer

2007

Magnetic Resonance Imaging

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Combined passive and active shimming for in vivo MR spectroscopy at high magnetic fields

Cited by 51 publications

References 27 publications

Slice‐selective FID acquisition, localized by outer volume suppression (FIDLOVS) for ¹H‐MRSI of the human brain at 7 T with minimal signal loss

Slice‐selective FID acquisition, localized by outer volume suppression (FIDLOVS) for ¹H‐MRSI of the human brain at 7 T with minimal signal loss

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

1H-MRS of the macaque monkey primary visual cortex at 7 T: strategies and pitfalls of shimming at the brain surface

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Combined passive and active shimming for in vivo MR spectroscopy at high magnetic fields

Cited by 51 publications

References 27 publications

Slice‐selective FID acquisition, localized by outer volume suppression (FIDLOVS) for 1H‐MRSI of the human brain at 7 T with minimal signal loss

Slice‐selective FID acquisition, localized by outer volume suppression (FIDLOVS) for 1H‐MRSI of the human brain at 7 T with minimal signal loss

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

1H-MRS of the macaque monkey primary visual cortex at 7 T: strategies and pitfalls of shimming at the brain surface

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Slice‐selective FID acquisition, localized by outer volume suppression (FIDLOVS) for ¹H‐MRSI of the human brain at 7 T with minimal signal loss

Slice‐selective FID acquisition, localized by outer volume suppression (FIDLOVS) for ¹H‐MRSI of the human brain at 7 T with minimal signal loss