Fast iterative pre‐emphasis calibration method enabling third‐order dynamic shim updated fMRI

Fillmer, Ariane; Vannesjo, S. Johanna; Pavan, Matteo; Scheidegger, Milan; Pruessmann, Klaas P.; Henning, A

doi:10.1002/mrm.25695

Cited by 22 publications

(45 citation statements)

References 26 publications

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“…These functions are used to create a linear time-invariant model of all coil-field dynamics which can then be inverted to obtain pre-emphasis waveforms that null all unwanted dynamics. This approach has been used to achieve 0th–3rd order shim updating with settling times on the order of 1 ms (Vannesjo et al, 2016) or 2 ms (Fillmer et al, 2016). Field camera-monitored dynamic 1st–3rd order SH shimming has been used to acquire fMRI time series EPI slices (Fillmer et al, 2016) that show no visible eddy current artifacts.…”

Section: Overview Of B0 Shim Field Generation Hardwarementioning

confidence: 99%

“…This approach has been used to achieve 0th–3rd order shim updating with settling times on the order of 1 ms (Vannesjo et al, 2016) or 2 ms (Fillmer et al, 2016). Field camera-monitored dynamic 1st–3rd order SH shimming has been used to acquire fMRI time series EPI slices (Fillmer et al, 2016) that show no visible eddy current artifacts. Notably, limitations on shim amplifier voltage and current headroom prevented the full benefit of the pre-emphasis from being realized in these experiments.…”

Section: Overview Of B0 Shim Field Generation Hardwarementioning

confidence: 99%

“…This result suggests that the shim currents are relatively stable during the EPI readouts; however, the experiment did not compare tSNR with and without the SH insert coil present in the bore. For the case of dynamic SH shimming, Fillmer et al (2016) shows resting-state fMRI correlation maps acquired with pre-emphasized higher-order dynamic SH shimming, but the paper does not report the tSNR compared to baseline data acquired with static 1st–2nd order shimming. This is an important figure of merit that would be useful to measure in future work on dynamic SH shimming.…”

Section: Overview Of B0 Shim Field Generation Hardwarementioning

confidence: 99%

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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%

Section: Overview Of B0 Shim Field Generation Hardwarementioning

confidence: 99%

Section: Overview Of B0 Shim Field Generation Hardwarementioning

confidence: 99%

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

2018

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“…Therefore, only three translation and three rotation parameters

τ = [x, y, z, θ, φ, ρ]

need to be found to find the optimal positions of all the field probes. The optimization problem from Equation then becomes:

\min_{τ = [x, y, z, θ, φ, ρ]} \sum_{p = 1}^{N_{p}} {‖ {bold-italicf}_{m e a s} - γ G false(r (bold-italicτ) false) ‖}^{2} .

…”

Section: Methodsmentioning

confidence: 99%

Constrained optimization for position calibration of an NMR field camera

Chang

Nassirpour

Eschelbach

et al. 2017

Magnetic Resonance in Med

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“…An advantage of MC shim arrays are their intrinsic similarity to multi‐element receive and transmit RF coil arrays. As a natural extension of this latter concept, a merging both of RF and shim functions into a single device has recently been proposed by several groups . The advantage of such a combined device is the prospect for efficient packaging into a single closely coupled (tight fitting) coil array.…”

Section: Introductionmentioning

confidence: 99%

Comparison of new element designs for combined RF‐shim arrays at 7 T

Winkler¹,

Warr

Stockmann

et al. 2018

Concepts Magn Reson

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Purpose: To identify novel concepts for RF-shim loop architectures suitable for 7T made of two concentric conducting loops fulfilling RF and DC functions, respectively, and to determine their relative SNR performance. The goal is to minimize interference between the two systems while making efficient use of the space closest to the body. Theory: We show by means of theoretical derivation of the frequency spectrum that the proposed two-loop structure exhibits an anti-resonant null and a resonant peak in the frequency domain. Methods: The proposed structure is comprised of two concentric wire loops either arranged as nested loops or in the form of a coaxial cable, in which the two conductors carry the RF and shim signals, respectively. We use theory, simulation, and phantom measurements to obtain frequency spectra and SNR maps for the proposed structures. Results: Retained SNR is found to be 75% in the coaxial loop and ranges from 57% to 67% in three different coaxial configurations. We have found both implementations to be a viable concept for the use in RF-shim devices if remaining SNR limitations can be overcome. Conclusions: We have investigated two new design modalities in 7T RF-shim coil design that separate the RF and shim conductors such that the previously proposed toroidal chokes are eliminated – thereby removing undesired additional loss, bulk, and design complexity.

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Fast iterative pre‐emphasis calibration method enabling third‐order dynamic shim updated fMRI

Abstract: The proposed method enables accurate pre-emphasis calibration, and therefore, the application of dynamic shim updating to fMRI.

Cited by 22 publications

References 26 publications

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

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

Constrained optimization for position calibration of an NMR field camera

Comparison of new element designs for combined RF‐shim arrays at 7 T

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