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Origin and correction of magnetic field inhomogeneity at the interface in biphasic NMR samples.

ABSTRACT: The use of susceptibility matching to minimize spectral distortion of biphasic samples layered in a standard 5 mm NMR tube is described. The approach uses magic angle spinning (MAS) to first extract chemical shift differences by suppressing bulk magnetization. Then, using biphasic coaxial samples, magnetic susceptibilities are matched by titration with a paramagnetic salt. The matched phases are then layered in a standard NMR tube where they can be shimmed and examined. Linewidths of two distinct spectral lines, selected to characterize homogeneity in each phase, are simultaneously optimized. Two-dimensional distortion-free, slice-resolved spectra of an octanol/water system illustrate the method. These data are obtained using a 2D stepped-gradient pulse sequence devised for this application. Advantages of this sequence over slice-selective methods are that acquisition efficiency is increased and processing requires only conventional software.

SUBMITTER: Martin BT

PROVIDER: S-EPMC3351538 | biostudies-literature | 2012 May

REPOSITORIES: biostudies-literature

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Origin and correction of magnetic field inhomogeneity at the interface in biphasic NMR samples.

Martin Bryan T BT Chingas G C GC McDougal Owen M OM

Journal of magnetic resonance (San Diego, Calif. : 1997) 20120309

The use of susceptibility matching to minimize spectral distortion of biphasic samples layered in a standard 5 mm NMR tube is described. The approach uses magic angle spinning (MAS) to first extract chemical shift differences by suppressing bulk magnetization. Then, using biphasic coaxial samples, magnetic susceptibilities are matched by titration with a paramagnetic salt. The matched phases are then layered in a standard NMR tube where they can be shimmed and examined. Linewidths of two distinc ...[more]

PMID: 22459062

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Project description:ObjectiveTo correct for image distortions produced by standard Fourier reconstruction techniques on low field permanent magnet MRI systems with strong [Formula: see text] inhomogeneity and gradient field nonlinearities.Materials and methodsConventional image distortion correction algorithms require accurate [Formula: see text] maps which are not possible to acquire directly when the [Formula: see text] inhomogeneities also produce significant image distortions. Here we use a readout gradient time-shift in a TSE sequence to encode the [Formula: see text] field inhomogeneities in the k-space signals. Using a non-shifted and a shifted acquisition as input, [Formula: see text] maps and images were reconstructed in an iterative manner. In each iteration, [Formula: see text] maps were reconstructed from the phase difference using Tikhonov regularization, while images were reconstructed using either conjugate phase reconstruction (CPR) or model-based (MB) image reconstruction, taking the reconstructed field map into account. MB reconstructions were, furthermore, combined with compressed sensing (CS) to show the flexibility of this approach towards undersampling. These methods were compared to the standard fast Fourier transform (FFT) image reconstruction approach in simulations and measurements. Distortions due to gradient nonlinearities were corrected in CPR and MB using simulated gradient maps.ResultsSimulation results show that for moderate field inhomogeneities and gradient nonlinearities, [Formula: see text] maps and images reconstructed using iterative CPR result in comparable quality to that for iterative MB reconstructions. However, for stronger inhomogeneities, iterative MB reconstruction outperforms iterative CPR in terms of signal intensity correction. Combining MB with CS, similar image and [Formula: see text] map quality can be obtained without a scan time penalty. These findings were confirmed by experimental results.DiscussionIn case of [Formula: see text] inhomogeneities in the order of kHz, iterative MB reconstructions can help to improve both image quality and [Formula: see text] map estimation.

Project description:Purpose[Formula: see text] mapping is a powerful tool for studying osteoarthritis (OA) changes and bilateral imaging may be useful in investigating the role of between-knee asymmetry in OA onset and progression. The quantitative double-echo in steady-state (qDESS) can provide fast simultaneous bilateral knee [Formula: see text] and high-resolution morphometry for cartilage and meniscus. The qDESS uses an analytical signal model to compute [Formula: see text] relaxometry maps, which require knowledge of the flip angle (FA). In the presence of [Formula: see text] inhomogeneities, inconsistencies between the nominal and actual FA can affect the accuracy of [Formula: see text] measurements. We propose a pixel-wise [Formula: see text] correction method for qDESS [Formula: see text] mapping exploiting an auxiliary [Formula: see text] map to compute the actual FA used in the model.MethodsThe technique was validated in a phantom and in vivo with simultaneous bilateral knee imaging. [Formula: see text] measurements of femoral cartilage (FC) of both knees of six healthy participants were repeated longitudinally to investigate the association between [Formula: see text] variation and [Formula: see text].ResultsThe results showed that applying the [Formula: see text] correction mitigated [Formula: see text] variations that were driven by [Formula: see text] inhomogeneities. Specifically, [Formula: see text] left-right symmetry increased following the [Formula: see text] correction ([Formula: see text] = 0.74 > [Formula: see text] = 0.69). Without the [Formula: see text] correction, [Formula: see text] values showed a linear dependence with [Formula: see text]. The linear coefficient decreased using the [Formula: see text] correction (from 24.3 ± 1.6 ms to 4.1 ± 1.8) and the correlation was not statistically significant after the application of the Bonferroni correction (p value > 0.01).ConclusionThe study showed that [Formula: see text] correction could mitigate variations driven by the sensitivity of the qDESS [Formula: see text] mapping method to [Formula: see text], therefore, increasing the sensitivity to detect real biological changes. The proposed method may improve the robustness of bilateral qDESS [Formula: see text] mapping, allowing for an accurate and more efficient evaluation of OA pathways and pathophysiology through longitudinal and cross-sectional studies.

Dataset Information

Origin and correction of magnetic field inhomogeneity at the interface in biphasic NMR samples.

Publications

Origin and correction of magnetic field inhomogeneity at the interface in biphasic NMR samples.

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