Ultrafast localized two-dimensional magnetic resonance correlated spectroscopy via spatially encoded technique: Ultrafast Localized COSY Spectroscopy

Localized one-dimensional single voxel magnetic resonance spectroscopy without J coupling modulations

PURPOSE

To acquire single voxel localized one-dimensional ¹ H magnetic resonance spectroscopy (MRS) without J coupling modulations, free from amplitude and phase distortions.

METHODS

A pulse sequence, named PRESSIR, is developed for volume localized MRS without J modulations at arbitrary echo time (TE). The J coupling evolution is suppressed by the J-refocused module that uses a 90° pulse at the midpoint of a double spin echo.

RESULTS

The localization performance of the PRESSIR sequence was tested with a two-compartment phantom. The proposed sequence shows similar voxel localization accuracy as PRESS. Both PRESSIR and PRESS sequences were performed on MRS brain phantom and pig brain tissue. PRESS spectra suffer from amplitude and phase distortions due to J modulations, especially under moderate and long TEs, while PRESSIR spectra are almost free from distortions.

CONCLUSION

The PRESSIR sequence proposed herein enables the acquisition of single voxel in-phase MRS within a single scan. It allows an enhanced signal intensity of J coupling metabolites and reducing undesired broad resonances with short T2s while suppressing J modulations. Moreover, it provides an approach for direct measurement of nonoverlapping J coupling peaks and of transverse relaxation times T2s. Magn Reson Med 76:1661-1667, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

Fast hybrid multi-dimensional NMR methods based on ultrafast 2D NMR

Conventional multi-dimensional (nD) NMR experiments are characterized by inherent long acquisition durations, while ultrafast (UF) NMR makes it possible to reduce to a few hundreds of milliseconds the overall acquisition duration of a complete nD NMR dataset. Although extremely promising for a number of specific applications, the UF strategy suffers from significant limitations compared with its conventional counterpart. The main limitations concern the sensitivity, the resolution, and the accessible spectral width. However, when the targeted applications are compatible with an acquisition duration between a few seconds and a few minutes, hybrid UF techniques can be used to improve the performance of UF nD NMR while remaining faster than conventional acquisitions. Much better results in terms of signal-to-noise ratio can be achieved with the multi-scan single-shot approach or with interleaved acquisitions. Even more, for the same experimental duration, and in the case of homonuclear 2D NMR, the multi-scan single-shot approach has a much higher precision than conventional 2D NMR. Interleaved 2D NMR overcomes the drawbacks of single-scan UF NMR in terms of spectral width and provides spectra for which the quality is not significantly different from that obtained with conventional 2D NMR. Finally, high spectral qualities have been demonstrated from hybrid conventional/UF 3D approaches capable of recording a whole 3D spectrum in the time needed to record a conventional 2D spectrum. This mini-review aims at describing the principles, the recent advances and the latest applications of these hybrid techniques. Copyright © 2015 John Wiley & Sons, Ltd.

High-resolution localized spatiotemporal encoding correlated spectra under inhomogeneous magnetic fields via asymmetrical gradient encoding/decoding

Applications of conventional localized nuclear magnetic resonance correlated spectroscopy are restrained by long acquisition times and poor performance under inhomogeneous magnetic fields. Here, a method that combines the spatiotemporal encoding technique with the localization technique and implements the encoding and decoding in unison with suitable asymmetrical gradients is proposed to obtain high-resolution localized correlated spectra under inhomogeneous fields in greatly reduced times. Experiments on phantom solutions prove its insensitivity to linear field inhomogeneities along three orthogonal axes. Moreover, this method is applied to adipose study of marrow tissue with resolution improvements. The proposed method may offer promising perspectives for fast analyses of biological tissues.

Reverse detection for spectral width improvements in spatially encoded dimensions of ultrafast two-dimensional NMR spectra

Recently, the spatially encoded technique has been broadly used in the fast analyses of chemical systems and real-time detections of chemical reactions. In spatially encoded ultrafast 2D spectra, spectral widths and resolution in spatially encoded dimensions are contradictive, leading to the risk of insufficient spectral widths when providing satisfactory resolution values for all resonances. Here, a method named as reverse detection is proposed to improve the spectral width in the spatially encoded dimension. Experimental results show that spectral width improvements are at least twofold with reverse detection solely, and more improvements can be expected along with the gradient-controlled folding method. The proposed method can be applied to almost any spatially encoded scheme with echo planar spectroscopic imaging--like detection module and may promote wide applications of ultrafast 2D spectroscopy techniques in chemical analyses.