Yanqin Lin

High-resolution MRS in the presence of field inhomogeneity via intermolecular double-quantum coherences on a 3-T whole-body scanner

Signals from intermolecular double‐quantum coherences (iDQCs) have been shown to be insensitive to macroscopic field inhomogeneities and thus enable acquisition of high‐ resolution MR spectroscopy in the presence of large inhomogeneous fields. In this paper, localized iDQC 1H spectroscopy on a whole‐body 3‐T MR scanner is reported. Experiments with a brain metabolite phantom were performed to quantify characteristics of the iDQC signal under different conditions. The feasibility of in vivo iDQC high‐resolution MR spectroscopy in the presence of large intrinsic and external field inhomogeneity (in the order of hundreds of hertz) was demonstrated in the whole cerebellum of normal volunteers in a scan time of about 6.5 min. Major metabolite peaks were well resolved in the reconstructed one‐dimensional spectra projected from two‐dimensional iDQC acquisitions. Investigations on metabolite ratios, signal‐to‐noise ratio, and line width were performed and compared with results obtained with conventional point‐resolved spectroscopy/MR spectroscopy in homogeneous fields. Metabolite ratios from iDQC results showed excellent consistency under different in vitro and in vivo conditions, and they were similar to those from point‐resolved spectroscopy with small voxel sizes in homogeneous fields. MR spectroscopy with iDQCs can be applied potentially for quantification of gross metabolite changes due to diseases in large brain volumes with high field inhomogeneity. Magn Reson Med, 2010.

Partial homogeneity based high-resolution nuclear magnetic resonance spectra under inhomogeneous magnetic fields

In nuclear magnetic resonance (NMR) technique, it is of great necessity and importance to obtain high-resolution spectra, especially under inhomogeneous magnetic fields. In this study, a method based on partial homogeneity is proposed for retrieving high-resolution one-dimensional NMR spectra under inhomogeneous fields. Signals from series of small voxels, which characterize high resolution due to small sizes, are recorded simultaneously. Then, an inhomogeneity correction algorithm is developed based on pattern recognition to correct the influence brought by field inhomogeneity automatically, thus yielding high-resolution information. Experiments on chemical solutions and fish spawn were carried out to demonstrate the performance of the proposed method. The proposed method serves as a single radiofrequency pulse high-resolution NMR spectroscopy under inhomogeneous fields and may provide an alternative of obtaining high-resolution spectra of in vivo living systems or chemical-reaction systems, where performances of conventional techniques are usually degenerated by field inhomogeneity.

InGaN/GaN quantum wells on self-organized faceted GaN islands: Growth and luminescence studies

The growth behavior and luminescence properties of InGaN/GaN quantum wells (QW) on in situ self-organized GaN islands of various distinct smooth sidewall faceting are simultaneously investigated and directly compared. The QW thickness is found to be specific polar angle dependent, leading to variations in QW thickness on multifaceting islands. As a result, by color tuning through island shaping and modifications of the InGaN/GaN QWs on the faceted islands, polychromatic emissions are achieved.

An Intermolecular Single-Quantum Coherence Detection Scheme for High-Resolution Two-Dimensional J -resolved Spectroscopy in Inhomogeneous Fields

A new pulse sequence based on intermolecular single-quantum coherences (iSQCs) is proposed to achieve high-resolution two-dimensional (2D) J-resolved spectra in inhomogeneous fields via three-dimensional (3D) acquisition. Since the iSQC evolution period and spin echo evolution period in this sequence are intrinsically insensitive to magnetic field inhomogeneities, high-resolution 2D J-resolved spectra can be recovered from nuclei in inhomogeneous fields by projecting the 3D data onto the 2D plane. Analytical expressions of the resulting signals were derived assuming the secular dipole–dipole interaction. Analyses of a solution sample placed in a deliberately unshimmed magnetic field and of a biological sample with intrinsic field inhomogeneities were performed. The results show that this sequence provides an attractive and efficient way to eliminate the influence of field inhomogeneities on 2D J-resolved spectra, which is potentially useful for characterizing complex chemical materials and studying biological metabolites in inhomogeneous fields.

Measuring JHH values with a selective constant-time 2D NMR protocol

Proton-proton scalar couplings play important roles in molecule structure elucidation. However, measurements of JHH values in complex coupled spin systems remain challenging. In this study, we develop a selective constant-time (SECT) 2D NMR protocol with which scalar coupling networks involving chosen protons can be revealed, and corresponding JHH values can be measured through doublets along the F1 dimension. All JHH values within a network of n fully coupled protons can be separately determined with (n-1) SECT experiments. Additionally, the proposed pulse sequence possesses satisfactory sensitivity and handy implementation. Therefore, it will interest scientists who intend to address structural analyzes of molecules with overcrowded spectra, and may greatly facilitate the applications of scalar-coupling constants in molecule structure studies.

High-resolution absorptive intermolecular multiple-quantum coherence NMR spectroscopy under inhomogeneous fields

Intermolecular multiple-quantum coherence (iMQC) is capable of improving NMR spectral resolution using a 2D shearing manipulation method. A pulse sequence termed CT-iDH, which combines intermolecular double-quantum filter (iDQF) with a modified constant-time (CT) scheme, is designed to achieve fast acquisition of high-resolution intermolecular zero-quantum coherences (iZQCs) and intermolecular double-quantum coherences (iDQCs) spectra without strong coupling artifacts. Furthermore, double-absorption lineshapes are first realized in 2D intermolecular multi-quantum coherences (iMQCs) spectra under inhomogeneous fields through a combination of iZQC and iDQC signals to double the resolution without loss of sensitivity. Theoretically the spectral linewidth can be further reduced by half compared to original iMQC high-resolution spectra. Several experiments were performed to test the feasibility of the new method and the improvements are evaluated quantitatively. The study suggests potential applications for in vivo spectroscopy.

High-resolution NMR spectra in inhomogeneous fields utilizing the CRAZED sequence without coherence selection gradients

Coherence selection gradients have been considered as indispensable for high-resolution NMR spectroscopy in inhomogeneous fields utilizing the CRAZED-type sequences. However, our experimental results demonstrate that these gradients can be omitted if an appropriate phase cycling is applied. The measured linewidth of reconstructing 1D high-resolution spectral peaks does not depend on the dipolar correlation distance determined by the coherence selection gradients, but is only affected by diffusion and T(2) relaxation. This finding suggests the need to reconsider the mechanism for the iMQC-based high-resolution spectroscopy.

Line broadening interference for high-resolution nuclear magnetic resonance spectra under inhomogeneous magnetic fields.

Nuclear magnetic resonance spectroscopy serves as an important tool for analyzing chemicals and biological metabolites. However, its performance is subject to the magnetic-field homogeneity. Under inhomogeneous fields, peaks are broadened to overlap each other, introducing difficulties for assignments. Here, we propose a method termed as line broadening interference (LBI) to provide high-resolution information under inhomogeneous magnetic fields by employing certain gradients in the indirect dimension to interfere the magnetic-field inhomogeneity. The conventional spectral-line broadening is thus interfered to be non-diagonal, avoiding the overlapping among adjacent resonances. Furthermore, an inhomogeneity correction algorithm is developed based on pattern recognition to recover the high-resolution information from LBI spectra. Theoretical deductions are performed to offer systematic and detailed analyses on the proposed method. Moreover, experiments are conducted to prove the feasibility of the proposed method for yielding high-resolution spectra in inhomogeneous magnetic fields.

Ultrafast localized two-dimensional magnetic resonance correlated spectroscopy via spatially encoded technique.

To speed up acquisition of localized two‐dimensional (2D) correlated spectroscopy (LCOSY).

A simultaneous multi-slice selective J-resolved experiment for fully resolved scalar coupling information

Proton-proton scalar coupling plays an important role in molecular structure elucidation. Many methods have been proposed for revealing scalar coupling networks involving chosen protons. However, determining all JHH values within a fully coupled network remains as a tedious process. Here, we propose a method termed as simultaneous multi-slice selective J-resolved spectroscopy (SMS-SEJRES) for simultaneously measuring JHH values out of all coupling networks in a sample within one experiment. In this work, gradient-encoded selective refocusing, PSYCHE decoupling and echo planar spectroscopic imaging (EPSI) detection module are adopted, resulting in different selective J-edited spectra extracted from different spatial positions. The proposed pulse sequence can facilitate the analysis of molecular structures. Therefore, it will interest scientists who would like to efficiently address the structural analysis of molecules.