Yulan Lin

Undersampled MRI reconstruction with patch-based directional wavelets

Compressed sensing has shown great potential in reducing data acquisition time in magnetic resonance imaging (MRI). In traditional compressed sensing MRI methods, an image is reconstructed by enforcing its sparse representation with respect to a preconstructed basis or dictionary. In this paper, patch-based directional wavelets are proposed to reconstruct images from undersampled k-space data. A parameter of patch-based directional wavelets, indicating the geometric direction of each patch, is trained from the reconstructed image using conventional compressed sensing MRI methods and incorporated into the sparsifying transform to provide the sparse representation for the image to be reconstructed. A reconstruction formulation is proposed and solved via an efficient alternating direction algorithm. Simulation results on phantom and in vivo data indicate that the proposed method outperforms conventional compressed sensing MRI methods in preserving the edges and suppressing the noise. Besides, the proposed method is not sensitive to the initial image when training directions.

High-Resolution 2D J-Resolved Spectroscopy in Inhomogeneous Fields with Two Scans

A scheme via spatially encoded intermolecular zero-quantum coherences was proposed for high-resolution 2D J-resolved spectra in inhomogeneous fields with high acquisition efficiency. Compared to a recent paper (Pelupessy et al. Science, 2009, 324, 1693-1697), the novel method can obtain chemical shifts and J multiplicity patterns directly.

Ultrafast 2D COSY with constant-time phase-modulated spatial encoding

Recently ultrafast techniques enable 2D NMR spectra to be obtained in a single scan. They have been successfully applied for 2D COSY, TOCSY, DOSY, HMQC, and J-resolved spectra. In this paper, two alternative ultrafast 2D COSY methods (g-COSY and gDQF-COSY) based on continuous constant-time phase-modulated spatial encoding were proposed. Theoretical expressions of the resulting signals were deduced. Experiments were performed to verify our theoretical analysis and the feasibility of the methods. Comparisons between the experimental results of our methods and those of the previous real-time phase-modulated spatial encoding method demonstrate that the signal-to-noise ratio and resolution of the 2D COSY spectra are improved, and a good 2D COSY spectrum is easier to achieve by using our methods.

High-resolution NMR spectroscopy in inhomogeneous fields

High-resolution NMR spectroscopy, providing information on chemical shifts, J coupling constants, multiplet patterns, and relative peak areas, is a mainstream tool for analysis of molecular structures, conformations, compositions, and dynamics. Generally, a homogeneous magnetic field is a prerequisite for obtaining high-resolution NMR information. Magnetic field inhomogeneity, whether from non-ideal experimental conditions or from intrinsic magnetic susceptibility discontinuities in samples, represents a hurdle for applications of high-resolution NMR. Numerous techniques have been proposed for measuring high-resolution NMR spectra free from the influence of inhomogeneous magnetic fields. Besides developments and improvements in NMR instrumentation, various types of experimental approaches have been established for recovering NMR information in inhomogeneous magnetic fields. Three main types are systematically described in this review. In addition, other high-resolution NMR approaches or data processing methods are also briefly described. All high-resolution NMR approaches covered in this review have individual advantages and disadvantages in practical applications, and no one technique is applicable to all practical circumstances. Hence, they are complementary for high-resolution NMR applications in inhomogeneous fields. The underlying mechanisms of these approaches are presented, together with analyses of their applicability and efficiency.

Intermolecular Zero Quantum Coherence in NMR Spectroscopy

Abstract Intermolecular zero-quantum coherences (iZQCs) originating from distant dipolar interactions between spins in different molecules provide an innovative way for nuclear magnetic resonance (NMR) spectroscopy. Since the field experienced by iZQC signals is in the dipolar correlation distance, it is naturally to apply iZQCs for resolution enhancement in NMR spectroscopy, especially under the spatially and/or temporally varying magnetic fields. Two theoretical frames, classical distant dipolar field and quantum-mechanical intermolecular multiple-quantum coherence treatments, are available for the description of iZQC signal evolution. A variety of iZQC spectroscopic techniques have been established and most of them take advantage of two-dimensional acquisition to recover a one-dimensional high-resolution spectrum with information on chemical shifts, relative peak areas, J -coupling constants, and multiplet patterns. In this review, two comprehensive descriptions of an iZQC signal are presented first. The existing iZQC techniques are then systematically described. The details and underlying mechanisms of these techniques are discussed. Finally, the in vivo applications of iZQC spectroscopic techniques are given.

Accurate Measurement of Small J Couplings

Abstract Small J coupling constants are closely relevant to the assignment of conformational preferences for medium- to large-sized molecules and the valuable structural information about the backbone dihedral angles of biological molecules. Thus, accurate measurement for small J coupling constants is extremely important, which has long been recognized. However, the accurate measurement of small J coupling constants is limited by the natural linewidth since the line splitting arising from small J couplings are usually merged into the related peaks. A cornucopia of NMR spectral techniques has been designed for the achievement of small J coupling information. Generally, the existing techniques can be divided into two types. One is to directly extract small J coupling constants without J multiplication, while another is on the basis of J multiplication. Performance of the existing techniques has been evaluated and the results show that these approaches can be adapted for a wide range of samples, ranging from complex chemical molecules to biological samples, even for the samples under inhomogeneous magnetic fields. These spectral editing techniques provide complementary strategies for the quantification of small J coupling constants. In this review, the mechanisms of these approaches are presented with the analysis of their corresponding applicability and efficiencies.

Flat pancake distant dipolar fields for enhancement of intermolecular multiple-quantum coherence signals

Intermolecular multiple-quantum coherences (iMQCs) originated from distant dipolar field (DDF) possess some appealing unique properties for magnetic resonance imaging (MRI). DDF is usually induced with continuous wave (i.e., sine- or square-wave) magnetization modulation in the whole sample. In this article, a spatially localized and enhanced DDF was optimally tailored in a thin slice with an adiabatic inversion pulse. Evidence was provided to show that careful tailoring of the spatially localized DDF can generate highly efficient iMQC signals, with more than two-fold enhancement compared to the conventional sine-wave magnetization modulation method, and 1.5 times of that with the square-wave modulation under the similar condition. Theoretical predictions, simulation results, and experimental verifications agree well with each other. Practical implementation of this approach for efficient iMQC MRI was explored.

Combining Fourier phase encoding and broadband inversion toward J-edited spectra

Nuclear magnetic resonance (NMR) spectra are often utilized for gathering accurate information relevant to molecular structures and composition assignments. In this study, we develop a homonuclear encoding approach based on imparting a discrete phase modulation of the targeted cross peaks, and combine it with a pure shift experiments (PSYCHE) based J-modulated scheme, providing simple 2D J-edited spectra for accurate measurement of scalar coupling networks. Chemical shifts and J coupling constants of protons coupled to the specific protons are demonstrated along the F2 and F1 dimensions, respectively. Polychromatic pulses by Fourier phase encoding were performed to simultaneously detect several coupling networks. Proton-proton scalar couplings are chosen by a polychromatic pulse and a PSYCHE element. Axis peaks and unwanted couplings are complete eradicated by incorporating a selective COSY block as a preparation period. The theoretical principles and the signal processing procedure are laid out, and experimental observations are rationalized on the basis of theoretical analyses.