Nuclear Magnetic Resonance with the Distant Dipolar Field

C. A. Corum
186 pages, 43 figures, Ph.D. Dissertation, University of Arizona, Optical Sciences Center

Distant dipolar field (DDF)-based nuclear magnetic resonance is an active research area with many fundamental properties still not well understood. Already several intriguing applications have developed, like HOMOGENIZED and IDEAL spectroscopy, that allow high resolution spectra to be obtained in inhomogeneous fields, such as in-vivo. The theoretical and experimental research in this thesis concentrates on the fundamental signal properties of DDF-based sequences in the presence of relaxation (T1 and T2) and diffusion. A general introduction to magnetic resonance phenomenon is followed by a more in depth introduction to the DDF and its effects. A novel analytical signal equation has been developed to describe the effects of T2 relaxation and diffusing spatially modulated longitudinal spins during the signal build period of an HOMOGENIZED cross peak. Diffusion of the longitudinal spins results in a lengthening of the effective dipolar demagnetization time, delaying the re-phasing of coupled anti-phase states in the quantum picture. In the classical picture the unwinding rate of spatially twisted magnetization is no longer constant, but decays exponentially with time. The expression is experimentally verified for the HOMOGENIZED spectrum of 100mM TSP in H2O at 4.7T. Equations have also been developed for the case of multiple repetition steady state 1d and 2d spectroscopic sequences with incomplete magnetization recovery, leading to spatially varying longitudinal magnetization. Experimental verification has been accomplished by imaging the profile. The equations should be found generally applicable for those interested in DDF-based spectroscopy and imaging.

Multiple-Quantum Vector Imaging

Abstract of talk 420 from ISMRM 2005 Meeting (need password for access)
L-S. Bouchard, W. S. Warren

A novel methodology based on measurements of the distant dipolar field, or intermolecular multiple-quantum coherences, is presented for tracking the vector orientation of parallel fiber bundles or other anisotropic structures in materials or biological tissues. The method uses a CRAZED sequence (Warren et al., Science 262:2005-9, 1993) with correlation gradient applied along the X, Y and Z axes. A subtraction |Gx|+|Gy|-|Gz| gives a signal when structural anisotropy is present and zero when the material is isotropic. For a material consisting of oriented fibers, the strength of the subtraction is related to the polar angle of the directional vector of the fibers. A comparison of X and Y gradients is related to the azimuthal angle. Experimental results in the petiole of celery show that anisotropy can be readily detected by this method. Tilting the sample with respect to the applied field, and rotating the sample in the transverse plane allow gave subtraction results that are consistent with theoretical calculations and demonstrate its applicability. The method can clearly detect structural anisotropy even for very weak gradient pulses well outside any diffusion-weighted regime. The structure sizes that can be detected are on the order of the length of the correlation distance. Thus, material heterogeneities can be detected on the tens of microns to millimeters scales and the method nicely complements currently existing techniques such as diffusion tensor imaging or high resolution MRI. This method finds potential applications in the materials and biomedical sciences. It could perhaps be of use in detecting tumor vascularity or for mapping trabecular bone anisotropy.

Effect of Hypercapnia on the iDQC Response

Abstract of talk 117 from ISMRM 2005 Meeting (need password for access)
A. Schäfer, S. Zysset, W. Heinke, H. E. Möller

To date the origin of the fMRI signal change based on intermolecular double quantum coherences (iDQC) is not well understood. The aim of this work was to investigate the sensitivity to global changes of the cerebral blood flow (CBF) of this method. Global CBF changes can be induced by changing the arterial partial pressure of carbon dioxide due to hypercapnia. While the number of activated voxels was significantly decreased, the percentage signal change was roughly seven-fold increased in iDQC-based imaging as compared to conventional BOLD contrast.

Selection of intra- or inter-molecular multiple-quantum coherences in NMR of highly polarized solution

Xiaoqin Zhu, Zhong Chen, Shuhui Cai and Jianhui Zhong
published in the Physica B, Condensed matter [0921-4526] yr:2005 vol:362 iss:1-4
pg: 286 -294


Abstract
In highly polarized scalar-coupled liquid systems, nuclear magnetic resonance signals from multiple-quantum coherences (MQCs) may be formed by inter-molecular dipolar and/or intra-molecular scalar couplings. Selection of specific signals can simplify the spectra, which may help us understand the underlying physical mechanisms in complex coupled spin systems. In this paper, a pulse sequence with three selective radio-frequency (RF) pulses and phase cycling was designed for this purpose. For an IpSq (p, q=1,2,3,…) spin system, there are three kinds of MQC signals, which originate from intra-molecular I–S, inter-molecular I–S, and inter-molecular S–S (or I–I) coherences. These three kinds of signals can be detected separately by proper phase cycling of RF pulses, which is independent of coupling constants. The intra- and inter-molecular MQC signals can also be detected separately with specific preparation periods, but this method is sensitive to coupling constants. Our theoretical predictions are in good agreement with experimental observations. The method proposed herein can be extended to heteronuclear cases.
Keywords: NMR; Inter-molecular MQCs; Intra-molecular MQCs; Dipolar couplings; Scalar couplings; Phase cycling
PACS: 76.60.−k; 76.80.+y; 75.40.Gb; 39.30.+w

Nuclear Magnetic Resonance Studies of Quadrupolar Nuclei and Dipolar Field Effects

Jeffry Todd Urban
Ph.D. Dissertation
Doctor of Philosophy in Chemistry
University of California, Berkeley
Advisor: Professor Alexander Pines
2004

Experimental and theoretical research conducted in two areas in the field of nuclear magnetic resonance (NMR) spectroscopy is presented: (1) studies of the coherent quantum-mechanical control of the angular momentum dynamics of quadrupolar (spin I > 1/2) nuclei and its application to the determination of molecular structure; and (2) applications of the long-range nuclear dipolar field to novel NMR detection methodologies. The dissertation is organized into six chapters. The first two chapters and associated appendices are intended to be pedagogical and include an introduction to the quantum-mechanical theory of pulsed NMR spectroscopy and the time dependent theory of quantum mechanics. The third chapter describes investigations of the solid-state multiple-quantum magic angle spinning (MQMAS) NMR experiment applied to I = 5/2 quadrupolar nuclei. This work reports the use of rotary resonance-matched radiofrequency irradiation for sensitivity enhancement of the I = 5/2 MQMAS experiment. These experiments exhibited certain selective line narrowing effects which were investigated theoretically. The fourth chapter extends the discussion of multiple quantum spectroscopy of quadrupolar nuclei to a mostly theoretical study of the feasibility of enhancing the resolution of nitrogen-14 NMR of large biomolecules in solution via double-quantum spectroscopy. The fifth chapter continues to extend the principles of multiple quantum NMR spectroscopy of quadrupolar nuclei to make analogies between experiments in NMR/nuclear quadrupolar resonance (NQR) and experiments in atomic/molecular optics (AMO). These analogies are made through the Hamiltonian and density operator formalism of angular momentum dynamics in the presence of electric and magnetic fields. The sixth chapter investigates the use of the macroscopic nuclear dipolar field to encode the NMR spectrum of an analyte nucleus indirectly in the magnetization of a sensor nucleus. This technique could potentially serve as an encoding module for the recently developed NMR remote detection experiment. The feasibility of using hyperpolarized xenon-129 gas as a sensor is discussed. This work also reports the use of an optical atomic magnetometer to detect the nuclear magnetization of Xe-129 gas, which has potential applicability as a detection module for NMR remote detection experiments.

NMR detection using laser-polarized xenon as a dipolar sensor

J. Granwehr, J.T. Urban, A.H. Trabesinger and A. Pines
Journal of Magnetic Resonance
Article in Press, Corrected Proof

Hyperpolarized ¹²⁹Xe can be used as a sensor to indirectly detect NMR spectra of heteronuclei that are neither covalently bound nor necessarily in direct contact with the Xe atoms, but coupled through long-range intermolecular dipole–dipole interactions. To reintroduce long-range dipolar couplings the sample symmetry has to be broken. This can be done either by using an asymmetric sample arrangement, or by breaking the symmetry of the spin magnetization with field gradient pulses. Experiments are performed where only a small fraction of the available ¹²⁹Xe magnetization is used for each point, so that a single batch of xenon suffices for the point-by-point acquisition of a heteronuclear NMR spectrum. Examples with ¹H as the analyte nucleus show that these methods have the potential to obtain spectra with a resolution that is high enough to determine homonuclear J couplings. The applicability of this technique with remote detection is discussed.