k, q & b - Moderne Kodierungsstrategien von Fourier-konjugierten Variablen der Bewegung in der Kernspinresonanz
- k, q & b - modern encoding strategies of movement fourier conjugate variables in nuclear magnetic resonance
Offer, Patrick Markus Dirk; Blümich, Bernhard (Thesis advisor); Raabe, Christian Julius (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2019
The emphasis in this dissertation was placed on extending known derivations, which are necessary for the accurate description of the nuclear magnetic resonance signal, to a higher number of dimensions and explicitly stating mathematical transformations of shortened literature known derivations. In this context, an approach for the systematic description of coherent motion of isochromates in an inhomogeneous field presented by Blümich was extended to n dimensions. In addition to the description of the basics of spectroscopy, diffusion and imaging, a general approach to encode different Fourier-conjugated terms was developed. This approach was extended to practically achievable gradient impulses by parameterization of the impulse, so that arbitrary combinations of consecutive pulses can be calculated automatically. An additional part of the dissertation deals with incoherent motion: It was discussed in detail, how Fourier-conjugated variables have to be encoded when anisotropic diffusion is present and the main directors for anisotropic diffusion are aligned randomly. Furthermore, it was shown how the single, double, triple and continuous diffusion encoding was implemented in Aachen on the Bruker spectrometers. Based on the parameterization of gradient impulses, generally valid solutions for trapezoidal gradient impulses were presented, which allows the generation of expressions for arbitrary diffusion encodings. The functionality of the pulse sequences was ensured by reference measurements on ultrapure water, a suspension of yeast cells and Aerosol OT. The investigation of two non-ionic surfactant systems using triple diffusion encoding and 2H-NMR spectroscopy quantitatively showed how the anisotropy of the lamellar phase depends on temperature and surfactant concentration: With increasing temperature and surfactant concentration the anisotropy increases, which is accompanied by fewer defects in the lamellae. This allows a clear differentiation of the transition from a gyroid structure - already known in the literature - to a (nearly) defect free lamellar structure. Furthermore, the robustness of the pulse sequence towards randomly aligned lamellae was demonstrated for triple diffusion coding: In contrast to previous publications, no idealized alignment of the lamellae had to be created. Since the measurements are performed in thermal equilibrium, this measurement method offers advantages over other methods for which this is not possible. Further measurements of this work deal with single or double diffusion encoding of water molecules located in a lattice of small polystyrene spheres. Using a model described by Callaghan et al., it was possible to quantify the size of the spheres, their distance and the reduction of the diffusion coefficient for single diffusion encoding. It could be shown that the face-centered cubic packing is a valid candidate for the arrangement of the polystyrene spheres. For different variants of double diffusion coding there are significant differences in the signal shape in the range of low coding times, in which the short pulse approximation is invalid, as well as for long diffusion times. Currently the observed effects cannot be described with any existing model and should be further investigated.
- DOI: 10.18154/RWTH-2019-05179
- RWTH PUBLICATIONS: RWTH-2019-05179