Biomedical Engineering - Theses

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    Techniques for signal acquisition, reconstruction and analysis in sodium magnetic resonance imaging
    Blunck, Yasmin ( 2018)
    Over the last decades, Magnetic Resonance Imaging (MRI) has undergone a high-paced development rendering it an extremely versatile imaging technique with revolutionary impact on medical practice and research. Typically, MRI acquisitions are sensitised to hydrogen-1 nuclei which facilitates high resolution images with unprecedented soft tissue contrast but lacks specificity as it only offers an indirect link to pathologies. The study of other MR-observable nuclei (so called x-Nuclei MRI) offers the potential to extend standard MRI to the detection of pathologies without apparent structural abnormalities and enhance therapy monitoring capabilities. Sodium as the second most abundant MR-observable nucleus and as a direct link to cell integrity and vitality via the Sodium-Potassium-Pump promises means for a quantitative in- sight into pathological processes. Despite its potential and its early beginnings in the 1980s, Sodium MRI is still considered a niche of MRI and plays no significant role in clinical routine. Its progression is hampered by its relatively low in vivo signal strength and challenging NMR-signal characteristics. While the former can be mitigated through the use of stronger magnetic fields, the latter dictates the acquisition principle but also provides great analysis potential. This thesis focusses on the second challenge, the Sodium NMR-signal, and exploits its aspects across the three main stages of an MRI experiment: signal acquisition, signal reconstruction, and signal analysis. Beginning with signal acquisition, this thesis introduces zGRF-RHE, an improved sequence timing concept for a reduction of encoding time. Compared with the conventional ultra-short echo time (UTE) sequence timing approach, the presented technique mitigates T2∗-induced blurring artefacts and improves image SNR. It illustrates a general sequence timing concept with applicability to any centre-out trajectory design. With regards to signal reconstruction, this work demonstrates a comprehensive investigation of Compressed Sensing (CS) reconstructions with a focus on parameter optimisation and an evaluation of accurate image intensity reconstructions. Lastly, concerning both acquisition and signal analysis, this thesis investigates the diverse Sodium NMR-signal characteristics. It presents an optimised multi-echo acquisition scheme together with a Rician-noise corrected parameter estimation approach facilitating a differentiation of underlying motion regimes. It provides a time-efficient measurement protocol from which multiple parameters are obtainable within the course of a standard Sodium-density weighted acquisition.