Master thesis : Multi-echo 23Na MRI Sequence Evaluation for Quantitative Mapping (including introduction to research methodology)

Abstract

Sodium magnetic resonance imaging (23Na MRI) is a promising imaging modality that provides information on tissue viability and cellular homeostasis beyond what can be obtained with conventional hydrogen MRI. However, its clinical and research applications remain challenging because of the low sodium concentration in vivo, its lower gyromagnetic ratio, and the rapid decay of the MR signal, which result in a low signal-to-noise ratio (SNR).

The first objective of this thesis was to optimize sodium MRI acquisition parameters in order to maximize SNR while maintaining clinically acceptable acquisition times and respecting specific absorption rate (SAR) constraints. A theoretical optimization framework based on the signal equation of spoiled gradient-echo imaging was developed and experimentally validated using homogeneous sodium phantoms. This approach enabled the determination of an optimal combination of repetition time, flip angle, and number of radial projections. The second objective was the development and evaluation of quantitative sodium relaxation mapping methods. Longitudinal relaxation times (T1) were estimated using the variable flip angle method, while effective transverse relaxation times (T2*) were obtained from multi-echo acquisitions combined with voxel-wise fitting procedures. The performance of conventional Ultra-Short Echo Time (UTE) imaging was compared with Density-Adapted Radial Acquisition (DARAD), a sampling strategy designed to improve acquisition efficiency and image quality in sodium MRI. Phantom experiments demonstrated that DARAD acquisitions produced more homogeneous parameter maps and narrower relaxation time distributions than conventional UTE acquisitions. The proposed methodology was subsequently applied to human brain imaging, where quantitative T1 and T2* maps were generated. The results suggest that density-adapted radial sampling improves the robustness and precision of sodium relaxation parameter estimation.

Overall, this work demonstrates the feasibility of quantitative sodium relaxation mapping at 7T and highlights the advantages of DARAD acquisitions for sodium MRI.