On the Simultaneous Mapping of Sodium Relaxation Parameters using Magnetic Resonance Fingerprinting

In this thesis, the development of a framework for simultaneous sodium (23Na) mapping of T1, T2l*, T2s*, T2* and ΔB0, based on magnetic resonance Fingerprinting (MRF), is presented. In initial experiments, the feasibility of 23Na MRF was investigated using a 2D MRF sequence with variable-rate selective excitation pulses and density adapted readout gradients at a static magnetic field strength of 7 T. The proposed technique was validated in simulations and phantom experiments by comparison of the MRF results to reference measurements. An ensuing in vivo study of the human brain was performed with five healthy volunteers, yielding results in good agreement with the literature. Subsequently, a 3D version of the pulse sequence was developed to increase the scan efficiency and an improved signal model for spin 3/2 nuclei, based on irreducible spherical tensor operators, was implemented. In an optimization step, a hybrid of single and double-echo readouts and a flip angle pattern optimized with the Cramer Rao lower bound were implemented. Phantom experiments yielded a mean deviation of the quantified relaxation times of 1.0% with respect to the references. A second in vivo study, conducted with the optimized 3D MRF framework, yielded good agreement to both the 2D MRF study and literature values. Mean values of T1 = (35.0 ± 3.2)ms, T2l* = (29.3 ± 3.8)ms and T2s* = (5.5 ± 1.3)ms were found in brain tissue, whereas T1 and T2* were (61.9 ± 2.8) ms and (46.3 ± 4.5)ms in cerebrospinal fluid. The culmination of all advancements proposed throughout this work enabled relaxometric sodium mapping of the human head within approximately 1/2 h with a nominal resolution of (5mm)3. The findings of this work suggest that 23Na MRF is a promising candidate to push sodium relaxometric mapping towards clinically feasible measurement times