Single-Voxel Proton Magnetic Resonance Spectroscopy in the Human Brain at 9.4 T: Methods and Applications

Abstract

Proton magnetic resonance spectroscopy (1H-MRS) is a non-invasive method allowing the detection as well as the quantification of several metabolites in the human brain. The introduction of ultra-high field (UHF) scanners (≥7 T) led to an increase of the signal-to-noise ratio and a higher frequency dispersion, hence better spectral resolution. These advantages promote the potential of MRS. Despite the significant advantages of UHF for MRS, several technical challenges (such as B1+efficiency and inhomogeneity, increased power deposition, chemical shift displacement etc.) must be addressed for the efficient utilization of these prospective benefits. The methods and techniques developed during this Ph.D. demonstrated the feasibility of metabolite cycling (MC) at 9.4 T, and the advantages of non-water suppressed MRS regarding frequency and phase fluctuations. The newly developed sequences (MC-STEAM and MC-semi-LASER) enabled the acquisition of reliable spectra with enhanced frequency resolution, both upfield and downfield of water in 1H spectra. Furthermore, the designed RF coils, hardware setup (power splitters, phase cables, etc.), as well as, the gained knowledge regarding the achievement of efficient transmit fields and can be utilized in future MRS studies and applications. As a result, the human brain macromolecular baseline was investigated revealing additional macromolecular peaks and information regarding their concentration levels. Moreover, the chemical exchange rates of the downfield metabolites, as well as, their correlation with the upfield peaks were examined contributing further to the assignment of the downfield peaks. Finally, the performed functional MRS studies in which the MC-semi-LASER sequences were used, demonstrated the potentials of UHF and MC regarding the simultaneous investigation of water and metabolites alterations during visual stimulation.