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Purpose To perform exchange-rate measurements on the in vivo human brain downfield spectrum (5–10 ppm) at 9.4 T and to compare the variation in concentrations of the downfield resonances and of known upfield metabolites to determine potential peak labels. Methods Non-water-suppressed metabolite cycling was used in combination with an inversion transfer technique in two brain locations in healthy volunteers to measure the exchange rates and T1 values of exchanging peaks. Spectra were fitted with a heuristic model of a series of 13 or 14 Voigt lines, and a Bloch–McConnell model was used to fit the exchange rate curves. Concentrations from non-water-inverted spectra upfield and downfield were compared. Results Mean T1 values ranged from 0.40 to 0.77 s, and exchange rates from 0.74 to 13.8 s−1. There were no significant correlations between downfield and upfield concentrations, except for N-acetylaspartate, with a correlation coefficient of 0.63 and P

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.

Purpose To improve the decoupling of a transceiver human head phased array at ultra-high fields (UHF, ≥ 7T) and to optimize its transmit (Tx) and receive (Rx) performance, a single-row eight-element (1 × 8) tight-fit transceiver overlapped loop array was developed and constructed. Overlapping the loops increases the RF field penetration depth but can compromise decoupling by generating substantial mutual resistance. Methods Based on analytical modeling, we optimized the loop geometry and relative positioning to simultaneously minimize the resistive and inductive coupling and constructed a 9.4T eight-loop transceiver head phased array decoupled entirely by overlapping loops. Results We demonstrated that both the magnetic and electric coupling between adjacent loops is compensated at the same time by overlapping and nearly perfect decoupling (below -30 dB) can be obtained without additional decoupling strategies. Tx-efficiency and SNR of the overlapped array outperformed that of a common UHF gapped array of similar dimensions. Parallel Rx-performance was also not compromised due to overlapping the loops. Conclusion As a proof of concept we developed and constructed a 9.4T (400 MHz) overlapped transceiver head array based on results of the analytical modeling. We demonstrated that at UHF overlapping loops not only provides excellent decoupling but also improves both Tx- and Rx-performance.