Increased Glutamate concentrations during prolonged motor activation as measured using functional Magnetic Resonance Spectroscopy at 3T.

Recent implications of glutamatergic signaling in a wide range of psychiatric disorders has highlighted the need to study the dynamics of glutamate (Glu) in the brain outside of steady state. A promising modality for doing so is functional Magnetic Resonance Spectroscopy (fMRS). Recent human studies at high magnetic fields (7T) have reported small but consistent changes in metabolite concentrations, in particular a 2-4% increase in Glu during visual and motor stimulation. While the origins of these changes remain the topic of ongoing research, the ability of fMRS to observe metabolites directly associated with neurotransmission and brain energetics could potentially aid our understanding of brain pathophysiology and the interpretation of functional imaging experiments. For this to happen, the current ultrahigh field results must be reproduced at lower, widely available clinical field strengths, in response to a wide variety of stimuli classes. Our goal herein was to investigate metabolite changes during a hand-clenching motor task at 3T, and to investigate the effect of the stimulation's temporal characteristics on the magnitude of the fMRS changes; specifically, we compared two block-designed functional activation paradigms, using short- and long-cycled clenching designs. Small but statistically significant increases in Glx=Glutamate+Glutamine (3.8%) and Glu (4.0%) concentrations were detected during the long-cycled design, while no statistically significant changes were observed during the short-cycled design. Activation during the long-cycled tasks was correlated to the frequency of clenching. We have also shown that using subject-level analysis in combination with a linear mixed model increases the observed effect size, and could help analyzing the weak MRS signals. Our results are in good agreement with the previous reports acquired at higher field systems, and support the viability of fMRS as a research tool at clinical field strengths, while also emphasizing the importance of the functional paradigm itself.

Analysis of magnetization transfer (MT) influence on quantitative mapping of T2 relaxation time.

PURPOSE: Multi-echo spin-echo (MESE) protocol is the most effective tool for mapping T2 relaxation in vivo. Still, MESE extensive use of radiofrequency pulses causes magnetization transfer (MT)-related bias of the water signal, instigated by the presence of macromolecules (MMP). Here, we analyze the effects of MT on MESE signal, alongside their impact on quantitative T2 measurements. METHODS: Study used 3 models: in vitro urea phantom, ex vivo horse brain, and in vivo human brain. MT ratio (MTR) was measured between single-SE and MESE protocols under different scan settings including varying echo train lengths, number of slices, and inter-slice gap. MTR and T2 values were extracted for each model and protocol. RESULTS: MT interactions biased MESE signals, and in certain settings, the corresponding T2 values. T2 underestimation of up to 4.3% was found versus single-SE values in vitro and up to 13.8% ex vivo, correlating with the MMP content. T2 bias originated from intra-slice saturation of the MMP, rather than from indirect saturation in multi-slice acquisitions. MT-related signal attenuation was caused by slice crosstalk and/or partial T1 recovery, whereas smaller contribution was caused by MMP interactions. Inter-slice gap had a similar effect on in vivo MTR (21.2%), in comparison to increasing the number of slices (18.9%). CONCLUSIONS: MT influences MESE protocols either by uniformly attenuating the entire echo train or by cumulatively attenuating the signal along the train. Although both processes depend on scan settings and MMP content, only the latter will cause underestimation of T2 .

Application of phase rotation to STRESS localization scheme at 3 T.

PURPOSE: Application of phase rotation to the STRESS (=STEAM+PRESS) localization scheme, to shorten echo time, minimize J-coupling dephasing and estimate B1+ inhomogeneity. STRESS (=STEAM + PRESS) simultaneously refocuses and acquires the double spin echo (SE123 ) and stimulated echo (STE- ) pathways, combining PRESS-like signal with lower chemical shift displacement as in STEAM. Phase rotation effectively separates coherence pathways, allows reduction of spoiling gradients moments leading to reduction in echo time. Implementing it in STRESS allows one to individually phase-correct SE123 and STE- prior to combination. Moreover, B1+ inhomogeneity can be assessed by comparing the measured ratio of resonance intensities of SE123 and STE- pathways to the simulated one. METHODS: In vivo spectra were acquired from a single voxel placed in the sensory-motor cortex of 10 healthy volunteers, using phase rotation-STRESS/PRESS/STEAM sequences at 3 T scanner. The phases of each slice-selective pulse were incremented by Δϕ1/2/3=22.5°/-45°/45°. RESULTS: Phase rotation-STRESS showed quantification accuracy (% Cramer Rao lower bounds) and reproducibility (% coefficients of variation) comparable to PRESS and STEAM, in both phantoms and in vivo study. Minimal echo time achieved was 13 ms. CONCLUSION: Phase rotation complements STRESS by reducing echo time, allowing processing of each pathway individually prior to addition and providing B1+ estimation in single voxel proton magnetic resonance spectroscopy. Magn Reson Med 79:2481-2490, 2018. © 2017 International Society for Magnetic Resonance in Medicine.