Detection of metabolite changes in response to a varying visual stimulation paradigm using short‐TE ¹H MRS at 7 T

Abstract

The two-fold benefit of H magnetic resonance spectroscopy (MRS) at high B fields - enhanced sensitivity and increased spectral dispersion - has been used previously to study dynamic changes in metabolite concentrations in the human brain in response to visual stimulation. In these studies, a strong visual on/off stimulus was combined with MRS data acquisition in a voxel location in the occipital cortex determined by an initial functional magnetic resonance imaging experiment. However, 1) to exclude the possibility of systemic effects (heartbeat, blood flow, etc.), which tend to be different for on/off conditions, a modified stimulation condition not affecting the target voxel needs to be employed, and 2) to assess important neurotransmitters of low concentration, in particular γ-aminobutyric acid (GABA), it may be advantageous to analyze steady-state, rather than dynamic, conditions. Thus, the aim of this study was to use short-TE H MRS methodology at 7 T to detect differences in steady-state metabolite levels in response to a varying stimulation paradigm in the human visual cortex. The two different stimulation conditions were termed voxel and control activation. Localized MR spectra were acquired using the SPECIAL (spin-echo full-intensity acquired localized) sequence. Data were analyzed using LCModel. Fifteen individual metabolites were reliably quantified. On comparison of steady-state concentrations for voxel versus control activation, a decrease in GABA of 0.05 mmol/L (5%) and an increase in lactate of 0.04 mmol/L (7%) were found to be the only significant effects. The observed reduction in GABA can be interpreted as reduced neuronal inhibition during voxel activation, whereas the increase in lactate hints at an intensification of anaerobic glycolysis. Differences from previous studies, notably the absence of any changes in glutamate, are attributed to the modified experimental conditions. This study demonstrates that the use of advanced H MRS methodology at 7 T allows the detection of subtle changes in metabolite concentrations involved in neuronal activation and inhibition.

Mixed-polarity random-dot stereograms alter GABA and Glx concentration in the early visual cortex

“…However, MRS also measures extracellular GABA, which maintains tonic cortical inhibition (Martin & Rimvall, 1993) and is unlikely to be altered by our experimental manipulations. While the magnitude of GABA difference we observed was consistent with previous work (Bednařík et al, 2015;Mekle et al, 2017), this may explain its meagre size (4%). The difference in Glx (2%) is also consistent with previous work (Bednařík et al, 2015;Mangia et al, 2007;Mullins, Rowland, Jung, & Sibbitt, 2005;Schaller et al, 2013).…”

Measurement of brain lactate during visual stimulation using a long TE semi‐LASER sequence at 7 T

“…Although the results of the initial studies were contradictory, consistent changes in Lac concentration upon visual stimulation have been reported at 7 T, benefitting from the increase in SNR and spectral resolution at ultra‐high field. The average rise in Lac level during visual stimulation was found to be in the range of 7% to 37%, maintained until the end of the stimulus in most studies. However, these 7 T studies were generally designed to follow the time courses of all neurometabolites detectable by 1 H MRS and were not specifically optimized for measurement of Lac.…”

Maximizing sensitivity for fast GABA edited spectroscopy in the visual cortex at 7 T

“…53 Similarly as obtained for functional lactate MRS, these may reveal even faster GABA fluctuations in the visual cortex. 54,55 Despite the short echo time, care must be taken in assuming constant levels of overlapping resonances and fitting accuracies. 49,56 Short echo time MRS is more heavily dependent on data quality and fitting constraints for the quantification of GABA.…”