Investigation of the metabolic changes in visual cortex due to visual stimulation using high field magnetic resonance spectroscopy at 7.0 T

Abstract

Objective To investigate the metabolic changes in the visual cortex due to visual stimulation using high field functional proton magnetic resonance spectroscopy at 7.0 T.A clear picture of brain metabolism and neurotransmitter activity during activation was expected to be established.Methods Nine healthy subjects participated in this study. All MR measurements were acquired using a 7.0 T MR system and a 16-channel SENSE head coil. An initial fMRI scan was performed prior to spectroscopic acquisition in order to determine the activated region in the visual cortex.A cubic ROI of 2 cm× 2 cm × 2 cm was positioned inside the activated region for functional MRS acquisition.A short TE STEAM sequence was used for acquiring the MRS data. The functional paradigm comprised 6.6 min baseline followed by 13.2 min of visual stimulation and 19.8 min recovery.Summed averaged spectra for visual stimulus off and visual stimulus on were analyzed separately using LC Model and internal reference of water. A Wilcoxon signed rank test was conducted to compare the metabolite changes. Results During stimulation,concentration in Asp [ ( 3.20 ± 0.28 ) μmol/g ],Gln [ ( 2.07 ± 0.10 ) μmol/g ] and Gly [ ( 1.65 ± 0.11 )μmol/g ] was found to be significantly decreased,compared with that of ( 3.52 ± 0.28 ),( 2.25 ± 0.10 ) and ( 1.85 ± 0.11 ) μmol/g in rest ( Z =- 2.073,- 2.073 and - 2.429,respectively,P ＜ 0.05 ).The level in Glu [ ( 11.50 ± 0.11 ) μmol/g],GSH[ (2.45 ± 0.10) μmol/g] and Lac[ (0.89 ± 0.05) μ.mol/g]due to neuronal activation was found to be significantly increased,versus resting concentration of ( 11.28 ± 0.11 ),( 2.28 ± 0.10 ) and (0.79 ± 0.05 ) μmol/g,respectively ( Z =2.521,2.310,2.016,respectively,P ＜0.05).Glc level [ (1.54 ± 0.23 )μmol/g] exhibited a tendency to decrease throughout the period of stimulation,compared with that of [ (1.78 ± 0.28 ) μmol/g ] in rest,but the decrease did not reach statistical significance (Z =- 1.897,P ＞ 0.05 ).Conclusions Using a novel visual stimulation paradigm and 1 H MRS at 7.0 T and LC Model,the metabolic response to activation have been able to be observed.The observed changes of Asp,Glc and Lac concentrations in response to visual stimulation suggests that sustained neuronal activation raises oxidative metabolism to a new steady state.The observation of increased Glu with decreased Gln during stimulation can be interpreted as a stimulus driven increase in excitatory neurotransmitter cycling.The elevated GSH in the visual cortex in response to visual stimulation is a new observation.Possible explanations for this include detoxification of reactive oxygen species. Key words: Magnetic resonance spectroscopy; Vision; Energy metabolism