Nathalie Just ^*

• Danish Research Centre for Magnetic Resonance, Copenhagen University Hospital, Copenhagen, Denmark

This study aimed to characterize Blood oxygen level-dependent (BOLD) effects in 1 H-MR spectra obtained during optogenetic activation of the rat forelimb cortex for the correction and estimation of accurate metabolite concentration changes.For an increased understanding of BOLD effects detected with fMRS and the optimized way to correct them, a 1 Hz line-narrowing effect was simulated.Then, 1 H-fMRS data acquired using STEAM at 9.4T in rats (n=8) upon optogenetic stimulation of the primary somatosensory cortex were used. Data were analyzed with MATLAB routines and LCModel.Uncorrected and corrected 1 H-MR spectra of simulated and in-vivo data were quantified and compared. BOLD-corrected difference spectra were also calculated and analyzed. The effects of stimulated and non-stimulated water on metabolite concentration quantification were also investigated.Results: Significant mean increases in water and NAA peak heights (+ 1.1% and +4.5%, respectively)were found accompanied by decreased linewidths (-0.5 Hz and -2.8%) upon optogenetic stimulation.These estimates were used for further definition of an accurate line-broadening factor (lb). Usage of an non-data driven lb introduced false-positive errors in metabolite concentration change estimates thereby altering the specificity of findings. Using different water scalings within LCModel, the water and metabolite BOLD contributions were separated.The linewidth-matching procedure using a precise lb factor remains the most performant approach for the accurate quantification of small (±0.3 µmol/g) metabolic changes in 1 H-fMRS studies. A simple and preliminary compartmentation of BOLD effects was proposed, which will require validation.