Observed BOLD effects on cerebral metabolite resonances in human visual cortex during visual stimulation: a functional (1)H MRS study at 4 T.

Using the localized spin-echo (1)H MRS technique, the water resonance and methyl resonance peaks of the cerebral metabolites N-acetylaspartate (NAA at 2.0 ppm) and phosphocreatine/creatine (Cr at 3.0 ppm) were studied in the human visual cortex to detect and quantify the blood oxygenation level dependent (BOLD) effect during visual stimulation at 4 T. Significant BOLD effects, which reflect the increases of spectral peak height (H) accompanied by the decreases of spectral linewidth (Deltaupsilon(1/2)), were observed in NAA (H: 2.5%; Deltaupsilon(1/2): -1.7%) and Cr (H: 3.1%; Deltaupsilon(1/2): -1.8%) as well as in water (H: 3.1%; Deltaupsilon(1/2): -2.3%). Because NAA and Cr mainly exist in the brain cells, the BOLD effects on these cerebral metabolite resonances only measure the susceptibility component spreading into the extravascular cellular compartment. In contrast, water is affected in the intra- and the extravascular compartments. Therefore, the water signal measures the BOLD effects in both compartments. BOLD responses in water were similar to those observed in metabolites. The similarity indicates that the susceptibility spreading into the extravascular parenchyma contributed significantly to the observed BOLD effects at 4 T. Finally, taking advantage of the higher NMR sensitivity at 4 T, the feasibility of measuring BOLD effects on cerebral metabolites by localized (1)H MRS is demonstrated.