Comparison of calibration strategies for the in vivo determination of absolute metabolite concentrations in the human brain by 31P MRS.

Cerebral concentrations of phosphorus metabolites can be assessed non-invasively by 31P MRS provided the metabolite signals are calibrated with the signal of a standard of known concentration. The reliability of the concentration estimates depends mainly on the strategy of calibration. Three strategies were compared by assessing the concentrations both in a test dummy and in the brain of volunteers. The first strategy utilized tissue water as an internal heteronuclear concentration standard. The second and third strategies used as phosphorus solution as an external homonuclear standard; this solution was either put into a reference bottle placed on top of the head or into a simulation phantom measured instead of the head. Localization was always achieved with the ISIS pulse sequence. The two external homonuclear strategies achieved a higher accuracy (mean error approximately 5%) and reproducibility (mean SD approximately 8%) of the concentration estimates than the internal heteronuclear strategy (mean error approximately 11%; mean SD approximately 15%).