Unveiling a hidden 31P signal coresonating with extracellular inorganic phosphate by outer-volume-suppression and localized 31P MRS in the human brain at 7T

Purpose: The study was undertaken to demonstrate that there is more than 1 component in the extracellular P-i P-31 signal (P-i(ex)) acquired from human head using nonlocalized P-31 MRS. Methods: Outer-volume-suppression (OVS) saturation and 1D/2D P-31 CSI were utilized to reveal the presence of an additional component in the P-i(ex) signal. Results: 67% of the head extracellular P-i signal was attenuated upon OVS saturation of the peripheral meningeal tissues, likely reflecting elimination of the P-i signal in the meningeal fluids (the blood and CSF). Localized 1D/2D CSI data provided further support for this assignment. Upon correction for the meningeal contribution, the extracellular P-i concentration was 0.51 +/- 0.07 mM, whereas the intracellular P-i was 0.85 +/- 0.10 mM. The extracellular pH was measured as 7.32 +/- 0.04 when using OVS, as compared to 7.39 +/- 0.03 when measured without OVS (N = 7 subjects). Conclusion: The extracellular P-i signal acquired from the human head using nonlocalized P-31 MRS contains a significant component likely contributed by peripheral blood and CSF in meninges that must be removed in order to use this signal as an endogenous probe for measuring extracellular pH and other properties in the brain.