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_i³¹P signal (P^ex_i) acquired from human head using nonlocalized ³¹P MRS. Methods: Outer-volume-suppression (OVS) saturation and 1D/2D ³¹P CSI were utilized to reveal the presence of an additional component in the (P^ex_i) 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 ³¹P 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.