Volumetric mapping of intra- and extracellular pH in the human brain using 31P MRSI at 7T

Purpose In vivo P-31 MRSI enables noninvasive mapping of absolute pH values via the pH-dependent chemical shifts of inorganic phosphates (P-i). A particular challenge is the quantification of extracellular P-i with low SNR in vivo. The purpose of this study was to demonstrate feasibility of assessing both intra- and extracellular pH across the whole human brain via volumetric P-31 MRSI at 7T. Methods 3D P-31 MRSI data sets of the brain were acquired from three healthy volunteers and three glioma patients. Low-rank denoising was applied to enhance the SNR of P-31 MRSI data sets that enables detection of extracellular P-i at high spatial resolutions. A robust two-compartment quantification model for intra- and extracellular P-i signals was implemented. Results In particular low-rank denoising enabled volumetric mapping of intra- and extracellular pH in the human brain with voxel sizes of 5.7 mL. The average intra- and extracellular pH measured in white matter of healthy volunteers were 7.00 +/- 0.00 and 7.33 +/- 0.03, respectively. In tumor tissue of glioma patients, both the average intra- and extracellular pH increased to 7.12 +/- 0.01 and 7.44 +/- 0.01, respectively, compared to normal appearing tissue. Conclusion Mapping of pH values via P-31 MRSI at 7T using the proposed two-compartment quantification model improves reliability of pH values obtained in vivo, and has the potential to provide novel insights into the pH heterogeneity of various tissues.