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

PURPOSE: In vivo (31) P MRSI enables noninvasive mapping of absolute pH values via the pH-dependent chemical shifts of inorganic phosphates (Pi ). A particular challenge is the quantification of extracellular Pi 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 (31) P MRSI at 7T. METHODS: 3D (31) P 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 (31) P MRSI data sets that enables detection of extracellular Pi at high spatial resolutions. A robust two-compartment quantification model for intra- and extracellular Pi 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 (31) P 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.