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

Abstract

Purpose In vivo ³¹P 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 ³¹P MRSI at 7T. Methods 3D ³¹P 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 ³¹P 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 ³¹P 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.