Purpose Relaxation‐compensated CEST‐MRI (i.e., the inverse metrics magnetization transfer ratio and apparent exchange‐dependent relaxation) has already been shown to provide valuable information for brain tumor diagnosis at ultrahigh magnetic field strengths. This study aims at translating the established acquisition protocol at 7 T to a clinically relevant magnetic field strength of 3 T. Methods Protein model solutions were analyzed at multiple magnetic field strengths to assess the spectral widths of the amide proton transfer and relayed nuclear Overhauser effect (rNOE) signals at 3 T. This prior knowledge of the spectral range of CEST signals enabled a reliable and stable Lorentzian‐fitting also at 3 T where distinct peaks are no longer resolved in the Z‐spectrum. In comparison to the established acquisition protocol at 7 T, also the image readout was extended to three dimensions. Results The observed spectral range of CEST signals at 3 T was approximately ±15 ppm. Final relaxation‐compensated amide proton transfer and relayed nuclear Overhauser effect contrasts were in line with previous results at 7 T. Examination of a patient with glioblastoma demonstrated the applicability of this acquisition protocol in a clinical setting. Conclusion The presented acquisition protocol allows relaxation‐compensated CEST‐MRI at 3 T with a 3D coverage of the human brain. Translation to a clinically relevant magnetic field strength of 3 T opens the door to trials with a large number of participants, thus enabling a comprehensive assessment of the clinical relevance of relaxation compensation in CEST‐MRI.

Purpose In vivo 31P 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 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 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 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.