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Although nearly all ³¹P resonances have pH dependence, the Pi−PCr difference is the most commonly used. The main reasons for using this system are: 1) its pKa = 6.77, lying centrally within the physiological range of pH values commonly encountered in tissues; 2) the chemical shift of PCr is nearly constant in this range; 3) the relationship is relatively linear between pH 6.2 and 7.2. When this method can be used, its accuracy is about 0.05 pH units.
The Pi−PCr method may not be applicable in several circumstances. In liver and kidney, for example, a PCr resonance is not normally detectable. PCr may also be absent in certain pathological conditions, including hypoxia, ischemia, and neoplasia. Sometimes the Pi peak is hard to detect (e.g. resting skeletal muscle) or be obscured by overlapping peaks (e.g., 2-3-DPG in heart MRS).
Befroy DE, Rothman DL, Petersen KF, Shulman GI. ³¹P-magnetization transfer magnetic resonance spectroscopy measurements of in vivo metabolism. Diabetes 2102; 61:2669-2678
Befroy DE, Shulman GI. Magnetic resonance spectroscopy studies of human metabolism. Diabetes 2011; 60:1361-1369.
Kemp GJ, Ahmad RE, Nicolay K, Prompers JJ. Quantification of skeletal muscle mitochondrial function by 31P magnetic resonance spectroscopy techniques: a quantitative review. Acta Physiol 2015; 213:107–144.
Moon RB, Richards JH. Determination of intracellular pH by ³¹P magnetic resonance. J Biol Chem 1973; 248:7276-7278. (first use of ³¹P MRS to measure pH in vitro).
Parasoglou P, Xia D, Chang G, Regatte RR. Three-dimensional saturation transfer ³¹P-MRI in muscles of the lower leg at 3.0 T. Sci Reports 2014; 4:5219.
Rata M, Giles SL, deSouza NM, et al. Comparison of three reference methods for the measurement of intracellular pH using ³¹P MRS in healthy volunteers and patients with lymphoma. NMR Biomed 2014; 27:158-162.
Thomas G. 31P Spectroscopy intracellular pH calculator. Philips NetForum Community, 2013. Download calculator here as Excel file. Download instructions here.
What is magnetization transfer?
What peaks are seen in the ³¹P spectrum and what do they mean?