³¹P Measurements

You made reference to the use of ³¹P MRS to measure cellular metabolism. How does this work?    

Estimation of Tissue pH

The chemical shifts of many ³¹P metabolites are responsive to local tissue pH. PCr is relatively immune to such changes, while Pi is highly sensitive. The chemical shift difference between PCr and Pi can therefore be used to estimate tissue pH. The relationship is relatively linear over normal physiological ranges where a chemical shift difference (Δδ) increase of 0.1 ppm corresponds to a pH increase of approximately 0.08 units. In tissues like liver and kidney where no PCr peak is detectable, a similar method using the shift between Pi and the α-ATP peak may be used. 

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Dynamic Metabolic Spectra

Rapidly acquired ³¹P spectra can be obtained during sustained muscular contraction or exercise. As shown below, during muscular contraction PCr is depleted to maintain constant ATP levels while Pi accumulates. Acidosis (lowered pH) develops as manifest by decreasing chemical shift between the Pi and PCr peaks. Upon session of contraction, metabolites return to their baseline levels.

From Befroy and Shulman (2011) under CC BY license

Saturation (Magnetization) Transfer

Narrow bandwidth RF-pulses can be applied to selectively saturate specific peaks in the ³¹P spectrum. If the ³¹P nucleus in a saturated metabolite exchanges chemically with another metabolite, the saturated nucleus retains and transfers its nulled magnetization state to the new host molecule, whose resonance is secondarily reduced. Such a technique is commonly used to monitor the creatinine kinase reaction (PCR + ADP ↔ ATP + Cr) as shown below. Here the γ-ATP peak is selectively irradiated, with saturation transfer secondarily reducing the size of the PCr and Pi peaks.

From Befroy and Shulman (2011) under CC BY license

Advanced Discussion (show/hide)»

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).

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References
     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.

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Related Questions
     What is magnetization transfer?  
     What peaks are seen in the ³¹P spectrum and what do they mean?

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