What can be measured with 31P MRS?
With 31P MRS, peaks from inorganic phosphate, phosphomonoesters, phosphodiesters, phosphocreatine, and the three phosphates of ATP can be observed. In additional to concentration measurements (typically using phosphocreatine as an internal standard), the intracellular free Mg2+ concentration can be derived from analysis of the ATP peaks, and the chemical shift between inorganic phosphate and phosphocreatine is sensitive to pH. 31P spectra from a frog muscle before and after exercise are shown below.
