Figure 1.

Example of ¹H and ³¹P MR spectra acquired at 1.5 T in a healthy volunteer muscle used to calibrate the water‐based method. H₂O, water; PCr, phosphocreatine; P_i, inorganic phosphate; α, β, γ, nucleoside triphosphates. Peaks of interest of each spectrum are labelled in green. For this particular example, the measured frequencies were $f_{{\mathrm{H}}_2\mathrm{O}}$ = 63.641294 MHz and f _PCr = 25.762264 MHz, giving k = 0.40480421. The frequency of water signal from the ¹H spectrum was multiplied by the experimentally derived constant k to create a virtual peak in the ³¹P spectrum, called the estimated PCr. Subsequently, pH was calculated using the Henderson–Hasselbalch equation and the chemical shift difference between the measured P_i and the estimated PCr.