³¹P MR spectroscopy of tumor pHe. (a) The ³¹P spectrum from mouse leg muscle injected with 3-APP can measure extracellular pH (pHe) from the 3-APP chemical shift and can measure intracellular pH (pHi) from the chemical shift of inorganic phosphate (). (b) The ³¹P chemical shift of 3-APP is correlated with pH as shown by an in vitro titration. (c) The pHe and pHi measured with 3-APP and in a tumor before (bottom) and after (middle) injection of 3-APP, and after euthanasia (top). Reproduced with permission from [41].