Endogenous CEST MRI of tumor pHi. (a) The asymmetry of the magnetization transfer ratio (), a measure of CEST, decreases after ischemia is induced in a rat model (black: ischemic region; green: nonischemic contralateral region; data represents the average and standard deviation of results with 7 rats). (b) This decrease in CEST, also represented as a change in the amide proton transfer ration (ΔAPTR), has been correlated with intracellular pH (pHi) as measured with ³¹P MR spectroscopy. (c) The can be mapped in a rat model of ischemia, (d) which can be converted into a pH map. Reproduced with permission from [59, 83, 84].