CEST MRI of tumor pHe with Yb-HPDO3A. (a) An in vivo CEST spectrum of the tumor mass before and after i.v. injection of the agent into a mouse model of B16-F10 melanoma. (b) A ratio of the two CEST effects is calibrated with pH at 33°C. Although the calibration is dependent on temperature, the temperature can be determined from the chemical shifts of the CEST effects. (c) An anatomical image shows the location of the subcutaneous tumor. (d) The pixelwise pHe map of the tumor shows a heterogenous distribution of pHe values with an average pHe of 5.8. Reproduced with permission from [61].