(a–c) Hyperpolarized 13C measurements (coloured) in an axial slice overlayed on anatomical proton images (greyscale). A calibration phantom containing 13C urea is visible in the upper left corner of the image. The signal stemming from the two simultaneously hyperpolarized and injected substances ZA (a) and urea (b) is well localized to the bladder. (c) The pH map calculated from changes in chemical shift of the 13C pH biosensor shows an acidic pH in the bladder of 6.48±0.02 (n=9 pixels, mean±s.d.). (d) The spectrum from a voxel at nominal resolution (black box) in the bladder shows ZA (fitted, green), urea (fitted, grey) and a noticeable amount of a decay product of ZA, PPH (not fitted, white). Scale bars, 1 cm.