(a,b) Hyperpolarized 13C biosensor pH maps (coloured) in an axial slice overlayed on proton images (greyscale). The electrode pH values are shown in white. ZA and urea were simultaneously hyperpolarized and injected into (a) six buffer phantoms and (b) two blood phantoms prepared at different pH values. In both a,b, a calibration phantom (see Methods section) was present. 13C biosensor pH map windows are based on a signal-level threshold. (c) 13C biosensor pH and electrode pH correlate well for experiments in different buffers (black circles and crosses), at elevated temperature (37 °C, blue crosses), at a 10 mM Ca2+ concentration (green crosses) and in blood (red squares). We determined a root mean square error of the fitting residuals (RMSE) of 0.10 pH units from the set of all in vitro experiments shown in Fig. 2c. Scale bars, 1 cm.