FIGURE 1.

A, Imaging mechanism: hyperpolarized [¹³C]pyruvate and urea are intravenously administered in a single bolus, then leave the vasculature and enter the exocellular space (indicated by the solid line). Pyruvate can enter the cell at a slower rate compared to the vascular delivery (indicated by the dashed line) and be metabolized into lactate, alanine, or CO₂/bicarbonate, evaluating multiple metabolic fluxes. In contrast, urea is predominately extracellular and metabolically inactive, serving as a perfusion sensing probe. (K^trans, vascular transfer constant; MCT, monocarboxylate transporter; LDH, lactate dehydrogenase; ALT, alanine transaminase; PDH, pyruvate dehydrogenase). B, Co‐polarization system set up: urea/water/EPA mixture (lower density) is loaded into cryovial first, then PA/EPA mixture is added (higher density), resulting in a homogeneous mixture of PA and urea with a molar ratio of 4:1 (PA, pyruvic acid; EPA, electron paramagmetic agent). C, Representative ¹³C NMR spectra of hyperpolarized [1‐¹³C]pyruvate and [¹³C, ¹⁵N₂]urea. The right spectrum zooms in the boxed region of the left spectrum, showing urea and Impurity II with relevant coupling constants (J _CN = 20.2 Hz, J _CN = 18.3 Hz, J _CC = 7.5 Hz)