Hyperpolarized ¹³C-pyruvate in vivo MRI performed over the postsyngeneic and allogeneic HSCT course. [1-¹³C]Pyruvic acid (30 μL), containing 15 mM OX063 and 2.5 mM gadolinium chelate, was hyperpolarized using the Hypersense DNP Polarizer (Oxford Instruments). After hyperpolarization was achieved, the sample was dissolved in 4.5 mL of heated alkaline buffer, ie, 40 mM 4-(2-hydroxyethyl)-1 piperazineethanesulfonic acid, 30 mM of NaCl, and 100 mg/L of EDTA. The hyperpolarized [1-¹³C]pyruvate solution (96 mM) was then administered through an IV tail vein catheter. ¹³C MRI studies were performed on a 3-T MR Solutions MRI using a custom ¹³C-¹H saddle coil for spectroscopic and anatomical imaging. (A) Schematic representation of the pyruvate-to-lactate conversion with the hyperpolarized ¹³C indicated in red. (B) Representative coronal anatomical MRI images overlaid with the ¹³C MRSI spectra for each voxel and an exemplary ¹³C spectrum indicating the lactate, alanine, and pyruvate peaks shown for a syngeneic (top) and allogeneic (bottom) animal at day 14. (C) MRSI images of syn (top) and allo (bottom) at day 14, indicating the signal intensity for lactate, pyruvate, as well as the combined lactate/pyruvate ratio. A region of interest was drawn around the liver based on the anatomical MRI, which was then used to quantify the signal intensity for each peak. (D) Lactate/pyruvate signal intensity in the liver over time after HSCT in syngeneic and allogeneic animals, n = 3 to 6 animals per group and time point, **P < .01 in a mixed-effect analysis with Sidak’s multiple comparison test. (E) Mouse plasma was collected via mandibular bleed at indicated time points in sodium fluoride/EDTA–coated microtubes. After centrifugation, an internal standard was added, and the lactate levels were quantified using ultra performance liquid chromatography–mass spectrometry, n = 4 to 17 per group and time point, **P < .01 in a mixed-effect analysis with Sidak’s multiple comparison test. All data are shown as mean ± SEM. ALT, alanine aminotransferase.