Investigating the origin of the 13C lactate signal in the anesthetized healthy rat brain in vivo after hyperpolarized [1‐13C]pyruvate injection

The goal of this study was to investigate the origin of brain lactate (Lac) signal in the healthy anesthetized rat after injection of hyperpolarized (HP) [1‐13C]pyruvate (Pyr). Dynamic two‐dimensional spiral chemical shift imaging with flow‐sensitizing gradients revealed reduction in both vascular and brain Pyr, while no significant dependence on the level of flow suppression was detected for Lac. These results support the hypothesis that the HP metabolites predominantly reside in different compartments in the brain (i.e., Pyr in the blood and Lac in the parenchyma). Data from high‐resolution metabolic imaging of [1‐13C]Pyr further demonstrated that Lac detected in the brain was not from contributions of vascular signal attributable to partial volume effects. Additionally, metabolite distributions and kinetics measured with dynamic imaging after injection of HP [1‐13C]Lac were similar to Pyr data when Pyr was used as the substrate. These data do not support the hypothesis that Lac observed in the brain after Pyr injection was generated in other organs and then transported across the blood–brain barrier (BBB). Together, the presented results provide further evidence that even in healthy anesthetized rats, the transport of HP Pyr across the BBB is sufficiently fast to permit detection of its metabolic conversion to Lac within the brain. Dynamic metabolic imaging with flow‐sensitizing gradients of hyperpolarized (HP) [1‑13C]pyruvate revealed reduction in both vascular and brain pyruvate, while no significant dependence on the level of flow suppression was detected for lactate. The presented results provide further evidence that transport of HP pyruvate into the brain and its subsequent conversion to lactate is limited by blood–brain barrier transport and that the majority of the HP lactate observed in brain was indeed produced there.