Evidence of ¹³ C-lactate oxidation in the human brain from hyperpolarized ¹³ C-MRI

Purpose: To test the hypothesis that lactate oxidation contributes to the ${}^{13}$ C-bicarbonate signal observed in the awake human brain using hyperpolarized ${}^{13}$ C MRI.

Methods: Healthy human volunteers (N = 6) were scanned twice using hyperpolarized ${}^{13}$ C-MRI, with increased radiofrequency saturation of ${}^{13}$ C-lactate on one set of scans. ${}^{13}$ C-lactate, ${}^{13}$ C-bicarbonate, and ${}^{13}$ C-pyruvate signals for 132 brain regions across each set of scans were compared using a clustered Wilcoxon signed-rank test.

Results: Increased ${}^{13}$ C-lactate radiofrequency saturation resulted in a significantly lower ${}^{13}$ C-bicarbonate signal (p = 0.04). These changes were observed across the majority of brain regions.

Conclusion: Radiofrequency saturation of ${}^{13}$ C-lactate leads to a decrease in ${}^{13}$ C-bicarbonate signal, demonstrating that the ${}^{13}$ C-lactate generated from the injected ${}^{13}$ C-pyruvate is being converted back to ${}^{13}$ C-pyruvate and oxidized throughout the human brain.