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

To test the hypothesis that lactate oxidation contributes to the ${\hspace{0.17em}}^{13}$ C-bicarbonate signal observed in the awake human brain using hyperpolarized ${\hspace{0.17em}}^{13}$ C MRI.

Healthy human volunteers (N = 6) were scanned twice using hyperpolarized ${\hspace{0.17em}}^{13}$ C-MRI, with increased radiofrequency saturation of ${\hspace{0.17em}}^{13}$ C-lactate on one set of scans. ${\hspace{0.17em}}^{13}$ C-lactate, ${\hspace{0.17em}}^{13}$ C-bicarbonate, and ${\hspace{0.17em}}^{13}$ C-pyruvate signals for 132 brain regions across each set of scans were compared using a clustered Wilcoxon signed-rank test.

Increased ${\hspace{0.17em}}^{13}$ C-lactate radiofrequency saturation resulted in a significantly lower ${\hspace{0.17em}}^{13}$ C-bicarbonate signal (p = 0.04). These changes were observed across the majority of brain regions.

Radiofrequency saturation of ${\hspace{0.17em}}^{13}$ C-lactate leads to a decrease in ${\hspace{0.17em}}^{13}$ C-bicarbonate signal, demonstrating that the ${\hspace{0.17em}}^{13}$ C-lactate generated from the injected ${\hspace{0.17em}}^{13}$ C-pyruvate is being converted back to ${\hspace{0.17em}}^{13}$ C-pyruvate and oxidized throughout the human brain.