¹⁵N-carnitine, a novel endogenous hyperpolarized MRI probe with long signal lifetime

Purpose: The purpose of this study was to investigate hyperpolarization and in vivo imaging of [¹⁵N]carnitine, a novel endogenous MRI probe with long signal lifetime. Methods: L-[¹⁵N]carnitine-d₉ was hyperpolarized by the method of dynamic nuclear polarization followed by rapid dissolution. The T₁ signal lifetimes were estimated in aqueous solution and in vivo following intravenous injection in rats, using a custom-built dual-tuned ¹⁵N/¹H RF coil at 4.7 T. ¹⁵N chemical shift imaging and ¹⁵N fast spin-echo images of rat abdomen were acquired 3 minutes after [¹⁵N]carnitine injection. Results: Estimated T₁ times of [¹⁵N]carnitine at 4.7 T were 210 seconds (in H₂O) and 160 seconds (in vivo), with an estimated polarization level of 10%. Remarkably, the [¹⁵N]carnitine coherence was detectable in rat abdomen for 5 minutes after injection for the nonlocalized acquisition. No downstream metabolites were detected on localized or nonlocalized ¹⁵N spectra. Diffuse liver enhancement was detected on ¹⁵N fast spin-echo imaging 3 minutes after injection, with mean hepatic SNR of 18 ± 5 at a spatial resolution of 4 × 4 mm. Conclusion: This study showed the feasibility of hyperpolarizing and imaging the biodistribution of HP [¹⁵N]carnitine.