Hyperpolarized 13C tracers: Technical advancements and perspectives for clinical applications

Enabling SENSE accelerated 2D CSI for hyperpolarized carbon-13 imaging

As hyperpolarized (HP) carbon-13 (13C) metabolic imaging is clinically translated, there is a need for easy-to-implement, fast, and robust imaging techniques. However, achieving high temporal resolution without decreasing spatial and/or spectral resolution, whilst maintaining the usability of the imaging sequence is challenging. Therefore, this study looked to accelerate HP 13C MRI by combining a well-established and robust sequence called two-dimensional Chemical Shift Imaging (2D CSI) with prospective under sampling and SENSitivity Encoding (SENSE) reconstruction. Due to the low natural abundance of 13C, the sensitivity maps cannot be pre-acquired for the reconstruction. As such, the implementation of sodium (23Na) sensitivity maps for SENSE reconstructed 13C CSI was demonstrated in a phantom and in vivo in the pig kidney. Results showed that SENSE reconstruction using 23Na sensitivity maps corrected aliased images with a four-fold acceleration. With high temporal resolution, the kidney spectra produced a detailed metabolic arrival and decay curve, useful for further metabolite kinetic modelling or denoising. Metabolic ratio maps were produced in three pigs demonstrating the technique's ability for repeat metabolic measurements. In cases with unknown metabolite spectra or limited HP MRI specialist knowledge, this robust acceleration method ensures comprehensive capture of metabolic signals, mitigating the risk of missing spectral data.

Real-time cell metabolism assessed repeatedly on the same cells via para-hydrogen induced polarization

Signal-enhanced or hyperpolarized nuclear magnetic resonance (NMR) spectroscopy stands out as a unique tool to monitor real-time enzymatic reactions in living cells. The singlet state of para-hydrogen is thereby one source of spin order that can be converted into largely enhanced signals of e.g. metabolites. Here, we have investigated a parahydrogen-induced polarization (PHIP) approach as a biological assay for in vitro cellular metabolic characterization. Here, we demonstrate the possibility to perform consecutive measurements yielding metabolic information on the same sample. We observed a strongly reduced pyruvate-to-lactate conversion rate (flux) of a Hodgkin's lymphoma cancer cell line L1236 treated with FK866, an inhibitor of nicotinamide phosphoribosyltransferase (NAMPT) affecting the amount of NAD⁺ and thus NADH in cells. In the consecutive measurement the flux was recovered by NADH to the same amount as in the single-measurement-per-sample and provides a promising new analytical tool for continuous real-time studies combinable with bioreactors and lab-on-a-chip devices in the future.

Evaluation of enzymatic and magnetic properties of γ-glutamyl-[1-13C]glycine and its deuteration toward longer retention of the hyperpolarized state

Dynamic nuclear polarization (DNP) is an emerging cutting-edge method of acquiring metabolic and physiological information in vivo. We recently developed γ-glutamyl-[1-13C]glycine (γ-Glu-[1-13C]Gly) as a DNP nuclear magnetic resonance (NMR) molecular probe to detect γ-glutamyl transpeptidase (GGT) activity in vivo. However, the detailed enzymatic and magnetic properties of this probe remain unknown. Here, we evaluate a γ-Glu-Gly scaffold and develop a deuterated probe, γ-Glu-[1-13C]Gly-d 2, that can realize a longer lifetime of the hyperpolarized signal. We initially evaluated the GGT-mediated enzymatic conversion of γ-Glu-Gly and the magnetic properties of 13C-enriched γ-Glu-Gly (γ-Glu-[1-13C]Gly and γ-[5-13C]Glu-Gly) to support the validity of γ-Glu-[1-13C]Gly as a DNP NMR molecular probe for GGT. We then examined the spin-lattice relaxation time (T 1) of γ-Glu-[1-13C]Gly and γ-Glu-[1-13C]Gly-d 2 under various conditions (D2O, PBS, and serum) and confirmed that the T 1 of γ-Glu-[1-13C]Gly and γ-Glu-[1-13C]Gly-d 2 was maintained for 30 s (9.4 T) and 41 s (9.4 T), respectively, even in serum. Relaxation analysis of γ-Glu-[1-13C]Gly revealed a significant contribution of the dipole-dipole interaction and the chemical shift anisotropy relaxation pathway (71% of the total relaxation rate at 9.4 T), indicating the potential of deuteration and the use of a lower magnetic field for realizing a longer T 1. In fact, by using γ-Glu-[1-13C]Gly-d 2 as a DNP probe, we achieved longer retention of the hyperpolarized signal at 1.4 T.