Upscaling the hyperpolarization sample volume of an automated hydrogenative parahydrogen-induced polarizer

Abstract

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) suffer from inherently low sensitivity due to the weak thermal polarization of nuclear spins. Parahydrogen-induced polarization (PHIP) offers a powerful route to enhance NMR signals by several orders of magnitude, enabling real-time metabolic imaging. However, PHIP implementations are often constrained by small sample volumes, limited automation, and complex high-pressure requirements. In this work, we present an upgraded, automated PHIP system capable of hyperpolarizing sample volumes of up to 2.2 mL, making it suitable for preclinical MRI applications. We developed several high-pressure reactors and multi-port NMR tube caps compatible with standard commercial 5, 10, and 16 mm glass tubes. Reactor designs were simulated and fabricated from chemically resistant polymers, ensuring mechanical safety at more than 30 bar. Using FLASH MRI, nutation, and CPMG sequences, we characterized magnetic field homogeneity and stability, establishing optimal sample dimensions (12.5/16 mm ID/OD glass tube, 20 mm height) with B0 inhomogeneity below 2.5 ppm and B1 inhomogeneity around 1%. A high level of injection reproducibility was confirmed (volume precision ~0.6%). Optimization of experimental parameters, including hydrogenation pressure, pH2 flow rate, and sample temperature, enabled rapid and efficient polarization transfer. At optimized conditions (20 bar pH2, 2 L/min flow, 55{\deg}C, 4 s bubbling time), up to 31.3% 1H polarization of two protons was achieved for deuterated ethyl acetate in acetone with the theoretical maximum of 50%. This level of polarization was achieved with a duty cycle of 80 s, and the standard deviation of the mean was below 6.8%.