Toward Next-Generation Molecular Imaging with a Clinical Low-Field (0.064 T) Point-of-Care MRI Scanner.

Low-field (LF) MRI promises soft-tissue imaging without the expensive, immobile magnets of clinical scanners but generally suffers from limited detection sensitivity and contrast. The sensitivity boost provided by hyperpolarization can thus be highly synergistic with LF MRI. Initial efforts to integrate a continuous-bubbling SABRE (signal amplification by reversible exchange) hyperpolarization setup with a portable, point-of-care 64 mT clinical MRI scanner are reported. Results from 1H SABRE MRI of pyrazine and nicotinamide are compared with those of benchtop NMR spectroscopy. Comparison with MRI signals from samples with known H2O/D2O ratios allowed quantification of the SABRE enhancements of imaged samples with various substrate concentrations (down to 3 mM). Respective limits of detection and quantification of 3.3 and 10.1 mM were determined with pyrazine 1H polarization (PH) enhancements of ∼1900 (PH ∼0.04%), supporting ongoing and envisioned efforts to realize SABRE-enabled MRI-based molecular imaging.

Anomalously Large Antiphase Signals from Hyperpolarized Orthohydrogen Using a MOF-Based SABRE Catalyst.

Hyperpolarized orthohydrogen (o-H2 ) is a frequent product of parahydrogen-based hyperpolarization approaches like signal amplification by reversible exchange (SABRE), where the hyperpolarized o-H2 signal is usually absorptive. We describe a novel manifestation of this effect wherein large antiphase o-H2 signals are observed, with 1 H enhancements up to ≈500-fold (effective polarization PH ≈1.6 %). This anomalous effect is attained only when using an intact heterogeneous catalyst constructed using a metal-organic framework (MOF) and is qualitatively independent of substrate nature. This seemingly paradoxical observation is analogous to the "partial negative line" (PNL) effect recently explained in the context of Parahydrogen Induced Polarization (PHIP) by Ivanov and co-workers. The two-spin order of the o-H2 resonance is manifested by a two-fold higher Rabi frequency, and the lifetime of the antiphase HP o-H2 resonance is extended by several-fold.