Catalysis-Assisted Signal Enhancement in NMR Imaging

Parahydrogen-induced polarization (PHIP) is an established hyperpolarization technique, but it is still predominantly used in combination with homogeneous hydrogenations in solution catalyzed by dissolved transition metal complexes. Our recent demonstrations of PHIP effects in heterogeneous hydrogenations catalyzed by a variety of catalyst types provide novel opportunities for continuously producing catalyst-free hyperpolarized fluids, which is very promising for various practical applications of NMR and MRI, including potential in vivo studies. In particular, hydrogenation of propene with parahydrogen produces hyperpolarized propane gas which was successfully imaged in various model objects in high magnetic fields. In addition, propane was found to exhibit spin states with lifetimes that exceed its T₁ time by a factor of ca. 8 in low magnetic fields. This made it possible to perform MR imaging of propane gas with sub-millimeter spatial and sub-second temporal resolution directly in the 47.5 mT field using either the spin-lock induced crossing (SLIC) sequence or deuterated propylene as the substrate in hydrogenation. Another remarkable emerging possibility is to develop a hypersensitive NMR-based tool for catalytic research, exemplified by the MRI study of an operating microreactor with the packed catalyst bed. Applications of parahydrogen to homogeneous catalytic processes are far from being exhausted either. Observation of PHIP effects in metal-free catalysis will be exemplified by the activation of parahydrogen by frustrated Lewis pairs of ansa-aminoboranes. SABRE technique is rapidly becoming a powerful and popular approach for hyperpolarizing suitable substances in solution. In addition to the observation of SABRE effects in high magnetic fields, we demonstrated recently a substantial ³¹P NMR signal enhancement for free and bound PPh₃ species upon their reversible interaction with a metal complex and parahydrogen in solution. The achieved improvement in sensitivity by more than two orders of magnitude at 7 T made it possible to perform a single-shot ³¹P MRI of a model object. Significant sensitivity improvement also allowed us to perform the low-field (47.5 mT) ¹H MRI of 100 mM pyridine solution, with SABRE effects generated either in situ or in the ~6 mT field with subsequent sample transfer to the 47.5 mT field for imaging.

Acknowledgments
Financial support from RFBR (grants 14-03-93183-MCX-a, 14-03-00374-a), RSCF (grants 14-13-00445, 14-35-00020), and the Council on Grants of the President of the Russian Federation (MK-1329.2014.3) is acknowledged.