Improving NMR and MRI Sensitivity with Parahydrogen

Abstract

Parahydrogen induced polarisation (PHIP) has wide utility in NMR and MRI as it can increase the sensitivity of both techniques. The transfer of spin order from parahydrogen to nuclei in the analyte leads to an increased magnetic response following interrogation by RF pulses. This spin transfer is catalysed by a homogeneous or heterogeneous catalyst. The increased magnetic response not only reduces the number of transients required to obtain the spectrum or image, but can also illuminate previously undetectable species present in solution. From its theoretical prediction to its experimental validation, PHIP has been applied in a range of different areas such as the structural analysis of complexes, understanding reaction mechanisms involving hydrogen and for the production of contrast agents for use in MRI. PHIP can also be readily combined with other techniques such as photochemistry which widens its field of applicability. In this review, we detail the properties of parahydrogen and the methods for its preparation and utilisation in homogeneous and heterogeneous based hydrogenation and non-hydrogenative reactions. Specific examples are explained for the application of PHIP in photochemical and hydroformylation reactions. Pulse sequences designed to be compatible with PHIP are described to exemplify how the increase in sensitivity can be increased even further by the interrogation of the magnetic states optimally. Finally, a section on the use of PHIP in the production of contrast agents suitable for MRI, and the monitoring of hydrogenation reactions using imaging techniques is discussed.