Optimizing Coherent Dynamics of Polarization Transfer in SABRE SHEATH

Abstract

Magnetic Resonance techniques are fundamentally limited by low signal intensities imparted by the small population differences between the states of interest at thermal equilibrium. Hyperpolarization artificially induces large, non-equilibrium population distributions to significantly enhance the available signal from low concentration compounds. This group of techniques has broadened the scope of MRI to include spatially and temporally resolved information about low concentration metabolites in pathologies like cancer, heart disease, and liver disease. However, the leading solution state hyperpolarization modality, dissolution Dynamic Nuclear Polarization (dDNP), is limited by high instrument cost, long polarization times, and the need for specialized personnel to operate equipment. Signal Amplification By Reversible Exchange (SABRE) is an alternative hyperpolarization modality, first introduced in 2009, which offers an inexpensive alternative to DNP for solution state hyperpolarization. In SABRE, transient association of parahydrogen and the hyperpolarization target on a catalyst allows for polarization transfer into desired states. However, the transient nature of this interaction significantly complicates the dynamics of the system. As SABRE is a relatively new technique, our understanding of the dynamics of polarization transfer in this system is still immature and polarization levels with this technique still lags behind dDNP. The work presented here demonstrates that the previous theoretical explanation for polarization transfer in this system, based in avoided crossings is insufficient to describe the dynamics and drive experiment design and introduces alternative theoretical interpretations of the dynamics. We go on to introduce a physically accurate numerical model for SABRE dynamics and use this model to explore some of the infinite number of potential field sequences to induce more optimal polarization transfer. These new pulse sequences alter the effective Hamiltonian governing population transfer to improve enhancement of magnetized as well as singlet target states on a variety of clinically interesting compounds.