Once limited to the realm of fundamental physics experiments, hyperpolarized (HP) noble gases have been exploited to enhance magnetic resonance signals for a wide range of applications, including biomedical and clinical imaging. The purpose of this tutorial is to introduce the principles and practice of the primary methods of preparing hyperpolarized noble gases: spin-exchange optical pumping (SEOP) and metastability-exchange optical pumping (MEOP). Alternative approaches (e.g. dynamic nuclear polarization, DNP), will also be discussed.
Once limited to the realm of fundamental physics experiments, hyperpolarized (HP) noble gases have been exploited to enhance magnetic resonance signals for a wide range of applications, including biomedical and clinical imaging. The purpose of this tutorial is to introduce the principles and practice of the primary methods of preparing hyperpolarized noble gases: spin-exchange optical pumping (SEOP) and metastability-exchange optical pumping (MEOP). Alternative approaches (e.g. dynamic nuclear polarization, DNP), will also be discussed.
Spin-exchange optical pumping (SEOP) is a two-step process by which angular momentum from circularly polarized laser light is first transferred to the electronic spins of an alkali metal vapor, and then subsequently to the nuclear spins of the noble gas during gas-phase collisions. Over time, high bulk non-equilibrium spin polarization can accumulate in the nuclear spins of the noble gas, corresponding to an orders-of-magnitude increase in magnetic resonance sensitivity for MR spectroscopy and imaging. MEOP is a similar process, but one where no alkali metal vapor is present. Instead, electric discharge is used to create metastable helium-3 atoms, which can be optically polarized with a laser; over time, metastability exchange collisions then give rise to bulk hyperpolarized helium-3.
This tutorial lecture will focus on a number of key aspects governing the preparation and use of hyperpolarized noble gases for NMR and MRI, with a focus on biomedical applications. Following a brief introduction to the history of SEOP and MEOP and their early application to nuclear magnetic resonance nearly three decades ago, topics will include: fundamental principles of SEOP and MEOP; experimental aspects, methods, and instrumentation governing HP gas preparation; and “care and feeding” of HP gases to avoid premature loss of the hard-won but ultimately transient HP state. Particular attention will also be given to specific subjects of interest from my group’s laboratory as well as those of collaborators, including: stopped-flow SEOP; SEOP at the “extremes” of high xenon density and resonant photon flux; “traditional” (low-pressure/re-compression) MEOP; high-field MEOP; technological developments in high-power frequency-narrowed lasers; and the development of clinical-scale “hyperpolarizers” capable of delivering near-unity nuclear spin polarization for human use. Time permitting, highlights from recent biomedical / clinical MRI demonstrations of HP noble gases will be reviewed.