The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad.
This award will support a twenty-four-month research fellowship by Dr. Nicholas Whiting to work with Drs. Michael J. Barlow, Peter Morris, and Thomas Meersmann at the University of Nottingham in the United Kingdom and with Dr. Boyd Goodson at Southern Illinois University Carbondale in the United States.
?Hyperpolarized? (HP) noble gases can be used to enhance the magnetic resonance (MR) detection sensitivity for a host of potential and realized applications. Because the nuclear spin polarization achieved in conventional MR methods is usually on the order of ~10-5?10-6, the orders-of-magnitude higher spin polarization manifested by such HP gases can translate directly into correspondingly massive enhancements in MR detection sensitivity?thus enabling a wide range of novel experiments and approaches that would not be possible otherwise. One of the primary methods for generating HP gases is spin-exchange optical pumping (SEOP). SEOP is a two-step process where angular momentum is transferred from resonant, circularly polarized laser light to the electronic spins of an alkali metal vapor, and then subsequently transferred to the nuclear spins of a noble gas via collisions?allowing the nuclear spin polarization to accumulate over time. The fundamental physical processes underlying SEOP are surprisingly multifaceted and interdependent, and despite decades of research into the optical, atomic, and molecular physics of these processes, they have yet to be sufficiently explored; indeed, there is much that is not known or understood?particularly when SEOP is performed under key regimes and experimental conditions that are most relevant for MR applications.
The objectives of the proposed research are to gain a deeper fundamental understanding of SEOP processes involving selected alkali metals and noble gases, and to apply these insights to improve SEOP for MR applications requiring large amounts of highly spin-polarized gases. The proposed research will involve SEOP using both rubidium (Rb) and cesium (Cs) as alkali metals, and xenon-129 (129Xe) and krypton-83 (83Kr) as noble gas species (with the possibility to expand to 131Xe); while Rb/129Xe SEOP has been well characterized in the past (although not under the conditions proposed in this work), studies involving Cs/129Xe as well as efforts to boost the polarization and MR applications of the quadrupolar 83Kr/131Xe isotopes are still largely in their infancy. The proposed studies of fundamental SEOP processes at the Host institution include: the dependence of the alkali metal electronic spin polarization (PRb/PCs) distribution ?map? on noble gas type and density, cell temperature, total pressure, laser flux, spectral offset, etc. using optical electron spin resonance (ESR) spectroscopy; investigations of energy-transport mechanisms within the OP cell (as a function of the same parameters) using Raman spectroscopy to measure the rovibrational temperature of nitrogen gas; and studies of the corresponding nuclear spin polarization distribution (and gas dynamics) across the OP cell via low-field 3-D MR imaging. The Nottingham apparatus?which the PI will help complete?will for the first time allow the comparison of all of these experimental observables in real time. These experiments will not only provide new knowledge concerning SEOP, but will inform the optimization of conditions for generating HP gases for MR applications?including proposed experiments that will explore the use of highly spin-polarized gases for probing porous materials and surfaces. While the proposed research is fundamental in nature, the results?to be widely disseminated in conference presentations and publications?should have a direct impact on both emerging and established applications of HP gases; such applications are manifold and vary from studies of materials to clinical human MR imaging of lung spaces and tissues. The University of Nottingham is uniquely positioned to serve as host, given its vast resources in state-of-the-art experimental instrumentation, expert personnel, and strong history in MR innovation?providing vital training and experiences, as well as establishing long-lasting international collaborations, that will positively impact the PI throughout the duration of his professional research career.
The high nuclear spin polarization of ‘hyperpolarized’ (HP) noble gases (i.e., 3He, 83Kr, & 129Xe) can be used to increase the detection sensitivity for a wide range of NMR/MRI applications—as well as enable a number of crucial measurements in fundamental physics. HP gases are often produced via spin-exchange optical pumping (SEOP)—a process whereby the angular momentum from resonant, circularly polarized laser light is transferred onto the electron spins of an alkali metal vapour (i.e., Rb, Cs); this spin-polarization is then transferred onto the nuclear spins of the noble gas via gas-phase collisions. SEOP often results in nuclear spin polarization enhancements of 4-5 orders of magnitude compared to the very low ‘thermal-equilibrium’ polarizations that are achieved by conventional NMR/MRI. This improves the detection sensitivity for a variety of applications, including: material studies, examining protein interactions, uses as biomolecular tracers, and human lung imaging. The physical processes underlying SEOP are surprisingly complex and interdependent, and despite decades of research into the optical, atomic, and molecular physics of these processes, they have yet to be sufficiently explored within regimes most relevant for MR applications. One aspect of particular interest to the PI is to develop techniques that allow the production of large amounts of highly polarized 129Xe (as increases in xenon density result in decreases to polarization, due to non-spin-conserving collisions with the alkali metal atoms). The overall objectives of the proposed research are to gain a deeper fundamental understanding of SEOP processes involving selected alkali metals and noble gases, and to apply these insights to improve SEOP for MR applications requiring large amounts of highly spin-polarized gases. Towards these goals, the following activities have taken place since the fellowship was awarded: we have shown the benefits of using caesium (as opposed to rubidium) for SEOP—leading to an average 1.5x increase in 129Xe polarization (and per-atom spin-exchange rate) under similar conditions, with the biggest gains found at the highest xenon densities. While this effect has been previously predicted, we were the first to physically realize it, due to access to new laser technology [Whiting, et. al., Phys. Rev. A. 83 (2011)]. Current plans centre on scaling up this model to use higher power, frequency-narrowed lasers (Whiting, et. al., in preparation) and implementing Cs/129Xe SEOP in clinical polarisers. Operating under these conditions of high resonant laser flux and high xenon partial pressures leads to issues regarding the dissipation of heat from the optical pumping cell (as energy from the intense laser light is trapped in the rovibrational manifold of the nitrogen buffer gas). In situ ultralow-frequency Raman spectroscopy of the nitrogen in the SEOP cell revealed temperatures elevated hundreds of degrees above ambient; in some cases, temperatures were measured to be ~450 oC above the temperature to which the SEOP cell was externally heated. The most significant increases in the heat load occurred under conditions of high xenon partial pressures (and correspondingly lower nitrogen pressures), likely due to the inability of the small amounts of nitrogen to adequately dissipate the heat to the SEOP cell walls (Whiting, et. al., in preparation). This work also led to the development of an in-line module for collecting the Raman data; this module uses volume Bragg grating filters to notch out contributions from laser and Raleigh scattering, and allowed a ~23x improvement in detection sensitivity [Newton, et. al., in preparation]. Additional studies included: examining regimes of using high xenon partial pressures in flow-through polarisers; calibration of low-field 129Xe NMR spectroscopy using low-field detection of 1H in water; examining the use of aluminium as a cryogenic storage container for hyperpolarized xenon; SEOP simulations; and using optically-detected electron spin resonance to evaluate the alkali metal electronic spin polarization during optical pumping under a variety of experimental parameters. Knowledge gained from these studies contributed towards a novel clinical xenon polarizer that was planned, assembled, tested, and delivered by a consortium of universities (Southern Illinois University Carbondale, Vanderbilt University, University of Nottingham, Harvard University) to Brigham and Women’s Hospital (Boston, MA) to be used for pulmonary imaging. This polarizer is based on single batch-mode SEOP at high xenon partial pressures (as opposed to continuous flow, collection, and accumulation of small amounts of xenon), and the gas is collected without the (polarization-destroying) freeze/sublimate step (due to the xenon-rich mixture). This polarizer has achieved ‘record-high’ values of xenon polarizations at high xenon partial pressures [Nikolaou, et. al., in preparation]. Additional, next-generation polarisers are currently under construction at Vanderbilt University and the Queen’s Medical Centre (University of Nottingham). Results from this fellowship have already been disseminated in one journal publication [Whiting, et. al., Phys. Rev. A. 83 (2011)], with an additional five manuscripts in various stages of publication, and have been presented at international conferences and meetings on twelve occasions.
