The Efficiency Limits of Spin Exchange Optical Pumping Methods of 129Xe Hyperpolarization: Implications for in vivo MRI Applications by Matthew S. Freeman Graduate Program in Medical Physics Duke University Date: __________________ Approved: _____________________________ Bastiaan Driehuys, Supervisor _____________________________ James MacFall _____________________________ Chunlei Liu _____________________________ Joseph Lo _____________________________ Warren Warren Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate Program in Medical Physics in the Graduate School of Duke University 2015 ABSTRACT The Efficiency Limits of Spin Exchange Optical Pumping Methods of 129Xe Hyperpolarization: Implications for in vivo MRI Applications by Matthew S. Freeman Graduate Program in Medical Physics Duke University Date: __________________ Approved: _____________________________ Bastiaan Driehuys, Supervisor _____________________________ James MacFall _____________________________ Chunlei Liu _____________________________ Joseph Lo _____________________________ Warren Warren Abstract of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate Program in Medical Physics in the Graduate School of Duke University 2015 Copyright by Matthew Stouten Freeman 2015 Abstract Since the inception of hyperpolarized 129Xe MRI, the field has yearned for more efficient production of more highly polarized 129Xe. For nearly all polarizers built to date, both peak 129Xe polarization and production rate fall far below theoretical predictions. This thesis sought to develop a fundamental understanding of why the observed performance of large-scale 129Xe hyperpolarization lagged so badly behind theoretical predictions. This is done by thoroughly characterizing a high-volume, continuous-flow polarizer using optical cells having three different internal volumes, and employing two different laser sources. For each of these 6 combinations, 129Xe polarization was carefully measured as a function of production rate across a range of laser absorption levels. The resultant peak polarizations were consistently a factor of 2-3 lower than predicted across a range of absorption levels, and scaling of production rates deviated badly from predictions based on spin exchange efficiency. To bridge this gap, we propose that paramagnetic, activated Rb clusters form during spin exchange optical pumping (SEOP), and depolarize Rb and 129Xe, while unproductively scattering optical pumping light. When a model was built that incorporated the effects of clusters, its predictions matched observations for both polarization and production rate for all 6 systems studied. This permits us to place a limit on cluster number density of <2 × 109 cm-3. iv The work culminates with deploying this framework to identify methods to improve polarization to above 50%, leaving the SEOP cell. Combined with additional methods of preserving polarization, the polarization of a 300-mL batch of 129Xe increased from an average of 9%, before this work began, to a recent value of 34%. We anticipate that these developments will lay the groundwork for continued advancement and scaling up of SEOP-based hyperpolarization methods that may one day permit real-time, on-demand 129Xe MRI to become a reality. v to Happus may our adventure never end vi Contents Abstract ............................................................................................................................... iv List of Tables ...................................................................................................................... ix List of Figures ...................................................................................................................... x Acknowledgements ........................................................................................................... xiii 1. Introduction .....................................................................................................................1 2. Rb-129Xe Spin Exchange Optical Pumping ...................................................................11 3. Characterizing 129Xe SEOP Performance .....................................................................23 3.1 Possible Extensions to the Standard Model of SEOP ....................................34 4. Rb Nanoclusters in the SEOP environment .................................................................38 4.1 Hypothesizing Laser Induced Activation of Rb Clusters in SEOP Cells ......39 4.2 Modeling Activated Rb Clusters .....................................................................43 4.3 Steepest Descent Tuning of Cluster Parameters .............................................47 4.4 Comparison of Individual Clusters Effects .....................................................51 4.4.1 Spin Destruction ...............................................................................53 4.4.2 Broad Photon Scattering ..................................................................55 4.4.3 129Xe Relaxation ................................................................................56 4.5 Possible Presence of Activated Clusters in 3He Cells ......................................57 4.6 Limitations of Modeling Alkali Cluster Formation ........................................58 5. Rb Nanocluster Detection .............................................................................................59 6. Increasing 129Xe Polarization .........................................................................................81 6.1 Avoiding Magnetic Field Zero-Crossings .......................................................85 6.2 Cryogenic Accumulation, Capturing 129Xe and Minimizing Relaxation ........90 6.3 Optical Improvements at the SEOP Cell Interface ........................................96 6.4 Results ..............................................................................................................98 vii 6.5 Future considerations .....................................................................................102 Appendix A: Fundamental Spin Exchange Constants ....................................................106 References ........................................................................................................................107 Biography .........................................................................................................................112 viii List of Tables Table 1: Binary collision spin-destruction coefficients for Rb. .........................................14 Table 2: Coefficients determining the D1-absorption cross-section for Rb vapor. ..........15 Table 3: Characteristics of the optical cells used in this work. ..........................................26 Table 4: Proposed cluster scaling constant, cluster relaxation cross-sections, and scattering cross-section. .....................................................................................................51 ix List of Figures Figure 1: ventilation 129Xe MRI ...........................................................................................3 Figure 2: ADC 129Xe MRI ...................................................................................................4 Figure 3: dissolved 129Xe MRI .............................................................................................5 Figure 4: clinical production schedule .................................................................................5 Figure 5: optical pumping diagram ....................................................................................13 Figure 6: Rb D1-asborption cross-section.........................................................................16 Figure 7: modeled photon absorption................................................................................17 Figure 8: spin-exchange diagram .......................................................................................18 Figure 9: polarizer schematic .............................................................................................19 Figure 10: theoretical performance vs. reality ....................................................................22 Figure 11: flow circuit diagram ..........................................................................................23 Figure 12: research polarizer ..............................................................................................24 Figure 13: sample flow curves ............................................................................................28 Figure 14: standard model polarization .............................................................................29 Figure 15: standard model production...............................................................................31 Figure 16: polarization and cell size ...................................................................................33 Figure 17: cluster model polarization ................................................................................48 Figure 18: cluster model production ..................................................................................50 Figure 19: individual effects polarization ...........................................................................53 x Figure 20: individual effects production ............................................................................55