UNC-CH Physics and Astronomy Dissertation Defense

Michael Antonacci

“A Noble Solution: Dissolved-phase ¹²⁹Xe as a Magnetic Resonance Biosensor”

Hyperpolarized ¹²⁹Xe gas has long been considered an ideal nuclear magnetic resonance (NMR) probe for biological systems. Because of the large polarizable electron cloud of the ¹²⁹Xe atom, its nuclear spin is highly sensitive to its immediate chemical environment. The most common method used to polarize noble gases is spin-exchange optical pumping (SEOP). In the first part of this work, inefficiencies of the continuous-flow SEOP process are studied. Specifically, optical and NMR spectroscopic measurements are made to understand the effect of dark Rb on 129Xe polarization and to detect the presence of Rb nanoclusters, whose presence may reduce spin-exchange efficiency. An upper limit for the in-cell Rb nanocluster density of 8.3×10¹⁵ cm^-3 is set while dark Rb vapor is shown to have a detrimental effect on final ¹²⁹Xe polarization only at high cell temperatures and low production rates.

In the second part of this work, a novel application of hyperpolarized ¹²⁹Xe gas for magnetic resonance thermometry is described. First, a frequency referencing method for in vivo hyperpolarized ¹²⁹Xe magnetic resonance spectroscopy is developed and its general utility for dissolved-phase ¹²⁹Xe measurements is demonstrated. Then the referencing method is used to calibrate an absolute temperature probe based on the resonance frequency of the nuclear spin of lipid-dissolved ¹²⁹Xe. Finally, this temperature probe is used to detect new thermogenic mechanisms in adipose cells.