Using Raman Spectroscopy to Improve Hyperpolarized Noble Gas Production for Clinical Lung Imaging Techniques

Abstract

Spin-exchange optical pumping (SEOP) can be used to ``hyperpolarize{''} Xe-129 for human lung MRI. SEOP involves transfer of angular momentum from light to an alkali metal (Rb) vapor, and then onto Xe-129 nuclear spins during collisions; collisions between excited Rb and N-2 ensure that incident optical energy is nonradiatively converted into heat. However, because variables that govern SEOP are temperature-dependent, the excess heat can complicate efforts to maximize spin polarization-particularly at high laser fluxes and xenon densities. Ultra-low frequency Raman spectroscopy may be used to perform in situ gas temperature measurements to investigate the interplay of energy thermalization and SEOP dynamics. Experimental configurations include an ``orthogonal{''} pump-and-probe design and a newer ``inline{''} design (with source and detector on the same axis) that has provided a >20-fold improvement in SNR. The relationship between Xe-129 polarization and the spatiotemporal distribution of N-2 rotational temperatures has been investigated as a function of incident laser flux, exterior cell temperature, and gas composition. Significantly elevated gas temperatures have been observed-hundreds of degrees hotter than exterior cell surfaces-and variances with position and time can indicate underlying energy transport, convection, and Rb mass-transport processes that, if not controlled, can negatively impact Xe-129 hyperpolarization.