Large-scale 129Xe-Rb Spin-Exchange Optical Pumping

Spin-exchange optical pumping (SEOP) is a process by which the nuclear polarisation of xenon-129 (129Xe) can be enhanced by a factor 10^5 of its thermal equilibrium value, enabling clinical hyperpolarised (HP) gas magnetic resonance imaging (MRI). This thesis is concerned with accurately modelling SEOP and implementing informed polariser design upgrades in order to improve the production of HP-129Xe for clinical hyperpolarised gas MRI. This was pursued through the following projects: 1. Modelling SEOP using 1D and 3D simulations in order to understand the effect of realistic gas flow dynamics, temperatures, and Rubidium (Rb) source distribution on Rb vapour density, [Rb], and 129Xe polarisation, PXe. It was found that implementing an optimised presaturator leads to near-saturation homogeneous [Rb] across the optical cell, and high PXe. 2. Optimisation of atomic absorption spectroscopy (AAS) for direct [Rb] measurements across the optical cell. Determination of accuracy limits stemming from the breakdown of the Beer-Lambert law and line shape fitting at low spectral signal-to-noise ratios (SNR) was conducted over infrared and violet Rb valence electron transitions. It was observed that increasing Rb sources within the main cell body led to the most homogeneous [Rb] distribution across the cell, however Rb runaway and cell temperature heterogeneity were observed to limit [Rb] homogeneity. [Rb] was measured to be consistently lower than saturation [Rb], even at high laser absorption. In addition, the Rb-129Xe spin-exchange cross section, γ′, was calculated from the measured [Rb] values and was determined to be γ′ = (1.1 ± 0.1)×10^−21 m3s−1. 3. Evaluating the use of focused temperature control and Rb source distribution to optimise thermal management. These upgrades led to reduced Rb runaway susceptibility and improved cell temperature homogeneity and laser absorption. Characterisation and optimisation of the upgraded polariser for HP-129Xe production was performed. Optimal running conditions for 1L volume HP-Xe dose were PXe = 23.7%, and 29.6% at dose equivalence rates of DE = 725ml/hr and DE = 398ml/hr respectively. In addition, analytical and numerical models of SEOP were shown to diverge at high laser absorption and/or low gas flow rates. The discrepancy between theoretical and measured PXe values persisted after the incorporation of measured spin-exchange values from [Rb] measurements, suggesting the origin of this discrepancy extends to unexplored optical pumping and Rb spin-destruction considerations.