Helium-rich mixtures for improved batch-mode clinical-scale spin-exchange optical pumping of Xenon-129.

We present studies of spin-exchange optical pumping (SEOP) using ternary xenon-nitrogen-helium gas mixtures at high xenon partial pressures (up to 1330 Torr partial pressure at loading, out of 2660 Torr total pressure) in a 500-mL volume SEOP cell, using two automated batch-mode clinical-scale Xe hyperpolarizers operating under continuous high-power (~170 W) pump laser irradiation. In this pilot study, we explore SEOP in gas mixtures with up to 45% He content under a wide range of experimental conditions. When an aluminum jacket cooling/heating design was employed (GEN-3 hyperpolarizer), Xe polarization (%P) of 55.9 ± 0.9% was observed with mono-exponential build-up rate γ of 0.049 ± 0.001 min for the He-rich mixture (1000 Torr Xe/900 Torr He, 100 Torr N), compared to %P of 49.3 ± 3.3% at γ of 0.035 ± 0.004 min for the N-rich gas mixture (1000 Torr Xe/100 Torr He, 900 Torr N). When forced-air cooling/heating was used (GEN-2 hyperpolarizer), %P of 83.9 ± 2.7% was observed at γ of 0.045 ± 0.005 min for the He-rich mixture (1000 Torr Xe/900 Torr He, 100 Torr N), compared to %P of 73.5 ± 1.3% at γ of 0.028 ± 0.001 min for the N-rich gas mixture (1000 Torr Xe and 1000 Torr N). Additionally, %P of 72.6 ± 1.4% was observed at a build-up rate γ of 0.041 ± 0.003 min for a super-high-density He-rich mixture (1330 Torr Xe/1200 Torr He/130 Torr N), compared to %P = 56.6 ± 1.3% at a build-up rate of γ of 0.034 ± 0.002 min for an N-rich mixture (1330 Torr Xe/1330 Torr N) using forced air cooling/heating. The observed SEOP hyperpolarization performance under these conditions corresponds to %P improvement by a factor of 1.14 ± 0.04 at 1000 Torr Xe density and by up to a factor of 1.28 ± 0.04 at 1330 Torr Xe density at improved SEOP build-up rates by factors of 1.61 ± 0.18 and 1.21 ± 0.11 respectively. Record %P levels have been obtained here: 83.9 ± 2.7% at 1000 Torr Xe partial pressure and 72.6 ± 1.4% at 1330 Torr Xe partial pressure. In addition to improved thermal stability for SEOP, the use of He-rich gas mixtures also reduces the overall density of produced inhalable HP contrast agents; this property may be desirable for HP Xe inhalation by human subjects in clinical settings-especially in populations with heavily impaired lung function. The described approach should enjoy ready application in the production of inhalable Xe contrast agent with near-unity Xe nuclear spin polarization.