Purpose

Rapid production of large volumes of highly polarized ¹²⁹Xe on continuous-flow spin-exchange optical pumping (SEOP) ¹²⁹Xe polarizers is challenging due to the flow limitation imposed by the SEOP cell volume as the ¹²⁹Xe polarization decreases beyond a critical flow rate for which the 129Xe residency time in the SEOP cell is less than the ¹²⁹Xe-Rb spin-exchange time. Successful employment of a large cell volume (>1500 cm³) requires two main criteria: i) a highly homogeneous static B₀ field over the cell volume; and ii) a well-collimated, high-power optical pumping laser beam which is homogenous over the cell length. Presented here is a custom large-scale ¹²⁹Xe SEOP polarizer featuring a 3500 cm³ SEOP cell which is designed to work in a clinical setting.

Materials and Methods

¹²⁹Xe polarizer main components: Four square coils (100 cm width) with geometry based on the design of Merrit et al.¹ for optimized B₀ homogeneity at 3 mT; 3500 cm³ (dimensions 7.5 cm diameter, 80 cm length) cylindrical borosilicate SEOP cell filled with < 1 g rubidium located at the SEOP cell gas inlet. 200 W diode in combination with a beam expander to produce a 7.5 cm diameter circular beam; storage field (~0.2 T) and spiral glassware (8 rings, 4.5 cm diameter, 1 cm thickness) for collection of frozen Xe. ¹²⁹Xe SEOP flow parameters: A gas mixture of 3% Xe, 10% N2, 87% He was flowed through the cell at a volumetric flow rate of 600 sccm (standard cubic centimetres per minute) and collected in its solid state within a cryostat (spiral submerged in liquid nitrogen) for 50 min. The recovered gaseous Xe volume after submerging the spiral in hot water was 1000 mL, where the Xe was collected in pre-evacuated Tedlar bag. The oven housing the SEOP cell was regulated to T = 130 °C and the SEOP cell pressure was ~ 2 atm. These parameters were based on optimized ¹²⁹Xe polarization from previously reported volume/flow rate production maps.² The ¹²⁹Xe polarization was measured after 50 min of cryogenic collection using a polarization method described previously.3 Imaging parameters: Natural abundance Xe (26% ¹²⁹Xe) lung imaging parameters: 3D SSFP sequence at 3 T, TE/TR = 1.44 ms/4.6 ms; flip angle = 7°; bandwidth = ±8.5 kHz; FOV = 320x320x224mm; voxel size = 4.2x4.2x8 mm. Enriched Xe (86% ¹²⁹Xe) lung imaging parameters: 3D SSFP sequence at 3 T, TE/TR = 1.38 ms/4.5 ms; flip angle = 10°; bandwidth = ±8.5 kHz; FOV = 320x320x218mm; voxel size = 4.2 mm³. Enriched Xe (86% ¹²⁹Xe) brain imaging parameters: 2D SPGR at 1.5 T; TR/TE = 34 ms/1. 6 ms; flip angle 12.5; bandwidth = ±2kHz; FOV = 220x220 mm; voxel size = 6.9 mm.

Results and Discussion

The ¹²⁹Xe polarization was measured to be 35 % at a Xe production rate of 1200 mL/h. A performance metric called dose equivalence rate has been defined in a previous study as DE rate = f₁₂₉P₁₂₉FR_Xe, where f₁₂₉ = isotopic fraction of ¹²⁹Xe (assumed as 1 in the calculation), P₁₂₉ = ¹²⁹Xe polarization and FR_Xe = Xe production rate.³ Using this formula with FR_Xe = 1200 mL/h and P129 = 35% yields a DE rate = 420 mL/h, considerably higher than those recorded previously in a recent review tabulating ¹²⁹Xe polarizer performances across the field.³ This high ¹²⁹Xe polarization value and fast production rate has enabled collection of 1000 mL Xe in 50 min for: i) ¹²⁹Xe lung imaging with naturally abundant Xe (Fig. 2) with an average image signal-to-noise ratio (SNR) = 80; ii) 129Xe lung imaging with an isotropic voxel resolution of 4.2 mm³ with an average image SNR = 40; and (iii) ¹²⁹Xe axial brain image with SNR = 30.

Conclusion

By optimizing the SEOP cell design and running parameters from previous models, we have demonstrated the generation of highly polarized ¹²⁹Xe with rapid production rates. With this system, we have demonstrated high-SNR lung imaging with naturally abundant Xe ($25/L) and high-SNR ¹²⁹Xe brain imaging has been made possible.

Acknowledgements

No acknowledgement found.