Hyperpolarized Xe-129

Hyperpolarized (HP) 129Xenon can be used as a contrast agent in MRI. The number of lung’s studies performed with HP 129Xe has increased in the last three years due to its low price and high availability compared with 3He. HP 129Xe also has some advantages over 3He, like solubility in lipids and blood allowing dynamic and functional MRI tests for ventilation and perfusion [1]. HP 129Xe is also used in our group in fundamental physics research like search for physics beyond the standard model in clock-comparison experiments [2,3] and search for the electric dipole moment of 129Xe as an example for time reversal violation [4] . 

The method to produce HP 129Xe is based on Spin Exchange Optical Pumping (SEOP) [5]. The achievable degree of polarization depends on the optical pumping cell configuration (Rb saturation), production rate (pressure, gas-flow, % 129Xe on the gas mixture), the magnetic field properties and the freezing/thaw method necessary for the separation of HP 129Xe from the buffer gases (N2, 4He). A sketch of a standard Xe-polarizer [6] is shown in Fig.1.

 

 

 

Fig.1: 129Xe is polarized via SEOP in a cylindrical pumping cell and then separated from the buffer gases (N2, 4He) in a cold finger kept at LN2 temperature

 

A mobile Xe-polarizer (Fig.2) was upgraded for fundamental physics applications with a production rate of 0.2 bar*l/h. The main components are:

- a  30 W diode laser (wavelength 794.7 ± 0.15 nm)                                                             

- a holding field is provided by 6 coils which produce a B=20 G and ΔB/B<10-3/cm)                                                                                                                     

- the pumping cell is a glass cylinder with a spiral-shaped Rb saturator inside of an oven at 100°C<t<200°C

- flow controller (200-300 ml/min), the flow is opposite to the direction of the laser propagation (counter-flow [7])

- spiral-shaped cold trap is inside a Halbach magnet (0.3 T) for the accumulation of HP 129Xe in solid state [8]

- a freeze-thaw system allows a short time for the solid-gas transition minimizing relaxation losses [6]                                                                          

- a gas filling unit for mixing HP 129Xe (Fig.3) with suitable buffer gases to reduce the Xe-Xe van der Waals molecules lifetime [9].

 Fig.2: Portable Xe-polarizer. The red arrow indicates the laser beam direction while the blue arrow indicates the flow’s direction 

 

 Fig.3: Filling system

 

Once produced, the HP 129Xe gas must be stored in vessels free of paramagnetic impurities in order to minimize the relaxation due to wall interactions. The relaxation time measurements (T1) are perform in a low field NMR-spectrometer (Fig.4) which allows measuring T1 at different storage conditions. Fig.5 shows the result of such a relaxation experiment. So far, the measured total relaxation times for HP 129Xe in GE180 glass cells of 10 cm diameter reach up to 180 minutes. 

 

Fig.4: Low field NMR spectrometer (B0=23 G)


Fig.5: Pure HP 129Xe realxation time measurement with the low field NMR spectrometer 

 

For medical applications, a 129Xe-polarizer with about 1 bar*l/h production rate is under construction. A spectrally narrowed pump-laser (wavelength 794.70±0.14 nm) is build which is well adapted to the pressure-broadened Rb absorption line. The free running laser diode-array has a power of 100 W and a spectral width of 2 nm. With help of an external cavity resonator (see Fig.6 and Fig.7) the spectral width was successfully reduced by a factor of ten with a final output power around 60 W [10].

 

 

 

Fig.6: Specrally narrowed laser construction 

 

 

Fig.7: Spectrum from a free running and a spectrally narrowed diode array laser


Literature

1.  J. Wolber et al., Proc. Natl. Acad. Sci. USA, 96 (1999) 3664–3669 

2.  C.Gemmel et. al., Phys Rev. D 82 (2010) 111901 (R)

3.  K.Tullney et al., Physical Review Letters 111 (2013) 100801 

4.  W.Heil et al., Annalen der Physik 525 (2013) 539-549

5.  B.M.Goodson, Journal of Magnetic Resonance 155 (2002) 157-216

6.  B.Driehuys et al., Appl.Phys.Lett. 69 (1996) 1668-1670

7.  H.Raich and P.Blümler, Conc. Magn. Reson. 36A  (2010) 211-222

8.  I.C.Ruset, S.Ketel and F.W..Hersman, Physical Review Letters 96 (2006) 053002

9.  B.Chann et al., Physical Review Letters 88 (2002) 113201

10. S.R.Parnell et al., Nucl. Instr. and Meth. in Phys. Res. A 598 (2009) 774-778