Monday, May 18, 2015

Systematic T1 improvement for hyperpolarized (129)xenon

Repetto M, Babcock E, Blumler P, Heil W, Karpuk S, Tullney K. Systematic T1 improvement for hyperpolarized (129)xenon. J Magn Reson. 2015;252(0):163-9.

The spin-lattice relaxation time T1 of hyperpolarized (HP)-(129)Xe was improved at typical storage conditions (i.e. low and homogeneous magnetic fields). Very long wall relaxation times T1(wall) of about 18h were observed in uncoated, spherical GE180 glass cells of slashed circle=10cm which were free of rubidium and not permanently sealed but attached to a standard glass stopcock. An "aging" process of the wall relaxation was identified by repeating measurements on the same cell. This effect could be easily removed by repeating the initial cleaning procedure. In this way, a constant wall relaxation was ensured. The Xe nuclear spin-relaxation rate 1/T1(Xe-Xe) due to van der Waals molecules was investigated too, by admixing three different buffer gases (N2, SF6 and CO2). Especially CO2 exhibited an unexpected high efficiency (r) in shortening the lifetime of the Xe-Xe dimers and hence prolonging the total T1 relaxation even further. These measurements also yielded an improved accuracy for the van der Waals relaxation for pure Xe (with 85% (129)Xe) of T1(Xe-Xe)=(4.6+/-0.1)h. Repeating the measurements with HP (129)Xe in natural abundance in mixtures with SF6, a strong dependence of T1(Xe-Xe) and r on the isotopic enrichment was observed, uncovering a shorter T1(Xe-Xe) relaxation for the (129)Xe in natural composition as compared to the 85% isotopically enriched gas.