Abstract
Hyperpolarized (hp) 129Xe and hp 83Kr for magnetic resonance imaging (MRI) are typically obtained through spin-exchange optical pumping (SEOP) in gas mixtures with dilute concentrations of the respective noble gas. The usage of dilute noble gases mixtures requires cryogenic gas separation after SEOP, a step that makes clinical and preclinical applications of hp 129Xe MRI cumbersome. For hp 83Kr MRI, cryogenic concentration is not practical due to depolarization that is caused by quadrupolar relaxation in the condensed phase. In this work, the concept of stopped flow SEOP with concentrated noble gas mixtures at low pressures was explored using a laser with 23.3 W of output power and 0.25 nm linewidth. For 129Xe SEOP without cryogenic separation, the highest obtained MR signal intensity from the hp xenon-nitrogen gas mixture was equivalent to that arising from 15.5±1.9% spin polarized 129Xe in pure xenon gas. The production rate of the hp gas mixture, measured at 298 K, was 1.8 cm3/min. For hp 83Kr, the equivalent of 4.4±0.5% spin polarization in pure krypton at a production rate of 2 cm3/min was produced. The general dependency of spin polarization upon gas pressure obtained in stopped flow SEOP is reported for various noble gas concentrations. Aspects of SEOP specific to the two noble gas isotopes are discussed and compared with current theoretical opinions. A non-linear pressure broadening of the Rb D1 transition was observed and taken into account for the qualitative description of the SEOP process.
Highlights
Nuclear magnetic resonance imaging (MRI) of the respiratory system using hyperpolarized 129Xe is increasingly attracting attention for clinical [1,2,3,4,5,6] and preclinical research [7,8]despite the associated lower signal intensities compared to the more established hp 3He MRI [9,10]
MRI applications is possible through stopped flow spin exchange optical pumping (SEOP) with high noble gas concentrations at low total gas pressures
Recompression of the hp gases after SEOP is a necessary step with this technique and preliminary work resulted to Papp~12:4% and Papp~2:6% after recompression
Summary
Nuclear magnetic resonance imaging (MRI) of the respiratory system using hyperpolarized (hp) 129Xe is increasingly attracting attention for clinical [1,2,3,4,5,6] and preclinical research [7,8]despite the associated lower signal intensities compared to the more established hp 3He MRI [9,10]. The isotope 83Kr possesses a nuclear electric quadrupole moment (eQ) that may enable hp 83Kr to be used as a surface sensitive contrast agent and biomarker [14,15] Both noble gas isotopes, 129Xe (nuclear spin I = 1/2) and 83Kr (I = 9/2), can be hyperpolarized through spin exchange optical pumping (SEOP) with alkali metal vapor [16,17,18,19]. For SEOP, the noble gases are typically diluted in helium - nitrogen mixtures and, in the case of 129Xe, the hp xenon is subsequently separated from the other gasses by a freezethawing cycle using a cold trap at 77 K [5,21,22,23] This process is not viable for hp 83Kr because of its rapid quadrupolar relaxation in the frozen state [24,25].
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