Hyperpolarized ⁸³Kr and ¹²⁹Xe NMR relaxation measurements of hydrated surfaces: Implications for materials science and pulmonary diagnostics

In this proof of principle work, a technique is introduced to study hydrated surfaces using hyperpolarized (hp) ⁸³Kr NMR spectroscopy. The longitudinal (T₁) relaxation of hp-⁸³Kr is shown to be extremely sensitive to the presence of adsorbed water on hydrophilic borosilicate and hydrophobic siliconized glass surfaces. The krypton surface relaxation is found to be largely independent of the total gas pressure applied to the studied materials, and the presented technique is therefore fairly robust. However, the relaxational properties of hp-⁸³Kr can be "tuned" by adjusting the composition of the optical pumping gas mixture. This effect may be important for practical applications such as hp-⁸³Kr MR imaging and can be achieved without sacrificing signal intensity. Complementary information to that of hp-⁸³Kr surface relaxation data can be obtained from hp-¹²⁹Xe relaxation measurements that are sensitive to the presence of paramagnetic surface sites. In contrast to the signal decay of hp-¹²⁹Xe, the longitudinal relaxation of ⁸³Kr is largely unaffected by paramagnetic impurities, and in some materials, ⁸³Kr and ¹²⁹Xe show comparable T₁ times that are caused by two completely different relaxation mechanisms. Finally, the relaxation times of ⁸³Kr in contact with bovine lung surfactant coated glass pores that are similar in size to mammalian alveoli are presented. The results suggest that in vivo MR studies may be feasible and could provide valuable information about changes in pulmonary surface chemistry.