Intermolecular Dipole-Dipole Relaxation of lz9Xe Dissolved in Water: Implications for Spin Polarization-Induced NOE in the Lungs

Introduction Hyperpolarized gas MRI has been shown capable of generating high-resolution images of air spaces. Although useful, these images carry only indirect information about the lung parenchyma itself. Spin polarization-induced nulcear Uverhauser effect (SPINOE) has been successfully used to create proton enhancements in few molecules that interact with xenon (1). Intriguing is therefore the prospect of lung parenchyma imaging based on xenon-tissue SPINOE. The SPINOE is possible when xenon relaxes through dipole-dipole interactions with the protons. However, a study by Stith et al. (2) shows that the mere existence of proton-induced enhancement of the xenon relaxation does not necessarily render the SPINOE observable. Other competing relaxation mechanisms can reduce the polarization transferred substantially. Therefore, the dipolar contribution to the xenon relaxation must be significant if SPINOE is to be detected. Here, the contribution of the dipolar interaction to the relaxation of 129Xe in water is examined. Existence of cross-relaxation between 129Xe and the water protons is demonstrated. The contribution of the dipolar coupling on the xenon relaxation is shown to be significant. The feasibility of xenon SPINOE in the lungs is discussed.