Establishing an accurate gas phase reference frequency to quantify Xe chemical shifts in vivo.

Purpose: Xe-129 interacts with biological media to exhibit chemical shifts exceeding 200 ppm that report on physiology and pathology. Extracting this functional information requires shifts to be measured precisely. Historically, shifts have been reported relative to the gas- phase resonance originating from pulmonary airspaces. However, this frequency is not fixed- it is affected by bulk magnetic susceptibility, as well as Xe- N-2, Xe- Xe, and Xe- O-2 interactions. In this study, we addressed this by introducing a robust method to determine the 0 ppm Xe-129 reference from in vivo data. Methods: Respiratory- gated hyperpolarized Xe-129 spectra from the gas- and dissolved- phases were acquired in four mice at 2T from multiple axial slices within the thoracic cavity. Complex spectra were then fitted in the time domain to identify peaks. Results: Gas- phase Xe-129 exhibited two distinct resonances corresponding to Xe-129 in conducting airways ( varying from - 0.6 +/- 0.2 to 1.3 +/- 0.3 ppm) and alveoli ( relatively stable, at -2.2 +/- 0.1 ppm). Dissolved- phase Xe-129 exhibited five reproducible resonances in the thorax at 198.4 +/- 0.4, 195.5 +/- 0.4, 193.9 +/- 0.2, 191.3 +/- 0.2, and 190.7 +/- 0.3 ppm. Conclusion: The alveolar Xe-129 resonance exhibits a stable frequency across all mice. Therefore, it can provide a reliable in vivo reference frequency by which to characterize other spectroscopic shifts. (C) 2016 International Society for Magnetic Resonance in Medicine