Correcting ¹²⁹Xe Gas Exchange MRI for Incidental Gas-Phase Excitation-Comparing Approaches, and Identifying...

Incidental excitation of the gas-phase resonance during ¹²⁹Xe gas exchange MRI introduces contamination that can bias membrane uptake and red blood cell (RBC) transfer metrics. Here, we defined the level of gas-phase contamination that is acceptable, and compared the dual-echo (Hahn) and spectroscopy-informed (Willmering) frameworks to correct for it.

Nine healthy volunteers underwent interleaved 3D radial dual-echo Dixon-based ¹²⁹Xe MRI and spectroscopy. The dissolved-phase RF pulse was shortened from its optimal value to increase incidental gas contamination. The Hahn and Willmering corrections were implemented and refined using a grid search over a contamination-dominated ROI to determine any platform-specific scale and phase adjustments. The effects of contamination phase and scale were tested by synthetically reintroducing gas signal at phase offsets that maximized defect- and high intensity-like artifacts. Membrane/gas and RBC/gas defect and high percentages were evaluated against thresholds derived from a healthy reference cohort to determine acceptable gas contamination.

Images acquired with the shortened pulse contained 24% ± 6% gas contamination. After applying empirically calibrated phase shifts (Hahn≈230°, Willmering≈49°) and no scale adjustment (optimal value≈1), both correction methods increased RBC SNR by ∼9% and reduced background noise. Phase- and scale-sweep experiments indicated that gas contamination becomes unacceptable once it exceeds ∼9% ± 3%, at which point membrane and RBC defect or high percentages surpass their thresholds.

Gas-phase contamination beyond ∼9% requires correction. After a one-time empirical phase calibration, this can be equally well achieved by both the Hahn and Willmering methods.