Single-breath Fractional Ventilation Derived From 129Xe MRI: Repeatability in Idiopathic Pulmonary Fibrosis
Du, K; Leewiwatwong, S; Huang, Y-CT; Driehuys, B; Mummy, D
Published in: American Journal of Respiratory and Critical Care Medicine
Rationale: Current methods for assessing ventilation heterogeneity via hyperpolarized 129Xe MRI generally rely on histogram-based techniques such as rescaling to arbitrary thresholds or image-based clustering to calculate the ventilation defect percent (VDP). These approaches are based on relative signal intensities and are not intuitively related to physiology. Here, we present a novel method to calculate fractional ventilation (FV) from single-breath 129Xe MRI and evaluate its repeatability compared to conventional 99th percentile histogram rescaling. Methods: 23 participants with idiopathic pulmonary fibrosis (IPF) (8F, age 74.5±5.7) underwent 3D radial ventilation imaging with RF-depolarization bias correction at baseline and after three months. FV was calculated for each participant by dividing the total xenon signal measured within the segmented lung mask by the known volume of the inhaled xenon dose, modified to account for anatomical dead space. The resulting “xenon signal to volume” ratio was then used to estimate voxel-wise FV, defined as the volume of xenon mixture per voxel (as estimated from the xenon signal) divided by the voxel volume. VDP using the FV method (VDP-FV) was computed as the percentage of the histogram ≥2sd below the healthy reference mean derived from a healthy reference cohort. Repeatability between baseline and 3-month follow-up was assessed using Bland-Altman analysis, the intraclass correlation coefficient (ICC), and the coefficient of repeatability (CR) for both the VDP-FV and VDP using the 99th percentile histogram normalization (VDP-99). Results: Mean FV in the healthy reference cohort was 0.32±0.16 vs. 0.24±0.11 in IPF at baseline (p=0.003). Mean FV in IPF at 3-months was 0.21±0.092 (p=0.18 vs. baseline). The baseline VDP-FV was 10.5±6.53% vs. 4.02±4.05% for VDP-99. The limits of repeatability (LoR) for VDP-FV were [−6.36%-10.80%], ICC=0.80 and CR=8.79, vs. LoR of [−6.86%-8.84%], ICC=0.53, and CR=8.02 for VDP-99. Conclusions: Our proposed FV method provides physiologically plausible estimates of regional fraction ventilation based on first principles, unlike conventional methods of assessing ventilation heterogeneity based on relative intensity. Despite VDP-FV having a larger average magnitude than VDP-99, both methods had similar LoR. However, the ICC for VDP-FV was strikingly higher than for VDP-99, indicating more consistent measurements over the 3-month time period. Work is ongoing to further improve the robustness and repeatability of this measurement and further characterize FV patterns in other populations.
