Spatial comparison of CT-based surrogates of lung ventilation with hyperpolarized Helium-3 and Xenon-129 gas MRI in patients undergoing radiation therapy

Purpose To develop and apply an image acquisition and analysis strategy for spatial comparison of CT-ventilation images with hyperpolarized gas MRI.

Methods 11 lung cancer patients underwent 129Xe and 3He ventilation MRI and co-registered 1H anatomical MRI. Expiratory and inspiratory breath-hold CTs were used for deformable image registration and calculation of three CT-ventilation metrics: Hounsfield unit (CTHU), Jacobian (CTJac) and specific gas volume change (CTSGV). Inspiration CT and hyperpolarized gas ventilation MRI were registered via same-breath anatomical 1H-MRI. Voxel-wise Spearman correlation coefficients were calculated between each CT-ventilation image and its corresponding 3He/129Xe-MRI, and for the mean values in regions of interest (ROIs) ranging from fine to coarse in-plane dimensions of 5x5, 10x10, 15x15 and 20x20, located within the lungs as defined by the same-breath 1H-MRI lung mask. Correlation of 3He and 129Xe-MRI was also assessed.

Results Spatial correlation of CT-ventilation against 3He/129Xe-MRI increased with ROI size. For example, for CTHU, mean±SD Spearman coefficients were 0.37±0.19/0.33±0.17 at the voxel-level and 0.52±0.20/0.51±0.18 for 20x20 ROIs, respectively. Correlations were stronger for CTHU than for CTJac or CTSGV. Correlation of 3He with 129Xe-MRI was consistently higher than either gas against CT-ventilation maps over all ROIs (p<0.05). No significant differences were observed between CT-ventilation vs 3He-MRI and CT-ventilation vs 129Xe-MRI.

Conclusion Comparison of ventilation-related measures from CT and registered hyperpolarized gas MRI is feasible at a voxel level using a dedicated acquisition and analysis protocol. Moderate correlation between CT-ventilation and MRI exists at a regional level. Correlation between MRI and CT is significantly less than between 3He and 129Xe-MRI, suggesting that CT-ventilation surrogate measures may not be measuring lung ventilation alone.