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 computed tomography (CT)-ventilation images with hyperpolarized gas magnetic resonance imaging (MRI).

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

Results: Spatial correlation of CT-ventilation against He/Xe-MRI increased with ROI size. For example, for CT, 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 20 × 20 ROIs, respectively. Correlations were stronger for CT than for CT or CT. Correlation of He with Xe-MRI was consistently higher than either gas against CT-ventilation maps over all ROIs (P < .05). No significant differences were observed between CT-ventilation versus He-MRI and CT-ventilation versus Xe-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 that between He and Xe-MRI, suggesting that CT-ventilation surrogate measures may not be measuring lung ventilation alone.