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-129) and helium-3 (He-3) ventilation MRI and coregistered proton (H-1) anatomic MRI. Expiratory and inspiratory breath-hold CTs were used for deformable image registration and calculation of 3 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 anatomic H-1-MRI. Voxel-wise Spearman correlation coefficients were calculated between each CT-ventilation image and its corresponding He-3-/Xe-129-MRI, and for the mean values in regions of interest (ROIs) ranging from fine to coarse in-plane dimensions of 5 x 5, 10 x 10, 15 x 15, and 20 x 20, located within the lungs as defined by the same-breath 1H-MRI lung mask. Correlation of He-3 and Xe-129-MRI was also assessed. Results: Spatial correlation of CT-ventilation against He-3/Xe-129-MRI increased with ROI size. For example, for CT HU, 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 x 20 ROIs, respectively. Correlations were stronger for CTHU than for CTJac or CTSGV. Correlation of He-3 with Xe-129-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-3-MRI and CT-ventilation versus Xe-129-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-3 and Xe-129-MRI, suggesting that CT-ventilation surrogate measures may not be measuring lung ventilation alone. (C) 2018 Elsevier Inc. All rights reserved.