Spatial Comparison of CT-Based Surrogates of Lung Ventilation With Hyperpolarized Helium-3 and Xenon-129 Gas MRI in Patients Undergoing Radiation Therapy.

Abstract

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). Eleven lung cancer patients underwent xenon-129 (¹²⁹Xe) and helium-3 (³He) ventilation MRI and coregistered proton (¹H) anatomic MRI. Expiratory and inspiratory breath-hold CTs were used for deformable image registration and calculation of 3 CT-ventilation metrics: Hounsfield unit (CT^HU), Jacobian (CT^Jac), and specific gas volume change (CT^SGV). Inspiration CT and hyperpolarized gas ventilation MRI were registered via same-breath anatomic 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 5 × 5, 10 × 10, 15 × 15, and 20 × 20, located within the lungs as defined by the same-breath ¹H-MRI lung mask. Correlation of ³He and ¹²⁹Xe-MRI was also assessed. Spatial correlation of CT-ventilation against ³He/¹²⁹Xe-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 × 20 ROIs, respectively. Correlations were stronger for CT^HU than for CT^Jac or CT^SGV. Correlation of ³He with ¹²⁹Xe-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 ³He-MRI and CT-ventilation versus ¹²⁹Xe-MRI. 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 ³He and ¹²⁹Xe-MRI, suggesting that CT-ventilation surrogate measures may not be measuring lung ventilation alone.