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 ( ¹²⁹ Xe) and helium-3 ( ³ He) ventilation MRI and coregistered proton ( ¹ 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 ^HU ), Jacobian (CT ^Jac ), and specific gas volume change (CT ^SGV ). Inspiration CT and hyperpolarized gas ventilation MRI were registered via same-breath anatomic ¹ H-MRI. Voxel-wise Spearman correlation coefficients were calculated between each CT-ventilation image and its corresponding ³ He-/ ¹²⁹ 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 ¹ H-MRI lung mask. Correlation of ³ He and ¹²⁹ Xe-MRI was also assessed.
Results: Spatial correlation of CT-ventilation against ³ He/ ¹²⁹ Xe-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 ³ He with ¹²⁹ 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 ³ He-MRI and CT-ventilation versus ¹²⁹ 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 ³ He and ¹²⁹ Xe-MRI, suggesting that CT-ventilation surrogate measures may not be measuring lung ventilation alone.
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