Free-breathing 3D pulmonary ventilation mapping at 0.55 T using stack-of-spiral out-in bSSFP.

Purpose: To develop a free-breathing pulmonary imaging technique that provides three-dimensional (3D) structural images and regional ventilation maps, and to evaluate its repeatability and accuracy compared with two-dimensional phase-resolved functional lung (PREFUL) and global tidal volume.

Methods: A free-breathing 3D stack-of-spiral out-in (SOS out-in) balanced steady-state free precession (bSSFP) sequence with self-navigators was designed to achieve 2-mm isotropic resolution in 5 min. Respiratory-resolved images were reconstructed using spatial L1 wavelet and temporal finite-difference constraints. The 3D ventilation maps were generated based on Jacobian determinant of the estimated nonrigid deformations. Six healthy volunteers were scanned in supine and prone positions. Ventilation maps were compared with two-dimensional PREFUL from two matched slices. Test-retest repeatability was assessed using Bland-Altman analysis. Correlations among the proposed method, PREFUL, and global tidal volume were evaluated.

Results: In healthy volunteers, the SOS out-in lung images provided sufficient vessel-parenchyma contrast and boundary sharpness to support accurate ventilation estimation. Regional ventilation measurements from 3D SOS out-in demonstrated good repeatability (relative differences < 10%). Ventilation maps from 3D SOS out-in strongly correlated with PREFUL on a slice-matched basis as well as with global tidal volume (R² > 0.7, p < 0.001).

Conclusion: The proposed method provides high-quality respiratory-resolved structural images and 3D ventilation mapping in a single 5-min scan at 0.55 T. Ventilation measurements are sensitive, consistent, and in good agreement with PREFUL and spirometry.