Image‐Derived Volumetry During Forced Expiration Using a <scp>3D</scp> Fermat Looped, Orthogonally Encoded Trajectory

PURPOSE: To enable MRI-based nongated 3D depiction of forced expiration with both high spatio-temporal resolution and fidelity of lung dynamics. METHODS: An undersampled 3D Fermat looped, orthogonally encoded trajectory (FLORET) was optimized for nonsegmented dynamic acquisitions during forced expiration and a corresponding model-based reconstruction pipeline promoting spatial and temporal total variation was implemented. The approach was first optimized and validated using a lung phantom, specifically constructed for this study and was then translated to in vivo measurements. This allowed high temporal resolution of 200 ms and isotropic spatial resolution of 5 mm. RESULTS: Volumes derived by segmenting the MR images of the phantom demonstrated excellent agreement with the air exchange measured directly at the phantom within the bore, therefore confirming the validity of the approach. In both phantom and in vivo experiments, the dynamic deflation process could be successfully depicted, confirming the potential for assessing forced expiration with 3D UTE FLORET MRI. From the segmented volumes in vivo, quantitative parameters were derived, including the duration of the dynamic phase (2.8 ± 0.2 s), the time to 90% deflation (1.8 ± 0.2 s), and the volume change within the first second (2.8 ± 0.2 L). CONCLUSION: The phantom and in vivo studies demonstrate, that the 3D FLORET trajectory can be utilized to study lung deflation during forced expiration with high temporal resolution of 200 ms and isotropic spatial resolution of 5 mm using a TV-regularized primal-dual SENSE reconstruction. Acquisitions were performed for discrete distinct time frames, that is, no segmented k-space filling across several respiratory cycles was used.