Figure 1.

Upper: Axial ultrashort echo-time MRI retrospectively gated to end-expiration (left) and end-inspiration (right) via the motion-modulated center of the MRI k-space. The edge of the tracheal lumen is highlighted in green and magenta, respectively. Lower: Airway surfaces generated from segmentations of the airway from the ultrashort echo-time MRI shown from the posterior aspect. The trachealis is concave in the end-expiration image. Trachea cross-sections are shown for each of the end-expiration (green) and end-inspiration (magenta) images. These cross-sections are used to calculate the cross-sectional area at the two points in the respiratory cycle.