Figure 2b. Phase-contrast imaging of the aorta. Magnitude (a) and phase (b) axial images show the ascending (white arrowhead) and descending (black arrowhead) aorta at the level of the main pulmonary artery (MPA in a). Flow encoding was superior to inferior. On the phase image (b), the ascending aorta and main pulmonary artery appear black and the descending aorta appears white due to the opposite directions of flow in these arteries.