Main pulmonary artery segment is usually concave on a normal CXR. Here it is seen as convex, bulging to the left. Left atrial appendage is seen as a bulge to the left below the main pulmonary artery segment. This bulge is also known as the third mogul sign; the first two moguls being the aorta and main pulmonary artery. A slight prominence of right atrial contour along the right heart border is also seen. Right pulmonary artery is also slightly prominent in the right hilum. Reticular lines below the right hilum gives a semblance of Kerley C lines.
Kerley C lines represent Kerley B lines seen en face. Kerley B lines are horizontal lines seen in the lower zones, representing edema of the interlobular septa. Kerley A lines are linear opacities extending from the hilum to the periphery, representing distended anastomotic channels between central and peripheral lymphatics .
RA: Right atrium; Ao: Aorta; MPA: Main pulmonary artery; LAA: Left atrial appendage