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The cardiac size is normal and the main pulmonary artery is dilated, as evidenced by a bulge along the upper end of the left cardiac border (MPA in the image below). If main pulmary artery segment alone is dilated, the diagnostic possibility is an idiopathic dilatation of pulmonary artery. In post stenotic dilatation of pulmonary artery, the left pulmonary artery is also dilated (seen as a shadow tracking down from the main pulmonary artery, within the cardiac silhouette). In pulmonary hypertension, both right and left pulmonary arteries are dilated, along with the main pulmonary artery. Another cause for a prominent main pulmonary artery segment is partial absence of the left pericardium. In that situation, the left atrial appendage also bulges out, just below the main pulmonary artery segment.
MPA: main pulmonary artery (post stenotic dilatation of main pulmonary artery in this case)