Figure 19c. Pulmonary atresia with VSD. LPA = left pulmonary artery, RPA = right pulmonary artery. (a) CT scan shows the confluent portion of the central pulmonary artery (arrowheads). (b) CT scan shows absence of the confluent portion of the central pulmonary artery (*). Each pulmonary artery was supplied by a corresponding PDA (not shown). (c) CT scans depict the major aortopulmonary collateral vessels that supply the branch pulmonary arteries. These collateral vessels arise from the descending thoracic aorta (dA).