Still frame from an angiogram with radiocontrast dye injected using a pigtail catheter kept in the right brachiocephalic artery showing major aortopulmonary collateral artery (MAPCA) arising from the right internal mammary artery (RIMA). RSA: right subclavian artery; Right CCA: right common carotid artery; Left CCA: left common carotid artery. Left subclavian artery is not visualised well as the dye reflux into the arch of aorta is not enough to opacify it and the proximal holes of the pigtail are beyond its origin. The second frame (below) gives a better picture of the tortuous branches of the MAPCA. MAPCAs are seen in severe forms of Tetralogy of Fallot and pulmonary atresia. MAPCAs usually arise from the descending aorta. Strictly speaking the collateral arising from RIMA is not a major “aorto” pulmonary collateral, though it can be considered a MAPCA in the wider sense of the meaning. When the lungs are supplied by multiple MAPCAs, they are unifocalised prior to definitive surgical repair of Tetralogy of Fallot. Connecting the distal end of MAPCAs to a single vessel is known as unifocalization. Collaterals to the pulmonary arterial branches can also arise from the bronchial arteries within the lungs. Hilar collaterals can also occur in pulmonary atresia.