Fig. 10From: CT imaging features of atrioventricular shunts: what the radiologist must knowIn normal embryological development, the OP closes and fuses with the endocardial cushions. The ostium secundum appears simultaneously, as illustrated in Fig. 2. An OP defect occurs when the OP fails to close and fuse with the endocardial cushions (asterisk), shown in frontal (F) and lateral (L) view. LA left atrium, RA right atrium, SP septum primum, OS ostium secundumBack to article page