1100 AJR:195, November 2010 with the left due to lung expansion) displace the flap valve against the septum secundum. Eventually in two thirds of the population, the flap valve fuses with the septum secundum creating a morphologic and physiologic sepa- ration of the atria [2]. The lack of fusion be- tween the flap valve and septum secundum re- sults in a probe patent foramen ovale (PFO) or PFO [3] (Fig. 5). Types of Atrial Septal Defects Atrial septal defects (ASDs) can be classi- fied as morphologic defects of the interatrial septum that directly communicate the right and left atria (ostium primum ASD, endocar- dial cushion defects, ostium secundum ASD) and as defects that do not involve the inter- atrial septum but physiologically behave like interatrial septal defects (sinus venosus ASD, unroofed coronary sinus). The different types of ASDs can be differentiated from one an- other on CT images by location (Fig. 6). Ostium Primum Atrial Septal Defects Ostium primum ASDs are considered the mildest form in the spectrum of endocardi- al cushion defects. They represent 2–3% of the ASDs and are commonly associated with Down syndrome. During embryologic devel- opment, the endocardial cushions form the medial aspects of the mitral and tricuspid valves, the portion of the atrial septum adja- cent to the atrioventricular valves, and the in- let portion of the ventricular septum. When the defect is isolated because of the failure of fusion between the free edge of the septum primum and the endocardial cushion, it is termed “ostium primum ASD.” If this defect is associated with abnormal development of Embryology and Developmental Defects of the Interatrial Septum Carlos A. Rojas 1 Ahmed El-Sherief Hector M. Medina Jonathan H. Chung Garry Choy Brian B. Ghoshhajra Suhny Abbara Rojas CA, El-Sherief A, Medina HM, et al. 1 All authors: Department of Cardiac Imaging, Harvard Medical School and Massachusetts General Hospital, 165 Cambridge St., CPZ-4-400, Boston, MA 02114-2696. Address correspondence to C. A. Rojas (caranrojas@gmail.com). Cardiopulmonary฀Imaging฀•฀Pictorial฀Essay CME This article is available for CME credit. See www.arrs.org for more information. AJR 2010; 195:1100–1104 0361–803X/10/1955–1100 © American Roentgen Ray Society C ongenital defects of the interatrial septum are the most common con- genital heart disease presenting in adulthood [1]. To better recognize their imaging appearances, it is necessary to understand the normal development of this re- gion. Cardiac CT can help determine the type of defect and associated abnormalities. Development of the Interatrial Septum The interatrial septum (Fig. 1) is the struc- ture that divides the primary atrium into the right and left atrial chambers. Starting the fifth week of gestation, the septum primum begins to develop, growing toward the en- docardial cushions. The progressively di- minishing space between the endocardial cushions and the septum primum is known as the ostium primum (Fig. 2). Before the septum primum fuses with the endocardi- al cushions, small perforations develop and coalesce in the cephalic portion of the sep- tum primum, which is known as the ostium secundum. In the meantime, to the right of the septum primum, the septum secundum starts to form as an invagination of the atrial wall. The septum secundum stops growing at the end of the seventh week of gestation, leaving a posterior and inferior gap known as the fossa ovalis. The lower portion of the septum primum persists into adulthood and is known as the flap valve (Fig. 3). Exces- sive apoptosis (i.e., programmed cell death) of the cephalic portion of the septum pri- mum or incomplete development of the sep- tum secundum results in an ostium secun- dum ASD (Fig. 4). Changes in intracardiac pressure at birth (i.e., a drop in right heart pressures compared Keywords: atrial septal defects, cardiac CT, CT angiography, interatrial septum, MRI, patent foramen ovale, patent fossa ovale DOI:10.2214/AJR.10.4277 Received January 14, 2010; accepted after revision April 6, 2010. OBJECTIVE. The various types of atrial septal defects (ASDs) can be differentiated on the basis of their imaging appearance on MDCT. CONCLUSION. It is fundamental for the cardiac imager to understand the embryologic development of the interatrial septum and the morphogenic differences of ASDs. Rojas et al. Defects of the Interatrial Septum Cardiopulmonary Imaging Pictorial Essay Downloaded from www.ajronline.org by 52.73.204.196 on 05/17/22 from IP address 52.73.204.196. Copyright ARRS. For personal use only; all rights reserved