OBJECTIVE: We sought to demonstrate the distinguishing features between interarterial and intraseptal courses of an anomalous left coronary artery from the right sinus of Valsalva (RSV) on X-ray angiography, using an ex vivo model. BACKGROUND: An anomalous left main coronary artery (LMCA) arising from the RSV can take prepulmonary, retro-aortic, interarterial (IA) or intraseptal (IS) courses, of which only the IA course is associated with sudden death. Anomalous LMCA is usually identified during catheter angiography. On Xray angiography, IA and IS courses have common characteristics that makes their distinction challenging. We hypothesized that the cranialcaudal orientation of the vessel on X-ray angiography allows these pathways to be distinguished, and tested this hypothesis using an ex vivo heart model. METHODS: Plastic tubing was inserted along the IA and IS courses in an ex vivo normal pig heart. X-ray imaging in standard views and MRI on a 3-T scanner were performed. RESULTS: In a normally formed heart, an anomalous LMCA with IA path must take a cephalad course, superior to the pulmonary valve. Conversely, an IS vessel will pass caudally, at or below the level of the infundibular septum. These findings were demonstrated in the X-ray angiograms and confirmed by magnetic resonance imaging. CONCLUSIONS: X-ray angiography can differentiate IA and IS courses of an anomalous LMCA in the normally formed heart. This may obviate the need for further cross-sectional imaging in many cases.