Below the Aortic Annulus: Multiphase CT Features of an Interventricular Membranous Septal Aneurysm

We present interesting diagnostic images from a 77-year-old man with chronic coronary artery disease, a history of coronary artery bypass grafting (CABG), and no documented history of myocardial infarction who underwent coronary computed tomography angiography (CCTA) to assess graft patency. No operative report, previous medical documentation, or earlier cardiac imaging studies were available. CCTA demonstrated patent grafts from the left internal mammary artery to the left circumflex coronary artery (LIMA-LCx) and from the ascending aorta to a diagonal branch (aorto-diagonal graft). A stump-like protrusion of the ascending aorta was considered most consistent with the residual proximal segment of an occluded saphenous vein graft, presumably previously directed to the right coronary artery. In addition, an incidental broad-necked, partially lobulated, contrast-filled outpouching measuring approximately 28 × 17 × 29 mm was identified within the basal membranous interventricular septum. Multiplanar and multiphase reconstructions demonstrated a subannular neck communicating directly with the left ventricular outflow tract (LVOT), preservation of the muscular interventricular septum, separation from the sinus of Valsalva, and phase-dependent rightward protrusion toward the basal right ventricle. These anatomical features favored an interventricular membranous septal aneurysm (IVMSA). Because no CMR was performed, remote clinically silent ischemic injury could not be completely excluded. Overall, the lesion was considered most consistent with IVMSA; its congenital or developmental origin and temporal evolution could not be established in the absence of prior imaging and clinical documentation. This case highlights the diagnostic value of thin-section, multiphase CCTA for characterizing a septal outpouching and distinguishing IVMSA morphology from a sinus of Valsalva aneurysm and acquired postoperative or ischemic alternatives.