Cor triatriatum masquerading as a cardiac mass: the critical role of cardiac magnetic resonance

A 32-year-old woman presented with a 2-year history of intermittent chest tightness and haemoptysis. Transthoracic echocardiography (TTE, Panel H) revealed a hypoechoic mass near the right pulmonary vein orifices with no detectable flow. Computed tomography angiography (CTA, Panels E–G, white arrows) showed a low-density lesion in the left atrium, absence of contrast opacification in the right pulmonary veins, and numerous perivenous collateral vessels (see Supplementary data online, Video S1). The initial diagnosis was a left atrial mass affecting the right pulmonary veins. Cardiac magnetic resonance (CMR) definitively diagnosed cor triatriatum. The cine sequence demonstrated left atrium divided by a linear low-signal membrane into an accessory chamber (AC) and true atrium (TA). All right pulmonary veins drained into the AC, demonstrating disordered flow, while left veins drained into the TA (Panel A, white arrow; Supplementary data online, Video S2). On perfusion imaging, the AC and right pulmonary veins showed no early filling (Panel B, white arrow) but slow contrast wash-in, with signal intensity eventually matching the TA on both late perfusion and delayed enhancement (Panels C–D, white arrow; Supplementary data online, Videos S3 and S4).