Findings: Non-gated CTA of the chest: Filling defect in the right superolateral aspect of the left atrium (LA) and extending to the right superior pulmonary vein (RSPV) Gated CTA of the chest. Thin septum (thin red arrow) appears to divide the LA into 2 unequal sized chambers. Smaller chamber excluded by membrane shows delayed, relatively poor enhancement and a filling defect (thick red arrow). The smaller chamber communicates with the RSPV. The larger chamber shows normal enhancement. Cardiac MR, Bright blood true SSFP (Steady State Free Precession). A thin septum is noted within the LA. The proximal, smaller chamber appears to receive blood flow from the right superior pulmonary vein.