Case 29 Answer

nice job everyone!

Answer: left atrial appendage thrombus

There isn’t a lot to work with her but as you all noted, there is an obvious filling defect in one of the chambers of the heart, which as you all noted is actually the left atrial appendage (nice coronal view here - see the open arrow, not the persistent left SVC). 

Here is another case of thrombus in the atrium itselt. Nice, thorough write-up as well: http://www.imagingsciencetoday.com/teaching-case/cardiothoracic/20100707/left-atrial-thrombus-1515.html