This is the same frame with a harder window, and there is a large lucent
defect in the left atrium. This is a classic left atrial myxoma. Calcification
in a myxoma is common enough but I have not noticed before that they can
be of slightly low density.
Image 3 is to remind us that occasionally the right atrium can host a myxoma
(courtesy Dr Mike Rubens). Just as on the left side, the tumour is pedunculated
onto the atrial septum.
See a video version.