The correct answer is:

(Right) coronary artery aneurysm

The structure you see is a large aneurysm of the right coronary artery. Atherosclerosis accounts for most aneurysms in adults, while Kawasaki disease is responsible in younger patients and children. The (proximal) right coronary artery is most frequently involved.

Imaging Tip:
You will find an echo-free "cavity" on the echocardiogram with flow within the structure (color Doppler). If you rotate around the structure, you can follow the course of the vessel, which is often tortuous. First, look for the structure's connection to the ascending aorta (coronary ostium)! There you will usually find higher flow velocities. Left-side contrast will reach the left heart before it opacifies the vessel.

(Right) coronary artery aneurysm

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Thomas Binder and the 123sonography team