This is one of the most dangerous complications that can occur after an acute myocardial infarction, a pseudoaneurysm of the left ventricle.  This is in fact a contained rupture.  There has been so much damage to the myocardium that it has split open, and blood is only held in place by the thin outer connective tissue of the epicardium.  The endocardium and myocardium are not present around the margins of this out-pouching, only epicardium. Try to find the leak before going on to the labeled image.

Cardiac CT with contrast