Pericardial effusions occur when fluid collects in the pericardial space (a normal pericardiac sac contains approximately 30-50 ml of fluid).   CT makes the diagnosis extremely easy, but is usually obtained to try and clarify the cause of an effusion rather than to confirm the diagnosis. Pericardial effusions are a frequent incidental finding in unwell hospitalised patients.   http://radiopaedia.org/articles/pericardial-effusion