Common causes of pericardial effusion include heart failure,
renal insufficiency, infection (bacterial, viral, or tuberculous), neoplasm (carcinoma of lung or breast, or lymphoma), and injury (from trauma or myocardial infarction).
You didn't mention what tests were given. May require a CT scan or MR of the pericardial anatomy (outer sac covering the heart) to properly characterize and delineate pericardial lesions such as pericardial thickening, pericardial mass, etc.