Pericardial effusion: Lateral radiograph of the chest demonstrating the pericardial fat (yellow line) and epicardial fat (red line) with opaque density separating them, indicating pericardial effusion. See oreo cookie for comparison.
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