The vast majority of cardiac silhouette enlargement is going to be from cardiomegaly, but occasionally it will be from a pericardial effusion. There are a few signs (e.g. water bottle sign, Oreo cookie sign), but often no differentiation can be made.

Echocardiography is the easiest modality to assess for pericardial effusion (this patient was clinically suspected of having aortic dissection, hence the CT angiogram).