The delineation of the normal fatty septa within the subcutaneous tissue by a dense fluid suggests extravasated contrast media, which in this case has spread quite impressively around the elbow, up the arm and down the forearm. 

Extravasation  of contrast media is a well-known complication of contrast-enhanced CT scanning, which has become more common since the introduction of power injectors which can introduce large volumes contrast material in a short period of time.