Case 41 Answer

Big ups to those who answered! Everyone got it correct!

This was a bit of a eye test. The subcutaneous air was actually a hint.  Check out the diaphragms, are they continuous?

Answer: Continuous diaphragm sign - ie Pneumomediastinum

short write ups:

http://radiopaedia.org/articles/continuous-diaphragm-sign

http://residents.thoracicrad.org/BasicSign/CDS.html

nice archival paper on pneumomediastinum signs: http://www.ajronline.org/doi/pdf/10.2214/ajr.166.5.8615238