Retropharyngeal abscess


This case demonstrates the need to keep a careful eye on prevertebral tissues on plain film and also the difficulty in determining which space the infection lies in. 

An important differential is calcific tendinitis of the longus colli muscles, however, the presence of extensive enhancement and lymph node enlargement should dissuade you from raising this as a possibility even in the presence of calcification at C1/2 level.