• free air around mediastinal structures
  • blunt or penetrating trauma, esophageal perforation, iatrogenic, pulmonary infections, gas forming organisms, cocaine inhalation, air extension from pneumothorax
  • air per se is of no clinical significant in mediastinum, maybe mild substernal pain
  • ring around artery sign: air around usually right pulmonary artery
  • sail sign: elevation of thymus
  • air anterior to pericardium on lateral xray
  • continuous diaphragm sign: air trapped posterior to pericardium
  • tubular artery sign: air adjacent to major branches of aorta
  • double bronchial wall sign: air around bronchus depicts walls clearly
  • medial pneumothorax: air bounded on lateral side by thin margin of pleura
  • Mach effect: seen on convex surfaces. abrupt change in density between heart and lung. This lucency around convex borders. No opaque line seen as in pneumomediastinum