Mach bands and pneumomediastinum.
A thin line of apparent radiolucency is a frequent companion outline of the heart borders and the aortic knob. This usually represents a "negative" Mach band. On occasion, such lines can be confused with pneumomediastinum or pneumopericardium, especially if only a single postero-anterior (PA) film is available. Criteria are presented which enable the radiologist to distinguish between these possibilities without depending on other findings.
D015502:Absorptiometry, Photon; D006321:Heart / Q000000981:diagnostic imaging*; D006801:Humans; D008478:Mediastinal Emphysema / Q000000981:diagnostic imaging*; D008482:Mediastinum / Q000000981:diagnostic imaging; D011026:Pneumopericardium / Q000000981:diagnostic imaging