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.