Imaging description

Pseudopneumomediastinum can result from Mach bands, normal anatomic structures, or a paratracheal air cyst (tracheal diverticulum).

Mach bands, first described by Ernst Mach in 1865, are optical edge-enhancement phenomena secondary to lateral inhibition in the retina [1]. Mach bands appear as a region of lucency adjacent to structures with convex borders and can be distinguished from true pneumomediastinum as Mach bands do not have an opaque line (Figures 30.1–30.3). For a more detailed description of the Mach effect, see Case 79.

Normal anatomic structures can occasionally mimic pneumomediastinum. As described by Zylak and colleagues, the superior aspect of the major fissure or the anterior junction line may appear as a white line, mimicking pneumomediastinum. This tends to occur with lordotic positioning [2].

Tracheal diverticula are present in 3–4% of the US population [3]. These benign entities are typically located on the right at the level of the thoracic inlet. Communication between the trachea and diverticulum is seen in only 8–35% of cases [3, 4]. The rounded shape and characteristic location will prevent confusion of tracheal diverticulum with pneumomediastinum (Figures 30.4 and 30.5).