Membranes are transverse flaps that obstruct flow. They likely represent malformed valve tissue. Venography (A) with the catheter cephalad to the membrane appears normal. With the catheter withdrawn centrally (B), the membrane becomes visible (arrows). It is postulated that the catheter has pushed the membrane open against the lateral wall in Figure 7A.
Figure 6: Web in Azygous:
Webs are small tangles of tissue within the lumen of the vein possibly remnants of the cardinal vein maturation into the adult veins. 1. Arrows point to small areas of echogenic material. They may represent webs. However single areas of abnormal thickening of a valve could cause such an appearance. 2. The longitudinal view illustrates that this tissue has the random nature of a web (white circle) rather than of a valve.
