A membrane is a horizontal band of tissue, like a valve that extends transversely across the vein as well. On the longitudinal view, septum shows as relatively thin lines of tissue running parallel to the venous wall. They may be very difficult to visualize on IVUS because of their horizontal nature.
Web (Figure 6)
Webs are bands of tissue remaining within the lumen of the vein. They are thought to be sequellae of the de-differentiation/re-differentiation of the veins during the transition from the fetal cardinal system into the adult system. Webs are another abnormality that is often not recognizable by venography because of its location within the lumen. Webs are represented as echogenic material within the vein lumen that is persistent and located in the same location during several interrogations with the IVUS probe. They are most commonly seen within the azygous vein and less commonly in the jugular system. On longitudinal view they are usually seen as random areas of high echogenicity.
