Normal Azygous venous system (Figure 2)
The azygous vein is a critical conduit that drains the thoracic and, to some degree, the lumbar spinal venous circulation. The central and radial spinal veins, draining via vertebral plexuses and anterior and posterior medullary veins ultimately form intervertebral veins that connect with intercostal veins before draining into the azygous and hemiazygous veins. These spinal veins and plexuses are connected directly to the venous drainage of the brainstem and the cervical and lumbar spinal cord, via anterior and posterior medial spinal veins and dural plexuses.
The azygous vein is a continuation of the right ascending lumbar vein; the hemiazygous vein is a continuation of the left ascending lumbar vein or a branch of the left renal vein. The accessory hemiazygous vein is a continuation of the upper left intercostal veins and may drain into the left innominate vein or into the azygous vein. Valves are typically present in the arch of the azygous vein; they may be multiple.
On IVUS the azygous vein is usually a rounded structure with little sonolucency in the wall as all veins have minimal muscularis. At the level of the aorta, the vein may be flattened or narrowed. One sees segmental veins entering it; segmental arteries are posterior to the vein and may temporarily indent the vein. At the junction of the ascending azygous vein with the arch of the azygous, a large draining vein may enter it. This is the continuation of the accessory hemiazygous vein. Valves are usually present within the arch of the azygous vein. When thickened they are visible, echogenic and show limited motion.
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