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Azygos vein

The azygos vein is a major longitudinal venous channel of the thorax that provides an important collateral pathway between the superior vena cava (SVC) and inferior vena cava (IVC). It is formed by the union of the right ascending lumbar vein and the right subcostal vein, ascending through the posterior thorax along the right side of the vertebral column.

The azygos vein arches anteriorly over the root of the right lung at the level of T4 before draining into the superior vena cava. Along its course, it receives numerous tributaries, including the hemiazygos vein, accessory hemiazygos vein, intercostal veins, esophageal veins, bronchial veins, and mediastinal veins.

Clinically, the azygos system is crucial in providing an alternative route for venous return if the IVC or SVC is obstructed. It can become dilated in conditions such as SVC syndrome, right heart failure, portal hypertension, or congenital anomalies.

Synonyms

  • Vena azygos

  • Thoracic longitudinal vein

  • Azygos system main trunk

Function

  • Provides collateral venous drainage between the IVC and SVC

  • Drains blood from the thoracic wall, posterior intercostal veins, bronchial circulation, and mediastinum

  • Serves as a venous bypass route in case of caval obstruction

  • Acts as an important anatomical landmark in thoracic surgery and imaging

Tributaries

  • Hemiazygos vein (left lower thorax)

  • Accessory hemiazygos vein (left upper thorax)

  • Right posterior intercostal veins

  • Esophageal veins

  • Bronchial veins

  • Mediastinal and pericardial veins

MRI Appearance

T1-weighted images:

  • Lumen appears as a signal void (black) due to venous flow

  • Vessel wall hypointense, with hyperintense mediastinal fat outlining the vein

T2-weighted images:

  • Flowing blood shows signal void

  • Venous thrombus or slow flow may appear as intermediate to hyperintense signal within the lumen

STIR (Short Tau Inversion Recovery):

  • Suppresses mediastinal fat, improving contrast between the azygos vein and surrounding tissues

  • Acute thrombosis or perivascular edema appears hyperintense

T1 Post-Contrast (Gadolinium-enhanced MRI):

  • Azygos vein lumen enhances brightly and homogeneously

  • Filling defects (thrombus, tumor invasion) appear as non-enhancing intraluminal regions

  • Useful for evaluating venous obstruction, thrombosis, or collateralization

MRA (Magnetic Resonance Angiography):

  • Contrast-enhanced MRA provides 3D visualization of the azygos system, hemiazygos, and accessory hemiazygos veins

  • Demonstrates dilated collaterals, thrombosis, or anomalous venous return

  • Important for pre-surgical planning in thoracic and mediastinal pathology

CT Appearance

Contrast-Enhanced CT (CT Venography):

  • Azygos vein opacifies with intravenous contrast, allowing clear visualization of its course and tributaries

  • Dilatation, obstruction, thrombosis, or anomalous drainage patterns are well demonstrated

  • Enlarged azygos vein may mimic a mediastinal mass, requiring careful recognition

  • Multiplanar and 3D reconstructions show anastomoses with hemiazygos and collateral circulation

  • CTA/CTV is the gold standard for venous mapping in SVC syndrome or IVC obstruction

CT images

Azygos vein ct axial image -img-00000-00000

CT images

Azygos vein ct axial image -img-00000-00000_00001

MRI images

Azygos vein MRI axial image -img-00000-00000

MRI images

Azygos vein MRI coronal image -img-00000-00000