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Hemiazygos vein

The hemiazygos vein is an important component of the azygos venous system, providing venous drainage of the lower left thoracic wall and left posterior intercostal veins. It originates from the left ascending lumbar vein or the left renal vein, ascends along the left side of the vertebral column, and typically crosses the midline around T8–T9 vertebral level to join the azygos vein.

The hemiazygos vein forms a collateral venous pathway between the superior vena cava (SVC) and inferior vena cava (IVC). It communicates with the accessory hemiazygos vein superiorly and often with lumbar and left renal veins inferiorly.

Clinically, the hemiazygos vein is significant in SVC obstruction, where it provides an alternative route for venous return, and in imaging, where it can be mistaken for mediastinal pathology if dilated.

Synonyms

  • Vena hemiazygos

  • Hemiazygos branch of azygos system

  • Inferior hemiazygos vein

Function

  • Drains the left posterior intercostal veins (9th–11th)

  • Provides venous return from the lower left thoracic wall and mediastinum

  • Serves as a collateral channel between the IVC and SVC via the azygos system

  • Plays a role in venous compensation during SVC syndrome or IVC obstruction

Tributaries

  • Left ascending lumbar vein

  • Left renal vein (occasionally)

  • Left subcostal vein

  • Left posterior intercostal veins (usually 9th–11th)

MRI Appearance

T1-weighted images:

  • Appears as a tubular signal void (black lumen) along the left vertebral column

  • Wall is hypointense; surrounding mediastinal fat provides contrast

  • T2-weighted images (normal):

  • Hemiazygos vein appears as a dark signal void due to continuous venous flow

  • Best visualized against the hyperintense background of mediastinal fat and adjacent tissues

  • Thrombosis may appear as intermediate to high signal within the lumen

T2 TRUFISP (cardiac/respiratory-gated):

  • Hemiazygos vein visualized as a bright enhancing lumen with steady venous flow

  • Excellent for showing venous patency and collateral circulation

  • Demonstrates dilatation, compression, or thrombosis clearly

STIR (Short Tau Inversion Recovery):

  • Suppresses fat, improving visualization against posterior mediastinal fat

  • Perivascular edema or inflammatory changes appear hyperintense

T1 Post-Contrast (Gadolinium-enhanced MRI):

  • Hemiazygos vein enhances homogeneously and brightly

  • Filling defects represent thrombus or extrinsic compression

  • Useful for venous mapping in thoracic collateral pathways

MRA (Magnetic Resonance Angiography):

  • Contrast-enhanced MRA delineates the course, tributaries, and connection with azygos vein

  • Helpful in identifying collateral venous pathways in SVC syndrome

CT Appearance

Contrast-Enhanced CT (CT Venography):

  • Hemiazygos vein opacifies brightly, appearing as a tubular enhancing structure along the left vertebral column

  • Crossing at T8–T9 to the azygos vein is well seen on axial and coronal views

  • Identifies dilatation, thrombosis, or collateral circulation

  • Important for differentiating a dilated hemiazygos vein from posterior mediastinal mass or lymphadenopathy

CT image

Hemiazygos vein anatomy CT AXIAL image -img-00000-00000

MRI image

Hemiazygos vein anatomy MRI AXIAL image -img-00000-00000