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Thoracic duct

The thoracic duct is the largest lymphatic vessel in the body, measuring 35–45 cm in length. It begins in the abdomen as the cisterna chyli at the level of L1–L2, ascends through the aortic hiatus of the diaphragm into the posterior mediastinum, and travels upward between the aorta and azygos vein, posterior to the esophagus. It terminates at the junction of the left subclavian and internal jugular veins, where it empties lymph into the venous circulation.

The thoracic duct drains lymph from the entire body below the diaphragm, as well as from the left thorax, left upper limb, and left head and neck. It transports lymph rich in proteins, immune cells, and lipids (chyle). Clinically, the thoracic duct is important in conditions such as chylothorax, lymphatic obstruction, trauma, malignancy, and congenital anomalies.

Synonyms

  • Left lymphatic duct

  • Main lymphatic duct

  • Ductus thoracicus

Function

  • Drains lymph from:

    • Both lower limbs

    • Abdomen and pelvis

    • Left thorax, left upper limb, left head and neck

  • Returns lymph and chyle to the venous system at the left venous angle

  • Maintains fluid balance, immune surveillance, and fat absorption

  • Provides collateral lymphatic drainage in cases of localized obstruction

MRI Appearance

T1-weighted images:

  • Thoracic duct appears as a thin, tubular low-signal structure running vertically through the posterior mediastinum

  • Lymph (fluid content) is usually hypointense relative to surrounding fat

T2-weighted images:

  • The duct and its contents appear hyperintense (bright) due to fluid-rich lymph

  • Best sequence for direct visualization of duct course, caliber, and dilatation

  • Pathologies (e.g., obstruction, chyle leak) appear as enlargement or discontinuity

STIR (Short Tau Inversion Recovery):

  • Suppresses fat signal, enhancing visibility of the duct against mediastinal fat

  • Thoracic duct and lymph appear bright hyperintense

  • Very useful in detecting chylothorax, leakage, or lymphatic malformations

T1 Post-Contrast (Gadolinium-enhanced):

  • The duct wall may enhance subtly; lumen usually shows little to no direct enhancement

  • Leakage sites or abnormal communications with pleura/peritoneum may be seen indirectly

  • Useful in identifying tumor infiltration or inflammatory changes

CT Appearance

Contrast-Enhanced CT (CECT):

  • Thoracic duct appears as a thin tubular soft tissue density structure posterior to the esophagus and between the aorta and azygos vein

  • Contrast may not normally opacify the duct unless specialized lymphangiographic techniques are used (e.g., intranodal or pedal lymphangiography with lipiodol)

  • Pathological conditions include:

    • Chylothorax → extravasated fluid in pleural cavity

    • Obstruction/dilatation → ductal enlargement

    • Tumor infiltration → irregular walls or mass effect

  • 3D reconstructions can help in mapping the duct in oncologic or surgical planning

MRI images

Thoracic duct MRI axial image -img-00000-00000

MRI images

Thoracic duct MRI CORONAL image -img-00000-00000

CT image

Thoracic duct CT axial image -img-00000-00000