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Left main bronchus

The left main bronchus is one of the two primary bronchi arising from the tracheal bifurcation (carina) at the level of T4–T5 vertebrae and the sternal angle. It is longer (≈5 cm) and narrower than the right main bronchus and courses inferolaterally under the aortic arch toward the left lung hilum, where it divides into the left superior lobar bronchus and left inferior lobar bronchus. The left main bronchus passes anterior to the esophagus, thoracic aorta, and left pulmonary artery, making it an important anatomical and radiological landmark.

Synonyms

  • Left primary bronchus

  • Left mainstem bronchus

  • Bronchus principalis sinister

Function

  • Conducts air between the trachea and left lung

  • Ensures ventilation of both lobes of the left lung

  • Provides a conduit for mucus clearance via mucociliary transport

  • Serves as a landmark in bronchoscopy, CT, and thoracic surgery

CT Appearance

Lung Window:

  • Seen as a large air-filled, hypodense (black) tubular structure arising from the tracheal carina and coursing toward the left lung hilum

  • Walls are normally thin and well defined

  • Pathology: wall thickening (bronchitis, tumor infiltration), luminal narrowing (extrinsic compression by lymph nodes or vessels), or intraluminal obstruction (mucus, tumor, foreign body)

Mediastinal Window:

  • Bronchial wall appears as a soft tissue rim with clear margins against adjacent mediastinal fat and vessels

  • Useful for identifying peribronchial lymphadenopathy, extrinsic vascular compression (e.g., enlarged left atrium, aortic aneurysm), or tumor invasion

Contrast-enhanced CT (CECT):

  • Clearly delineates the bronchial wall and its relation to adjacent mediastinal structures (aortic arch, pulmonary artery, esophagus)

  • Detects endobronchial masses, peribronchial spread of carcinoma, and vascular compression

  • HRCT provides detailed assessment of wall thickness, lumen caliber, and branching pattern

MRI Appearance

T1-weighted images:

  • Air-filled lumen appears as a signal void (black)

  • Bronchial wall shows low signal intensity

  • Surrounded by hyperintense mediastinal fat, which enhances visualization

T2-weighted images:

  • Air-filled lumen remains a signal void

  • If fluid, mucus, or pus fills the bronchus, it appears bright hyperintense

  • Inflamed or thickened bronchial walls show intermediate to hyperintense signal

T1 Post-Contrast (Gadolinium-enhanced):

  • Normal wall shows thin, mild homogeneous enhancement

  • Pathological findings (tumors, granulomas, or infiltrative disease) appear as thickened, irregular, heterogeneously enhancing walls or masses

MRI image

Leftt main bronchus anatomy mri coronal image -img-00000-00000

CT images

Left main bronchus anatomy CT axial image -img-00000-00000_00001

CT images

Left main bronchus anatomy CTcoronal image -img-00000-00000