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Left anterior segmental bronchus (B3a, B3b, B3c)

The left anterior segmental bronchus (B3) is a segmental branch of the left upper lobar bronchus, supplying the anterior segment of the left upper lobe. It commonly divides into three subsegmental branches: B3a (apical-anterior), B3b (lateral-anterior), and B3c (medial-anterior), which ventilate different subregions of the anterior upper lobe. The B3 bronchus courses anteroinferiorly from the upper lobar bronchus toward the anterior chest wall. Because of its anatomical relationship, it is an important structure in bronchoscopy, pulmonary surgery (segmentectomy), and imaging interpretation of upper lobe pathology.

Synonyms

  • B3 bronchus

  • Left anterior bronchus

  • Anterior segmental bronchus of left upper lobe

Function

  • Conducts air to the anterior segment of the left upper lobe

  • Provides ventilation for gas exchange in the anterior subapical and subcostal lung fields

  • Subdivided into B3a, B3b, B3c, each supplying distinct subsegments

  • Serves as a surgical and imaging landmark for pulmonary resections and bronchoscopic mapping

CT Appearance

Lung Window:

  • Seen as air-filled, hypodense tubular branches coursing anteroinferiorly from the left upper lobar bronchus toward the anterior lung field

  • Normally thin-walled and patent

  • Pathology: wall thickening (bronchitis), luminal obstruction (mucus plug, endobronchial tumor), or tree-in-bud opacities (infection spreading distally)

Mediastinal Window:

  • Bronchial wall visualized as a thin soft tissue rim

  • Better assessment of peribronchial soft tissue masses, lymphadenopathy, or extrinsic compression

  • Helps differentiate endobronchial vs. extrinsic lesions

Contrast-enhanced CT (CECT):

  • Enhances bronchial wall and adjacent vessels

  • Useful for evaluating endobronchial lesions, peribronchial tumor spread, or mediastinal vascular compression

  • HRCT demonstrates detailed subsegmental branching (B3a, B3b, B3c) and luminal caliber

MRI Appearance

T1-weighted images:

  • Lumen appears as a signal void (black) when air-filled

  • Bronchial wall shows low signal intensity, contrasting with peribronchial fat (hyperintense)

T2-weighted images:

  • Air-filled lumen remains signal void

  • If fluid, mucus, or secretions are present → bright hyperintense signal

  • Bronchial wall thickening or infiltration shows intermediate-to-high signal intensity

T1 Post-Contrast (Gadolinium-enhanced):

  • Normal bronchial wall shows mild thin rim enhancement

  • Pathological lesions (tumors, inflammation, granulomas) appear as enhancing, irregular thickened walls or nodular intraluminal masses

CT images

Left anterior segmental bronchus (B3a, B3b, B3c) anatomy CT coronal image -img-00000-00000