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Left apicoposterior bronchus (B1+2a, B1+2b)

The left apicoposterior bronchus (B1+2) is a combined segmental bronchus of the left upper lobe, formed by fusion of the apical (B1) and posterior (B2) bronchi, unlike the right lung where they remain separate. It arises from the left superior lobar bronchus and ascends dorsocranially to ventilate the apicoposterior segment of the left upper lobe. This bronchus usually divides into two subsegments: B1+2a (apical) and B1+2b (posterior). It is clinically important as a common site of tuberculosis, bronchogenic carcinoma, and pneumonia, and is a key landmark in bronchoscopy and segmentectomy.

Synonyms

  • B1+2 bronchus

  • Left apicoposterior segmental bronchus

  • Apicoposterior bronchus of left upper lobe

Function

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

  • Ensures ventilation and gas exchange in the superior and posterior regions of the left lung

  • Aids in mucus clearance from the apicoposterior segment

  • Serves as a landmark in surgical resection and imaging interpretation

CT Appearance

Lung Window:

  • B1+2 appears as a branching air-filled (hypodense, black) tubular structure arising from the left upper lobar bronchus and coursing toward the lung apex and posterior upper lobe

  • Normally has thin, smooth walls

  • Pathology: wall thickening (bronchitis), luminal obstruction (mucus plug, tumor), or “tree-in-bud” nodularity (endobronchial infection)

Mediastinal Window:

  • Bronchial wall is seen as a thin soft tissue rim surrounded by mediastinal structures

  • Provides clearer assessment of adjacent vessels, nodes, or masses compressing the bronchus

  • Endobronchial masses appear as soft tissue density within the lumen

Contrast-enhanced CT (CECT):

  • Enhances the bronchial wall and surrounding vessels

  • Useful for identifying stenosis, extrinsic compression, peribronchial lymphadenopathy, or bronchogenic carcinoma

  • High-resolution CT (HRCT) clearly shows branching, lumen caliber, and segmental subdivisions (B1+2a and B1+2b)

MRI Appearance

T1-weighted images:

  • Air within the lumen appears as a signal void (black)

  • Bronchial wall shows low signal intensity, with adjacent peribronchial fat appearing hyperintense

T2-weighted images:

  • Air-filled lumen remains signal void

  • Fluid, pus, or mucus within the lumen appears bright hyperintense

  • Thickened or diseased bronchial walls show intermediate to hyperintense signal

T1 Post-Contrast (Gadolinium-enhanced):

  • Normal bronchial wall may show thin, uniform rim enhancement

  • Pathological states (tumor, granulomatous inflammation, infection) show irregular thickened or nodular enhancing walls

  • Enhances detection of endobronchial lesions and peribronchial spread

MRI image

Right apical segmental bronchus (B1) anatomy CT coronal image -img-00000-00000