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Right middle lobe bronchus

The right middle lobe bronchus (RML bronchus) arises from the intermediate bronchus (continuation of the right main bronchus after the superior lobar bronchus branches off). It courses anteroinferiorly and then divides into two segmental bronchi: the medial segmental bronchus (B5) and the lateral segmental bronchus (B4). These branches ventilate the medial and lateral segments of the right middle lobe, located between the horizontal and oblique fissures. The RML bronchus is clinically significant because of its narrow lumen, long course, and dependent position, making it particularly prone to obstruction, infection, and atelectasis.

Synonyms

  • RML bronchus

  • Middle lobar bronchus

  • Bronchus to right middle lobe

Function

  • Conducts air to the medial and lateral segments of the right middle lobe

  • Maintains ventilation and gas exchange in the middle lobe

  • Plays a role in mucus clearance, although its anatomical orientation predisposes it to stasis and recurrent infection

  • Serves as an important anatomical landmark in bronchoscopy and thoracic surgery

CT Appearance

Lung Window:

  • RML bronchus is seen as an air-filled, hypodense (black) tubular structure branching from the intermediate bronchus and coursing anteriorly

  • Normally has a thin, smooth wall

  • Pathology: wall thickening (bronchitis), endobronchial obstruction (tumor, foreign body), atelectasis (collapse of middle lobe), or tree-in-bud opacities (infection)

Mediastinal Window:

  • Bronchial wall appears as a thin soft tissue rim

  • Useful for evaluating peribronchial lymphadenopathy, extrinsic compression, or tumor infiltration

  • Differentiates between endobronchial lesions and mediastinal soft tissue masses

Contrast-enhanced CT (CECT):

  • Enhances the bronchial wall and surrounding vessels

  • Ideal for detecting endobronchial masses, stenosis, or peribronchial spread of carcinoma

  • HRCT clearly depicts bronchial caliber, patency, and relationship to fissures

MRI Appearance

T1-weighted images:

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

  • Bronchial wall is low signal intensity, outlined by hyperintense mediastinal and peribronchial fat

T2-weighted images:

  • Air-filled lumen remains a signal void

  • If fluid or mucus is present, it appears bright hyperintense within the bronchus

  • Thickened or diseased walls show intermediate to high signal intensity

T1 Post-Contrast (Gadolinium-enhanced):

  • Normal wall shows thin, mild rim enhancement

  • Pathology (infection, inflammation, carcinoma, granuloma) appears as thickened, irregularly enhancing bronchial walls or nodular intraluminal lesions

CT image

Right middle lobe bronchus anatomy CT coronal image -img-00000-00000