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Lateral segmental bronchus of right lung

The lateral segmental bronchus of the right lung, designated B4, is a branch of the right middle lobe bronchus, which itself arises from the intermediate bronchus (a continuation of the right main bronchus after giving off the upper lobe bronchus).

B4 specifically supplies the lateral segment (S4) of the right middle lobe, coursing laterally and slightly inferiorly toward the middle lobe pleural surface. It runs parallel to its companion lateral segmental pulmonary artery branch (A4) and is drained by the lateral segmental vein (V4), which contributes to the middle lobe vein and ultimately joins the right superior pulmonary vein.

Anatomically, B4 divides into subsegmental bronchi that ventilate the lateral aspect of the middle lobe, which is bounded superiorly by the horizontal fissure and inferiorly by the oblique fissure. This bronchus is an important surgical and radiologic landmark, particularly in segmentectomy, lobectomy, and preoperative planning for middle lobe resections.

Synonyms

  • Right B4 bronchus

  • Lateral bronchus of middle lobe

  • Lateral segmental bronchus (S4 bronchus)

Function

  • Ventilates the lateral segment (S4) of the right middle lobe

  • Works in conjunction with A4 (artery) and V4 (vein) as part of the bronchovascular bundle

  • Provides air conduction for gas exchange in the lateral portion of the middle lobe

  • Serves as a key anatomical unit in segmental lung surgery

Branches

  • Subsegmental bronchi to the subdivisions of S4

  • Collateral ventilation links with anterior segment (S3) of upper lobe and medial segment (S5) of middle lobe

MRI Appearance

T1-weighted images:

  • Bronchial lumen shows as a hypointense (dark) tubular structure with bright peribronchial fat

  • Best seen with high-resolution cardiac-gated thoracic MRI

T2-weighted images:

  • Bronchial lumen appears hypointense, surrounded by hyperintense lung parenchyma

  • Secretions or mucus plugging appear hyperintense

STIR:

  • Fat suppression increases conspicuity of bronchial walls

  • Inflammatory thickening or peribronchial edema appears bright

T1 Post-Gadolinium:

  • Normal bronchial lumen does not enhance

  • Bronchial wall may enhance if inflamed, e.g., bronchitis, tumor infiltration

MRI Non-Contrast Cardiac-Gated 3D Imaging:

  • Allows 3D reconstruction of bronchial tree, showing B4 as a lateral branch of the middle lobe bronchus

  • Useful for preoperative airway mapping and congenital anomaly detection

CT Appearance

High-Resolution CT (HRCT) / CT Pulmonary Angiography:

  • B4 is visualized as a lateral segmental bronchus branching from the middle lobe bronchus

  • Course can be traced toward lateral pleura of the middle lobe

  • Multiplanar and 3D reconstructions clearly depict relationship with A4 (artery) and V4 (vein)

  • HRCT is excellent for detecting bronchiectasis, obstruction, tumors, or congenital anomalies in the middle lobe

CT image

Lateral segmental bronchus of right lung CT axial image -img-00000-00000