Lung Fissures and Accessory Structures

Lung Fissures

• Interlobar fissures separate lobes in the lungs.

• Minor fissure separates the right upper lobe (RUL) from right middle lobe (RML) and is seen on frontal and lateral views as a horizontal line.

• Major (oblique) fissures seen only on lateral radiographs as oblique lines; if fluid-filled, they appear on frontal views as concave curvilinear opacities in the lateral hemithorax.

• Major fissure separates the RUL/RML in the right lung from the right lower lobe (RLL). -Separates the left upper lobe (LUL) from the left lower lobe (LLL) in the left lung.

• Accessory fissures include:

  • Azygos fissure (less than 1%): present with an azygos lobe
  • Superior accessory fissure (approximately 5%): separates superior and basal segments of RLL.
  • Inferior accessory fissure (approximately 12%, more common in right lung): divides medial basal segment from other basal segments.
  • Left minor fissure (approximately 8%): separates lingula from LUL.

Atelectasis Overview

• Atelectasis: loss of lung volume due to decreased aeration (collapse).

• Causes: bronchial obstruction, mucus plugging, external compression (e.g., small lung volumes or pleural effusions).

• Direct signs: from lobar volume loss (e.g., displacement of fissures).

• Indirect signs: due to volume loss on adjacent structures (e.g., vascular crowding, elevation of diaphragm).

• Air bronchograms: not seen in central bronchial obstruction atelectasis but can be seen in external compression atelectasis.

Mechanisms of Atelectasis

• Obstructive atelectasis: alveolar gas absorbed by blood, not replaced by inspired air due to bronchial obstruction.

• Can cause lobar atelectasis (complete lobe collapse).

• More rapid with supplemental oxygen (oxygen absorption faster than nitrogen).

• In children, airway obstruction often due to aspirated foreign objects, causing hyperexpansion of affected side.

• Subsegmental atelectasis: common after surgery or illness, due to mucus obstruction of small airways.

• Relaxation (passive) atelectasis: lung relaxation adjacent to intrathoracic lesions (e.g., pleural effusion, pneumothorax, pulmonary mass).

• Adhesive atelectasis: due to surfactant deficiency, common in neonatal respiratory distress syndrome and ARDS.

• Cicatricial atelectasis: volume loss from architectural distortion by fibrosis.

Lobar Atelectasis

• Usually caused by central bronchial obstruction (obstructive atelectasis), possibly due to mucus plugging or tumor.

• Acute lobar atelectasis: likely due to mucus plugging (more common in lower lobes, less common in LUL).

• Outpatient lobar atelectasis: must rule out obstructing central tumor.

• Characteristic appearances depending on affected lobe:

  • Displacement of fissures
  • Plate-like/triangular opacity from collapsed lung.
  • Vascular crowding
  • Diaphragm elevation
  • Rib crowding on affected side
  • Mediastinal shift to affected side
  • Overinflation of adjacent/contralateral lobes
  • Hilar displacement

Patterns of Lobar Atelectasis

• Illustrations show collapse direction of each lobe (RUL, LUL, RML, RLL, LLL) for frontal and lateral views.

Left Upper Lobe Atelectasis

• Left upper lobe collapse & luftsichel sign: frontal radiograph shows veil-like opacity.