Lobar Collapse of the Lung - Imaging Findings (X-Ray & CT)

What Is Lobar Collapse?

• Endobronchial obstruction (intrinsic): Bronchogenic carcinoma (most common in older smokers), mucous plugs, carcinoid, metastases, foreign bodies, granulomatous disease (TB, sarcoidosis)
• Extrinsic compression: Enlarged lymph nodes, aortic aneurysm, mediastinal masses
• Non-obstructive: Pleural effusion (passive/compressive atelectasis), pneumothorax, loss of surfactant (ARDS)

General Radiographic Signs of Lobar Collapse

Direct signs (volume loss):

• Fissure displacement - the key direct sign; fissures shift toward the collapsed lobe
• Displacement of pulmonary vessels and bronchi toward the collapsed area

Indirect signs (compensatory changes):

• Hyperinflation of remaining ipsilateral and contralateral lobes
• Ipsilateral hemidiaphragm elevation
• Mediastinal/tracheal shift toward the collapse
• Hilar displacement (upward for upper lobe, downward for lower lobe)
• Increased rib cage intercostal space narrowing ipsilaterally

Schematic overview of all lobar collapses

1. Right Upper Lobe (RUL) Collapse

X-Ray Findings:

• Frontal: Opacity in the right upper zone with upward displacement of the minor (horizontal) fissure. The hilum is pulled upward. The superior mediastinum shifts to the right. The trachea deviates to the right.
• Lateral: The minor fissure moves superiorly and anteriorly; the upper lobe collapses anterosuperiorly.
• "Golden S sign" (S-shaped or reverse S curve): seen when collapse is due to a central hilar mass - the lateral curve of the displaced fissure and the medial convexity of the tumor create the S configuration.

CT Findings:

• Triangular/wedge-shaped density in the anterosuperior right hemithorax
• Elevated minor fissure and mediastinal shift
• Well-demonstrates any endobronchial cause

2. Right Middle Lobe (RML) Collapse

X-Ray Findings:

• Frontal: Subtle loss of clarity of the right heart border (silhouette sign) due to the middle lobe lying adjacent to it. Triangular opacity adjacent to the right heart border pointing toward the hilum.
• Lateral (most useful view!): Classic "bowtie" or triangular band of opacity between the minor fissure (superiorly) and the major fissure (inferiorly). The two fissures converge anteriorly.

CT Findings:

• Flat/triangular collapsed lobe adjacent to the right heart border
• Both major and minor fissures approximate toward each other

3. Right Lower Lobe (RLL) Collapse

X-Ray Findings:

• Frontal: Triangular density behind the heart and right hemidiaphragm. The right hemidiaphragm outline may be partially preserved (because middle lobe remains between collapsed lower lobe and diaphragm). The right lower lobe pulmonary artery is not visible (no longer outlined by aerated lung).
• Lateral: Increased density in the posterior costophrenic angle; loss of silhouette of the right hemidiaphragm posteriorly.
• Superior triangle sign: Triangular density to the right of mediastinum (from displacement of anterior junctional structures) - do not confuse with RUL collapse.
• Rightward hilar displacement

CT Findings:

• Collapsed lobe lies posteromedially in the right hemithorax
• Fissure displaced posteriorly and medially
• CT mucous bronchogram sign: tubular low-attenuation branching structures within enhancing collapsed lobe

4. Left Upper Lobe (LUL) Collapse

X-Ray Findings:

• Frontal: Veil-like hazy opacity over the left upper hemithorax (not a well-demarcated border). The left hilum is elevated and the trachea deviates to the left.
• The aortic knuckle may be obscured (silhouette sign) - the left heart border may be partially preserved if the lingula is spared.
• Horizontal reorientation of the left mainstem bronchus.
• Luftsichel sign (crescent of air): Hyperinflated superior segment of the left lower lobe interposes between the aortic knuckle and the collapsed left upper lobe, creating a lucent crescent at the left apex - highly specific sign for LUL collapse.

CT Findings:

• Anteriorly displaced oblique fissure
• Triangular/slab-like density anterior to the left mediastinum
• "Pita bread" sign on axial CT - the collapsed LUL forms a flat slab anterior to the hilum

5. Left Lower Lobe (LLL) Collapse

X-Ray Findings:

• Frontal: Triangular density behind the heart (left-sided equivalent of RLL collapse). The medial left hemidiaphragm outline is lost (silhouette sign). The descending aorta outline may be preserved.
• Flat waist sign: With extensive LLL collapse, there is flattening of the contours of the aortic knuckle and main pulmonary artery due to cardiac rotation/displacement.
• Lateral: Increased density posteriorly; loss of the left hemidiaphragm posteriorly. Vertebral column appears progressively denser inferiorly (opposite of normal where upper vertebrae are denser).

CT Findings:

• Collapsed lobe lies posteromedially in left hemithorax
• Descending thoracic aorta may create a focal bulge in the contour of the collapsed lobe (Fig. 7.13)

CT: Special Signs & Advantages

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