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Collapse of Lung Lobes: X-ray and CT Findings

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Terminology

• "Collapse" = complete loss of volume of an entire lobe/lung
• "Atelectasis" = partial or complete loss, described by extent (subsegmental, sublobar, lobar) or by cause

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General Radiographic Principles

1. Increased opacity of the affected lobe
2. Volume loss - assessed by direct and indirect signs

Special Ancillary Signs

• Golden's S sign (reverse-S sign on the right): S-shaped fissure due to combined collapse + central mass, suggesting bronchogenic carcinoma as the cause
• Absence of air bronchograms: Suggests endobronchial obstruction rather than consolidation
• Luftsichel sign: "Air crescent" seen in left upper lobe collapse - aerated lung interposed between the aortic arch and the medial border of the collapse
• Juxtaphrenic peak sign: Small triangular density near hemidiaphragm in upper lobe collapse, due to reorientation of an inferior accessory fissure
• Shifting granuloma sign: A calcified granuloma changes position between serial films due to volume loss

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Schematic of All Lobar Collapses

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1. Right Upper Lobe (RUL) Collapse

Mechanism

Chest X-ray (PA view)

• Increased opacity at the right apex, adjacent to the mediastinum
• Horizontal fissure elevated with a concave inferior border
• Small convexity at the hilum (from pulmonary vessels) = early Golden's S sign
• In tight collapse: can mimic an apical pleural cap or mediastinal widening
• Compensatory hyperinflation of right middle and lower lobes + left upper lobe
• Right main bronchus and lower lobe bronchus take a more horizontal course

Chest X-ray (Lateral view)

• Both horizontal and oblique fissures displaced superiorly and medially
• Collapsed lobe forms a superior ill-defined wedge-shaped density

CT Findings

• Roughly triangular density with base anteriorly against chest wall and apex at hilum
• Focal bulge of lateral border = underlying mass (suggests carcinoma)
• Compensatory hyperinflation of RML, RLL, and even left upper lobe

X-ray Image

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2. Left Upper Lobe (LUL) Collapse

Key Difference from RUL Collapse

Chest X-ray (PA view)

• Veil-like increased density of the entire left hemithorax - most dense at the hilum, gradually fading laterally
• No clear inferior demarcation (unlike RUL collapse)
• Luftsichel sign: Paramediastinal lucency due to aerated lower lobe slipping upward between the collapse and the aortic arch
• Left heart border, left mediastinal outline, and aortic knuckle may be obscured (in less severe collapse)
• In more severe collapse, aortic knuckle may paradoxically become visible again (lower lobe hyperinflates superiorly)
• Increased angulation between left main bronchus and lower lobe bronchus
• On lateral: entire oblique fissure displaced anteriorly, parallel to chest wall. Increased retrosternal lucency.

CT Findings

• Triangular soft-tissue density, apex at upper lobe bronchus, base against anterior chest wall adjacent to left mediastinal border
• Lingular segment is a separate density closely opposed to the left heart border

Luftsichel Sign (X-ray + CT)

Left Upper Lobe Collapse (Frontal + Lateral X-ray)

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3. Right Middle Lobe (RML) Collapse

Chest X-ray (PA view)

• Findings can be very subtle on the frontal view - easy to miss
• Collapsed lobe lies against the right heart border → loss of right heart border silhouette
• May show only indistinctness of the right atrial border
• When collapse is parallel to the x-ray beam or the patient is in a lordotic position: triangular "sail-shaped" density adjacent to the heart
• The opacity itself may not be visible at all on the frontal view

Chest X-ray (Lateral view)

• Much more obvious on lateral: triangular density with approximation of the minor fissure and inferior major fissure, apex at hilum
• In severe collapse, fissures become nearly parallel - thin wedge of density between them

CT Findings

• Triangular-shaped density adjacent to the heart border
• Because collapse represents a flat sheet of tissue, only a small portion may be seen on each axial slice

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4. Right Lower Lobe (RLL) and Left Lower Lobe (LLL) Collapse

Direction of Collapse

Chest X-ray (PA view)

• Triangular density behind the heart (posteromedial)
• Medial portion of the hemidiaphragm obscured (no longer outlined by aerated lung)
• Hilar structures may be depressed

Chest X-ray (Lateral view)

• Posterior portion of the hemidiaphragm may not be visible
• Vertebral column appears progressively denser inferiorly (reversed normal transparency gradient)
• In more severe collapse, diaphragm may reappear as it is outlined by hyperinflated upper lobe

CT Findings

• Collapsed lobe seen as a posteromedial wedge of density
• Volume loss readily appreciated

Juxtaphrenic Peak Sign (LLL collapse shown)

Shifting Granuloma Sign (Before and After Lower Lobe Collapse)

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5. Combined Lobar Collapses

Right Middle + Right Lower Lobe Collapse

• Due to obstruction at the bronchus intermedius
• On frontal view: opacity extends to the right costophrenic angle
• On lateral view: opacity extends from front to back of the hemithorax

Combined RML + RLL X-ray and CT

Right Upper + Right Middle Lobe Collapse

• Less common - RUL and RML don't share a common bronchial origin
• Usually from carcinoma obstructing one bronchus + extrinsically compressing the other
• Appearance mimics left upper lobe collapse on both frontal and lateral radiographs

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Causes of Lobar Collapse (Summary)

In a middle-aged or elderly smoker, lobar collapse should be considered due to bronchogenic carcinoma until proven otherwise.

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CT vs X-ray: Advantages of CT

• No superimposition of structures
• Multiplanar reformats (coronal, sagittal, axial)
• Identifies the cause of collapse (e.g., endobronchial mass, mucous plug)
• Differentiates collapsed lung from pleural effusion
• Air bronchograms visible on CT even when absent on X-ray
• PET/CT can delineate tumour from post-obstructive collapse - useful for biopsy targeting and radiotherapy planning

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