# Lobar and Segmental Atelectasis Patterns

## Mechanisms of Atelectasis

### Five Pathophysiologic Types

Atelectasis, the partial or complete collapse of lung parenchyma, arises through five distinct mechanisms. Resorptive (obstructive) atelectasis is the most common in clinical practice and occurs when complete airway obstruction causes the distal air to be resorbed into the blood. Common causes include endobronchial tumors, mucus plugs, foreign bodies, and blood clots. Complete resorption occurs within hours when the patient is breathing room air and happens even faster with high-concentration supplemental oxygen, which is why patients on high-flow oxygen are particularly prone to rapid collapse behind an obstructing lesion.

Compressive atelectasis results from external compression of the lung by an adjacent space-occupying process such as a large pleural effusion, pneumothorax, large mass, or diaphragmatic hernia. Passive (relaxation) atelectasis is closely related and occurs when the lung collapses due to loss of normal negative intrapleural pressure, as seen with pneumothorax or large effusion. Round atelectasis is a specific variant in which folded lung tissue adjacent to pleural disease mimics a mass on imaging.

Adhesive atelectasis results from loss of surfactant function, causing alveolar instability and collapse. This mechanism underlies the atelectasis seen in acute respiratory distress syndrome (ARDS), respiratory distress syndrome of the newborn, pulmonary embolism, and post-radiation injury. Cicatrization atelectasis is volume loss caused by pulmonary fibrosis or scarring from prior granulomatous infection (tuberculosis, histoplasmosis), radiation fibrosis, or idiopathic pulmonary fibrosis.

## Imaging Signs of Atelectasis

### Direct Signs

The most reliable sign of atelectasis is displacement of interlobar fissures, with the fissure moving toward the collapsed lobe. Identifying the displaced fissure on both frontal and lateral radiographs is the key to confirming volume loss and identifying the affected lobe. The collapsed lobe itself shows increased opacity due to crowding of vessels and airless parenchyma, and the vascular structures within it are packed more closely together than normal.

### Indirect Signs

Several secondary findings support the diagnosis and help gauge the extent of volume loss. The mediastinum and trachea shift toward the side of collapse, a finding more pronounced with upper lobe atelectasis. The hilum shifts upward with upper lobe collapse and downward with lower lobe collapse. The ipsilateral hemidiaphragm may elevate, particularly with lower lobe or complete lung atelectasis. The remaining ipsilateral lobes compensate by overexpanding (compensatory hyperinflation), manifesting as increased lucency and wider spacing of vascular markings. Ipsilateral rib spaces narrow, and the heart and mediastinum shift toward the atelectatic side.

## Lobar Atelectasis Patterns

### Right Upper Lobe (RUL) Atelectasis

In right upper lobe collapse, the minor fissure shifts superiorly and medially, and the collapsed lobe becomes a wedge-shaped opacity along the superior mediastinum. The Golden S sign (or S sign of Golden) is a critically important finding: a reverse S-shaped curve of the minor fissure indicates that a central hilar mass, typically bronchogenic carcinoma, is causing the obstruction. The lateral convexity of the S represents the mass itself, while the medial concavity represents the collapsed lobe. The right hilum is elevated, the right middle and lower lobes show compensatory hyperinflation, and the trachea may deviate to the right.

### Left Upper Lobe (LUL) Atelectasis

Left upper lobe atelectasis has a distinctive pattern that differs from right upper lobe collapse because the left lung lacks a minor fissure. Instead, the major fissure swings anteriorly, and the collapsed left upper lobe drapes forward over the left hilum, producing a veil-like opacity over the left hemithorax. The luftsichel sign is a helpful clue: the hyperinflated superior segment of the left lower lobe interposes between the aortic arch and the collapsed upper lobe, creating a crescentic lucency along the aortic knob. The left hilum is obscured by the collapsed lobe. On the lateral view, the major fissure is displaced anteriorly and runs nearly parallel to the anterior chest wall.

### Right Middle Lobe (RML) Atelectasis

Right middle lobe atelectasis is notoriously subtle on the PA view. The minor fissure depresses and the lower portion of the major fissure elevates, but the resulting density may only be visible as a subtle opacity obscuring the right heart border (silhouette sign). The lateral view is essential for detection, where RML collapse appears as a wedge-shaped opacity with its apex at the hilum and its base at the anterior chest wall, bounded by the depressed minor fissure above and the elevated major fissure below. Because the right middle lobe is relatively small, volume loss signs may be minimal. Chronic or recurrent right middle lobe atelectasis is termed right middle lobe syndrome and may be caused by compressive lymphadenopathy, bronchiectasis, or chronic infection.

### Right Lower Lobe (RLL) Atelectasis

In right lower lobe collapse, the major fissure shifts posteriorly and inferiorly. On the frontal view, the collapsed lobe appears as a triangular opacity in the medial lower hemithorax behind the heart. The right hemidiaphragm may be obscured or elevated, and the right hilum is depressed. On the lateral view, increased opacity is seen over the lower thoracic vertebrae (the spine sign), and the major fissure is displaced posteroinferiorly.

### Left Lower Lobe (LLL) Atelectasis

Left lower lobe collapse follows a similar pattern to the right, with the collapsed lobe creating a triangular opacity behind the cardiac silhouette. The left hemidiaphragm is partially or completely obscured (silhouette sign). This finding is often subtle on the PA view and may only be apparent as loss of hemidiaphragm visibility and increased retrocardiac density. The flat waist sign describes the loss of normal contour of the aortopulmonary window and descending aorta due to mediastinal shift. The "sail sign" refers to the triangular density visible behind the heart with a straight lateral margin representing the displaced major fissure. The lateral view shows increased density over the lower vertebrae and posteroinferior fissure displacement.

### Complete Lung Atelectasis

When an entire lung collapses, the hemithorax becomes opacified with massive ipsilateral shift of the mediastinum. The hemidiaphragm is elevated and the ribs are crowded. The key distinguishing feature from total lung opacification due to a massive pleural effusion is the direction of mediastinal shift: complete atelectasis shifts the mediastinum toward the opacity (the collapsed lung "pulls"), while a massive effusion shifts the mediastinum away (the fluid "pushes").

## Lobar Atelectasis Summary

| Lobe | Fissure Displacement | Frontal Radiograph Pattern | Key Signs | Lateral View |
|------|---------------------|---------------------------|-----------|-------------|
| RUL | Minor fissure shifts superiorly and medially | Wedge-shaped opacity at superior mediastinum | Golden S sign (central mass), elevated hilum | Anterosuperior opacity |
| LUL | Major fissure swings anteriorly | Veil-like opacity over left hemithorax | Luftsichel sign (crescent at aortic knob), obscured hilum | Major fissure parallel to anterior chest wall |
| RML | Minor fissure depresses, lower major fissure elevates | Subtle; loss of right heart border (silhouette sign) | Right middle lobe syndrome if chronic | Wedge with apex at hilum, base anterior |
| RLL | Major fissure shifts posteroinferiorly | Triangular retrocardiac opacity, medial lower hemithorax | Depressed hilum, obscured diaphragm | Spine sign; posterior opacity |
| LLL | Major fissure shifts posteroinferiorly | Triangular retrocardiac density (sail sign) | Flat waist sign, loss of left hemidiaphragm silhouette | Spine sign; posterior opacity |
| Complete lung | All fissures lost | Opacified hemithorax with ipsilateral mediastinal shift | Shift TOWARD opacity (vs. effusion: shift AWAY) | Diffuse opacification |

## Round Atelectasis

### Imaging Features

Round atelectasis is a mass-like opacity along the pleura, typically in a posteroinferior location. Its hallmark is the comet tail sign (also called the vacuum cleaner sign), in which curved vessels and bronchi are seen sweeping into the mass. Adjacent pleural thickening, volume loss in the affected lobe, and an acute angle between the lesion and the pleural surface are additional characteristic features. Round atelectasis is almost always associated with prior asbestos exposure or pleural disease (effusion, empyema). When classic features are present on CT, the diagnosis can be made with confidence and biopsy is usually unnecessary.

## Atelectasis vs. Consolidation

### Key Distinguishing Features

| Feature | Atelectasis | Consolidation |
|---------|------------|---------------|
| Volume | Decreased (volume loss) | Normal or increased |
| Fissure Displacement | Toward opacity | None (may bulge away) |
| Mediastinal Shift | Toward affected side | None or away |
| Air Bronchograms | Usually absent (obstructive) | Typically present |
| Mass Effect | None (structures pulled toward) | Possible (structures pushed away) |

The fundamental distinction is volume loss. Atelectasis shows displaced fissures moving toward the opacity and mediastinal shift toward the affected side. Consolidation typically shows normal or increased volume, with air bronchograms, no fissure displacement, and possibly slight mass effect pushing structures away. Obstructive atelectasis typically lacks air bronchograms because the airway supplying the collapsed segment is blocked. If air bronchograms are present within an area of volume loss, adhesive or cicatrization atelectasis should be considered. Combined atelectasis and consolidation (obstructive pneumonitis) can occur distal to a bronchial obstruction.

## CT Evaluation of Atelectasis

### Advantages Over Radiography

CT offers several advantages over radiography for evaluating atelectasis. It can directly visualize the endobronchial obstruction, whether it is a mucus plug or tumor. It can differentiate atelectatic lung from adjacent pleural effusion, because atelectatic lung enhances with IV contrast while fluid does not. CT with contrast can identify a central mass causing obstructive atelectasis, showing the enhancing mass within the non-enhancing collapsed lung. CT also detects underlying lymphadenopathy and evaluates the remaining lung for additional pathology.

<image>A series of four PA chest radiograph diagrams showing the classic patterns of individual lobar collapse. Each diagram is a frontal view with the displaced fissure(s) drawn as dashed lines and the collapsed lobe shaded in gray. (1) Right upper lobe collapse: minor fissure elevated superiorly and medially with the collapsed lobe wedged against the superior mediastinum and the Golden S sign annotated when a central mass is present. (2) Left upper lobe collapse: veil-like opacity over the entire left hemithorax with the luftsichel sign (crescent of lucency along the aortic knob) labeled. (3) Right middle lobe collapse: subtle loss of the right heart border with a thin triangular opacity. (4) Left lower lobe collapse: triangular retrocardiac density with a straight lateral edge representing the displaced major fissure (sail sign) and loss of the left hemidiaphragm silhouette.</image>

<image>A CT axial image diagram of right middle lobe atelectasis. The collapsed right middle lobe is shown as a wedge-shaped area of increased attenuation between the displaced minor fissure (anterosuperior boundary) and the lower portion of the major fissure (posteroinferior boundary). The adjacent right lower lobe and right upper lobe are hyperinflated with decreased attenuation. The right heart border is in direct contact with the collapsed lobe. A small annotation indicates the absence of air bronchograms suggesting an obstructive etiology, with an arrow pointing to the narrowed or occluded right middle lobe bronchus.</image>

<image>A CT image diagram demonstrating round atelectasis. A peripheral, pleura-based mass-like opacity is shown in the left lower lobe posteriorly. Curved bronchi and vessels sweep into the mass forming the characteristic comet tail sign, which is labeled with arrows. Adjacent pleural thickening is visible. The lesion makes an acute angle with the thickened pleura. A note states that this pattern, when classic, is diagnostic and does not require biopsy.</image>

## Clinical Pearls

The direction of fissure displacement is the single most reliable indicator of atelectasis and identifies the affected lobe; the fissures should always be assessed first. The Golden S sign on a chest radiograph mandates CT with contrast and bronchoscopy, because it suggests a central obstructing mass, often bronchogenic carcinoma, causing upper lobe atelectasis. Left upper lobe atelectasis produces a veil-like opacity that is easily mistaken for a nonspecific "hazy left hemithorax" rather than recognized as collapse; the luftsichel sign is the key clue to the correct diagnosis. Right middle lobe atelectasis is notoriously subtle on the PA view, and the lateral view is essential for detection and characterization. The direction of mediastinal shift distinguishes complete lung atelectasis from massive pleural effusion: opacification with ipsilateral shift is atelectasis, while opacification with contralateral shift is effusion. Round atelectasis with classic CT features, including the comet tail sign, pleural thickening, and an acute pleural angle, can be diagnosed with confidence without biopsy, avoiding unnecessary invasive procedures.

## References

- Woodring JH, Reed JC. "Types and Mechanisms of Pulmonary Atelectasis." *Journal of Thoracic Imaging*, 1996
- Kattan KR, et al. "The Golden S Sign Revisited." *AJR*, 1980
- Proto AV, Tocino I. "Radiographic Manifestations of Lobar Collapse." *Seminars in Roentgenology*, 1980
- Brant WE, Helms CA. *Fundamentals of Diagnostic Radiology*, 5th ed. Wolters Kluwer, 2019
- Gurney JW. "Atypical Manifestations of Pulmonary Atelectasis." *Journal of Thoracic Imaging*, 1996