What does ground glass opacity (GGO) of the lung represent?

Ground Glass Opacity: Radiographic Definition and Pathophysiologic Significance

Ground glass opacity (GGO) represents a diffuse homogeneous increase in lung density on high-resolution CT where vessels and bronchial walls remain visible, indicating partial filling of alveolar spaces, thickening of alveolar walls or septal interstitium, or both. 1

Radiographic Definition

GGO is characterized by areas of hazy increased attenuation that preserve bronchial and vascular margins—this distinguishes it from consolidation, where bronchovascular structures become obscured. 2 The key diagnostic feature is that despite increased lung density, the underlying anatomic structures remain identifiable. 1

Pathophysiologic Mechanisms

GGO reflects three primary pathologic processes at the histologic level: 3

• Partial filling of alveolar spaces with fluid, cells, or other material 3
• Thickening of the alveolar walls or septal interstitium from inflammation or fibrosis 3
• Combined alveolar and interstitial involvement 3

Clinical Significance and Prognostic Value

The presence and pattern of GGO carries important diagnostic and prognostic implications:

• Isolated GGO without fibrotic features typically indicates active alveolitis with inflammatory cells in the alveolar septum or lumen, suggesting potentially reversible disease 1
• GGO with reticular lines and traction bronchiectasis/bronchiolectasis indicates established lung fibrosis 1, 4
• The extent of GGO correlates with physiologic impairment and helps predict treatment response 1
• GGO may regress with treatment in inflammatory conditions like desquamative interstitial pneumonitis, or progress to reticular opacity or honeycombing in fibrotic processes 1

Common Disease Associations

Interstitial Lung Diseases

Nonspecific interstitial pneumonia (NSIP) presents with patchy or diffuse GGO, typically bilateral and symmetric with lower lung predominance, progressing to irregular reticular opacities and traction bronchiectasis 5

Organizing pneumonia (OP) shows multifocal patchy alveolar opacities with peribronchovascular and/or peripheral distribution, often demonstrating the reversed halo sign 5, 1

Hypersensitivity pneumonitis (HP) demonstrates poorly defined small centrilobular nodules with bilateral GGO and mosaic attenuation 5

Diffuse alveolar damage (DAD) presents with extensive bilateral GGO and dependent airspace consolidation in the exudative phase 5

Drug-Related Pneumonitis

Various molecular targeting agents and immune checkpoint inhibitors produce GGO patterns mimicking NSIP, OP, HP, DAD, or simple pulmonary eosinophilia 5, 4

Infectious and Inflammatory Processes

Pneumocystis pneumonia typically shows diffuse bilateral perihilar infiltrates with patchy GGO and peripheral sparing 1

Pulmonary edema presents with hazy opacities and Kerley lines from expansion of connective tissue space around conducting airways and vessels 4

Alveolar hemorrhage manifests as bilateral patchy GGO in middle and lower lung zones 4

Diagnostic Approach Using Pattern Recognition

Critical Distinguishing Features

The "three-density pattern" (hypoattenuating, normal, and hyperattenuating lobules in close proximity) is highly specific for fibrotic hypersensitivity pneumonitis 5, 1

The "reversed halo sign" (GGO surrounded by a ring of consolidation) suggests organizing pneumonia or fungal infection 1

Mosaic attenuation with GGO indicates small airway disease, particularly in hypersensitivity pneumonitis 1

Peribronchovascular distribution most commonly indicates organizing pneumonia, hypersensitivity pneumonitis, or drug-related pneumonitis 6

Subpleural distribution is characteristic of COVID-19 pneumonia and organizing pneumonia 1

Distribution Analysis

Evaluate the anatomic distribution systematically: 4

• Diffuse vs. patchy involvement narrows the differential
• Peripheral vs. central predominance suggests specific etiologies
• Upper vs. lower lung zone distribution helps distinguish disease processes
• Unilateral vs. bilateral involvement affects diagnostic considerations

Associated Findings

Look for ancillary features that refine the diagnosis: 2

• Centrilobular nodules suggest hypersensitivity pneumonitis or respiratory bronchiolitis 1
• Traction bronchiectasis indicates fibrotic disease 5
• Septal thickening creates a "paving stone-like" appearance in COVID-19 or suggests pulmonary veno-occlusive disease 1
• Air trapping on expiratory imaging confirms small airways disease 5

Common Pitfalls

Extensive GGO (>30% of lung involvement) should prompt consideration of diagnoses other than idiopathic pulmonary fibrosis, as IPF typically shows more prominent reticular changes and honeycombing 1, 4

Physiologic processes can mimic pathologic GGO, including poor ventilation of dependent lung areas and expiratory phase imaging 7

Distinguish GGO from mosaic attenuation patterns caused by primary vascular diseases or airway abnormalities by evaluating vessel caliber and air trapping on expiratory scans 2