What does GGO (Ground-Glass Opacities) stand for on a CT (Computed Tomography) chest scan?

Ground-Glass Opacity (GGO) Definition

GGO stands for Ground-Glass Opacity, which is defined as hazy increased attenuation (density) of the lung on CT imaging where the underlying bronchial and vascular structures remain visible, distinguishing it from consolidation where these structures are obscured. 1

Radiologic Characteristics

• GGO appears as areas of increased lung density that are less opaque than consolidation, with preservation of bronchial and vascular margins visible through the hazy opacity 2, 3
• The term specifically describes a CT finding where lung parenchyma shows increased attenuation but vessels and airways can still be traced through the affected area, unlike consolidation which completely obscures these structures 1

Pathophysiologic Basis

GGO reflects several underlying processes at the histopathologic level:

• Partial filling of alveolar spaces with fluid, cells, or other material 2, 3
• Thickening of alveolar walls or septal interstitium from inflammation or fibrosis 2, 3
• Increased capillary blood volume in the affected lung regions 2
• Combination of alveolar filling and interstitial thickening occurring simultaneously 3

Clinical Significance in Diagnosis

• GGO is a nonspecific finding that requires correlation with distribution pattern, associated CT features, and clinical context to narrow the differential diagnosis 1, 2
• When GGO occurs with reticular lines and traction bronchiectasis/bronchiolectasis, it indicates established lung fibrosis rather than pure inflammatory processes 4, 5
• Isolated GGO without fibrotic features typically indicates inflammatory cells in the alveolar septum or lumen (alveolitis), suggesting a potentially reversible process 4, 5
• The extent of GGO on CT correlates with physiologic impairment and helps predict treatment response 4, 5

Common Disease Patterns

GGO manifests in multiple interstitial lung disease patterns:

• Organizing pneumonia (OP): Multifocal patchy opacities with peribronchovascular and/or peripheral distribution 1
• Hypersensitivity pneumonitis (HP): Bilateral GGO with poorly defined centrilobular nodules and mosaic attenuation 1
• Nonspecific interstitial pneumonia (NSIP): Patchy or diffuse GGO progressing to reticular opacities with traction bronchiectasis 1
• Diffuse alveolar damage (DAD/ARDS): Extensive bilateral GGO with dependent consolidation 1
• Drug-related pneumonitis: Various patterns of GGO depending on the specific drug reaction pattern 1, 4, 5

Diagnostic Approach

• Evaluate the distribution (diffuse, patchy, peripheral, peribronchovascular, basal-predominant) to narrow differential diagnosis 4, 6
• Assess for associated findings including septal thickening, nodules, cysts, traction bronchiectasis, and air trapping on expiratory images 2, 3
• Consider vessel caliber within areas of decreased attenuation to distinguish infiltrative GGO from mosaic perfusion patterns due to vascular or airway disease 1, 2
• Obtain expiratory CT images when mosaic attenuation is present to differentiate air trapping (airway disease) from pure GGO (infiltrative disease) 1, 2

Important Caveats

• Extensive GGO (>30% of lung involvement) in suspected idiopathic pulmonary fibrosis should prompt consideration of alternative diagnoses such as hypersensitivity pneumonitis or drug toxicity 1, 4, 5
• Physiologic processes including poor ventilation of dependent lung areas and expiratory phase imaging can mimic pathologic GGO 7
• The "three-density pattern" (combination of GGO, normal lung, and lucent areas with decreased vascularity) is highly specific for fibrotic hypersensitivity pneumonitis 1, 4