Ground-glass opacification/opacity (GGO) is a descriptive term referring to an area of increased attenuation in the lung on computed tomography (CT) with preserved bronchial and vascular markings. It is a non-specific sign with a wide aetiology including infection, chronic interstitial disease and acute alveolar disease.

Although the term was first used as a radiograph descriptor, and this use being in previous iterations of the Fleischner glossary ⁷, it is currently (c. 2024) recommended to reserve the term for CT only ⁹.

'Ground glass' derives from an industrial technique in glassmaking where the surface of normal glass is roughened by grinding it.

Pathology

Aetiology

Ground-glass opacities have a broad aetiology, caused by any process that partially fills the alveolar space and is below the resolution of CT ⁹:

• normal expiration

• partial filling of air spaces with oedema, haemorrhage, or cellular fluid

• partial collapse of alveoli

• interstitial thickening

• fibrosis

• lepidic proliferation of neoplasm

Morphological forms

• focal ground-glass opacification (includes ground-glass nodules)

• diffuse ground-glass opacification (includes diffuse ground-glass nodules)

• isolated diffuse ground-glass opacification ⁵

Differential diagnosis

Broadly speaking, the differential for ground-glass opacification can be split into ⁵:

• infectious processes (opportunistic vs non-opportunistic)

• chronic interstitial diseases

• acute alveolar diseases

• other causes

To narrow down the differential diagnosis, following points may be of help ⁸:

Is the GGO pathological?

• if the membranous posterior wall of the trachea bows anteriorly, the scan has been performed in expiration and lung attenuation will be increased.

• dependent atelectasis is a common normal finding. If the extent is greater than normal, prone imaging can differentiate normal from abnormal

• if mosaic attenuation pattern is present, attenuation measurements help to distinguish normal from abnormal

What is the time course of the GGO?

• acute GGO lasts only days or weeks. In this setting, imaging is less important as the most common causes of acute GGO (infection, oedema, haemorrhage, ARDS, and non-fibrotic hypersensitivity pneumonitis) may have overlapping and non-specific features. The clinical features are the key to diagnosis

• chronic GGO may remain relatively unchanged for many weeks and even years. In these case the spatial distribution and additional imaging findings are important

• recurring GGO (e.g., haemorrhages in vasculitis), is a potential pitfall when assessing the time course

• recent bronchoalveolar lavage may alter the attenuation in both directions (i.e., increasing attenuation through remaining fluid, and decreasing attenuation after treating pulmonary alveolar proteinosis)

What is the spatial distribution?

• upper zone predominant GGO may occur in hypersensitivity pneumonitis, respiratory bronchiolitis and sarcoidosis

• mid and lower zone predominant GGO may occur in non-specific interstitial pneumonia, desquamative interstitial pneumonia, pulmonary oedema, and aspiration

• GGO in the peripheral zone may occur in non-specific interstitial pneumonia, desquamative interstitial pneumonia, organising pneumonia and eosinophilic pneumonia

• GGO in the central zone may occur in hypersensitivity pneumonitis, diffuse alveolar haemorrhage, pulmonary alveolar proteinosis, pulmonary oedema, and aspiration

Infections

Opportunistic

• pneumocystis pneumonia (PCP/PJP)

• cytomegalovirus (CMV) pneumonia

• herpes simplex virus (HSV) pneumonia

• respiratory syncytial virus (RSV) bronchiolitis: type of infectious bronchiolitis

• other infectious causes

Non-opportunistic

• viral pneumonia ⁶

  • human coronaviruses

    • COVID-19

    • Middle East respiratory syndrome coronavirus (MERS-CoV) infection

    • severe acute respiratory syndrome (SARS)

  • herpesviridae

Chronic interstitial diseases

• eosinophilic pneumonias: ground-glass opacification can be seen in many of the eosinophilic pneumonias but is most commonly seen in ²:

  • simple pulmonary eosinophilia (SPE): nodules with a GGO halo

  • idiopathic hypereosinophilic syndrome (IHS): nodules with a GGO halo

  • acute eosinophilic pneumonia (AEP): bilateral patchy areas of GGO with interlobular septal thickening

  • eosinophilic drug reactions: peripheral airspace consolidation and GGO

• idiopathic interstitial pneumonias ³

  • non-specific interstitial pneumonia: GGO with linear or reticular markings, micronodules, consolidation, and microcystic honeycombing

  • usual interstitial pneumonia (UIP): focal GGO with macrocystic honeycombing, reticular opacities, traction bronchiectasis, and architectural distortion

  • cryptogenic organising pneumonia (COP): formerly bronchiolitis obliterans with organising pneumonia (BOOP); GGO with airspace consolidation and mild bronchial dilatation

  • exudative phase of acute interstitial pneumonia (AIP): diffuse lung consolidation with GGO

  • respiratory bronchiolitis-associated interstitial lung disease (RB-ILD): patchy GGO centrilobular nodules and bronchial wall thickening

  • desquamative interstitial pneumonia (DIP): GGO with linear or reticular opacities

  • lymphoid interstitial pneumonia (LIP): GGO often in association with perivascular cystic lesions, septal thickening, and centrilobular nodules

• sarcoidosis (pulmonary manifestations of sarcoidosis)

Acute alveolar disease

• alveolar oedema or pulmonary oedema

  • cardiogenic pulmonary oedema

  • adult respiratory distress syndrome (ARDS)

  • other causes of non-cardiogenic pulmonary oedema

• hypersensitivity pneumonitis: especially acute and subacute forms

Other causes

• neoplastic processes with a lepidic proliferation pattern

  • atypical adenomatous hyperplasia

  • localised adenocarcinoma

  • adenocarcinoma in situ or minimally invasive (formerly bronchoalveolar cell carcinoma)

• drug toxicity

Rare causes

• focal interstitial fibrosis: a non-neoplastic entity with a nodular ground-glass opacity that does not change over a long period of time; can be mistaken for a neoplastic process

• aspergillosis: a nodule with surrounding ground-glass opacity (CT halo sign) is rare except in severely immunocompromised patients

• thoracic endometriosis

• traumatic lung injury (pulmonary contusion)

• poisoning e.g. acute/subacute phase of paraquat poisoning

• pulmonary cryptococcus infection: solitary or multiple pulmonary nodules with or without peripheral GGO

• granulomatosis with polyangiitis

• Henoch-Schönlein purpura

• metal fume fever

• polymer fume fever

See also

• ground-glass nodules

• diffuse ground-glass nodules