Ground-glass opacification

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.

Ground glass opacification is also used in chest radiography to refer to a region of hazy lung radiopacity, often fairly diffuse, in which the edges of the pulmonary vessels may be difficult to appreciate ⁷.

The use of the term ground glass derives from the industrial technique in glassmaking whereby the surface of normal glass is roughened by grinding it. 

Pathology

Aetiology

Ground-glass opacities have a broad aetiology:

• normal expiration

  • on expiratory acquisitions, which can be detected if the posterior membranous wall of the trachea is flattened or bowed inwards

• partial filling of air spaces

• partial collapse of alveoli

• interstitial thickening

• inflammation

• oedema

• haemorrhage

• 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?

  • The configuration of the posterior tracheal wall helps in differentiating lack of ventilation from true pathology. Particularly, dependent atelectasis can pose a diagnostic difficulty which can be solved by repeating the scan while the patient lies prone.

  • Further, regular lung tissue may be misjudged as GGO when mosaic attenuation pattern is present.

• What is the time course of the GGO?

  • Acute GGO lasts only a few 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 unspecific features. Therefore, the patient’s presentation may have a greater impact on the differential diagnosis than the imaging appearance.

  • Chronic GGO may remain relatively unchanged for many weeks and even years. In cases where chronic GGO is the predominant pattern, reviewing the spatial distribution and additional imaging findings is of great importance.

  • Potential pitfalls when evaluating the time course of GGO is recurring GGO (e.g., haemorrhages in vasculitis), and recent bronchoalveolar lavage. 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, 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