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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 etiology 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.
Ground-glass opacities have a broad etiology:
partial filling of air spaces
partial collapse of alveoli
lepidic proliferation of neoplasm
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
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, edema, hemorrhage, 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., hemorrhages 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?
other infectious causes
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
usual interstitial pneumonia (UIP): focal GGO with macrocystic honeycombing, reticular opacities, traction bronchiectasis, and architectural distortion
cryptogenic organizing pneumonia (COP): formerly bronchiolitis obliterans with organizing 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
Acute alveolar disease
neoplastic processes with a lepidic proliferation pattern
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
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
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