Ground-glass opacity

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Definition

In both CT and chest radiographs, normal lungs appear dark due to the relative lower density of air compared to the surrounding tissues. When air is replaced by another substance (e.g. fluid or fibrosis), the density of the area increases, causing the tissue to appear lighter or more grey.[4]

Ground-glass opacity is most often used to describe findings in high-resolution CT imaging of the thorax, although it is also used when describing chest radiographs. In CT, the term refers to one or multiple areas of increased attenuation (density) without concealment of the pulmonary vasculature. This appears more grey, as opposed to the normally dark-appearing (air-filled) lung on CT imaging. In chest radiographs, the term refers to one or multiple areas in which the normally darker-appearing (air-filled) lung appears more opaque, hazy, or cloudy. Ground-glass opacity is in contrast to consolidation, in which the pulmonary vascular markings are obscured.[3][5] GGO can be used to describe both focal and diffuse areas of increased density.[5] Subtypes of GGOs include diffuse, nodular, centrilobular, mosaic, crazy paving, halo sign, and reversed halo sign.[6]

Causes

The differential diagnosis for ground-glass opacities is broad. General etiologies include infections, interstitial lung diseases, pulmonary edema, pulmonary hemorrhage, and neoplasm. A correlation of imaging with a patient's clinical features is useful in narrowing the diagnosis.[6][7] GGOs can be seen in normal lungs. Upon expiration there is less air in the lungs, leading to a relative increase in density of the tissue, and thus increased attenuation on CT. Furthermore, when a patient lays supine for a CT scan, the posterior lungs are in a dependent position, causing partial collapse of the posterior alveoli. This leads to an increase in density of the tissue, resulting increased attenuation and a possible ground-glass appearance on CT.[3]

Infectious causes

In the setting of pneumonia, the presence of GGO (as opposed to consolidation) is a useful diagnostic clue. Most bacterial infections lead to lobar consolidation, while atypical pneumonias may cause GGOs. It is important to note that while many of the pulmonary infections listed below may lead to GGOs, this does not occur in every case.[2][6][7][8][9]

Bacterial

Viral