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Concept
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Résumé
Ground-glass opacity (GGO) is a finding seen on chest x-ray (radiograph) or computed tomography (CT) imaging of the lungs. It is typically defined as an area of hazy opacification (x-ray) or increased attenuation (CT) due to air displacement by fluid, airway collapse, fibrosis, or a neoplastic process. When a substance other than air fills an area of the lung it increases that area's density. On both x-ray and CT, this appears more grey or hazy as opposed to the normally dark-appearing lungs. Although it can sometimes be seen in normal lungs, common pathologic causes include infections, interstitial lung disease, and pulmonary edema.
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.
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. GGO can be used to describe both focal and diffuse areas of increased density. Subtypes of GGOs include diffuse, nodular, centrilobular, mosaic, crazy paving, halo sign, and reversed halo sign.
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.
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