Ground-glass opacity is a finding seen on chest x-ray or computed tomography imaging of the lungs. It is typically defined as an area of hazy Opacification or increased attenuation 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, the density of the area increases, causing the tissue to appear lighter or greyer. 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 without concealment of the pulmonary vasculature. This appears more grey, as opposed to the normally dark-appearing 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. 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.

Journal of Imaging and Interventional Radiology is the peer-reviewed journal of choice for interventional radiologists, radiologists, cardiologists, vascular surgeons, neurosurgeons, and other clinicians who seek current and reliable information on every aspect of interventional radiology.
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Ann Jose

Journal coordinator

Journal of Imaging and Interventional Radiology

intervradiology@longdomjournal.org