Diffuse ground-glass opacity of the lung. A guide to interpreting the high-resolution computed tomographic (HRCT) picture

The so-called ground glass pulmonary opacity is characterized by a slight increase in lung density, with persistent visibility of vascular structures and bronchial walls. If vessels are obscured, the term consolidation is preferred. This kind of pulmonary opacity, which may be patchy or diffuse, was well known in conventional radiology, but has been recently re-evaluated, following the increasingly widespread use of high resolution CT of the lung. Ground glass opacity is commonly observed in patients with early diffuse pulmonary infiltrative diseases. Though non-specific in itself, the sign is always very significant. Particularly, it could represent a useful sign of active and treatable abnormality in some diffuse pulmonary diseases, such as idiopathic pulmonary fibrosis and sarcoidosis. The ground glass opacity may also be observed in pulmonary edema, desquamative pneumonitis, Pneumocystis carinii pneumonia, alveolar proteinosis, hypersensitive pneumonitis and drug induced or radiation induced lung disease. This paper represents a contribution to the understanding of the pathologic bases of the ground glass pulmonary opacity and an introduction to its differential diagnosis.