Ground-glass opacity

In radiology, ground glass opacity (GGO) is a nonspecific finding on computed tomography (CT) scans that indicates a partial filling of air spaces in the lungs by exudate or transudate, as well as interstitial thickening or partial collapse of lung alveoli.

Differential diagnosis

The differential diagnosis of the many causes of GGO includes pulmonary edema, infections (including cytomegalovirus and Pneumocystis jirovecii pneumonia), various noninfectious interstitial lung diseases (such as hypersensitivity pneumonitis, Hamman-Rich syndrome), diffuse alveolar hemorrhage, and cryptogenic organizing pneumonia.[1]

Reversed halo sign

A reversed halo sign is a central ground-glass opacity surrounded by denser consolidation. Criteria include that the consolidation should form more than three-fourths of a circle and be at least 2 mm thick.[2] It is suggestive of cryptogenic organizing pneumonia,[3] but is only seen in about 20% of individuals with this condition.[2] It can also be present in lung infarction where the halo consists of hemorrhage,[4] as well as in infectious diseases such as paracoccidioidomycosis, tuberculosis, zygomycosis, and aspergillosis, as well as in granulomatosis with polyangiitis, lymphomatoid granulomatosis, and sarcoidosis.[5]


  1. Jannette Collins, MD; Eric J. Stern, MD (1998). "Ground glass opacity on CT scanning of the chest: What does it mean?" (PDF). Applied Radiology. Retrieved 2012-02-01.
  2. Radswiki; et al. "Reversed halo sign (lungs)". Radiopaedia. Retrieved 2018-01-02.
  3. Brett M. Elicker, W. Richard Webb (2012). Fundamentals of High-Resolution Lung CT: Common Findings, Common Patterns, Common Diseases, and Differential Diagnosis. Lippincott Williams & Wilkins. ISBN 9781469824796.
  4. Wu, George; Schmit, Berndt; Arteaga, Veronica; Palacio, Diana (2017). "Medical image of the week: pulmonary infarction- the "reverse halo sign"". Southwest Journal of Pulmonary and Critical Care. 15 (4): 162–163. doi:10.13175/swjpcc124-17. ISSN 2160-6773.
  5. Page 256 in: D Karthikeyan (2013). High Resolution Computed Tomography of the Lungs: A Practical Guide. JP Medical Ltd. ISBN 9789350904084.
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