CT Halo Sign

The CT halo sign refers to a rim of ground-glass attenuation surrounding a pulmonary nodule or mass. It was first described in invasive pulmonary aspergillosis, and was thought to be specific for that disease. Since the initial description, however, the CT halo sign has been described in a number of other conditions.

• Angioinvasive fungal infection: The most common condition with the CT halo sign in immunocompromised patients. In severely neutropenic patients. Most commonly Aspergillus, but can also be seen with Mucor, Candida, and Coccidioides. Most frequently seen high in the early stages of disease. Ground-glass attenuation halo represents alveolar hemorrhage.
• Bronchoalveolar carcinoma: Mentioned separately because it is the most common condition with the CT halo sign in immunocompetent patients. The tumor can demonstrate lepidic growth, in which tumor cells grow along alveolar walls without destruction of the underlying architecture. Ground-glass attenuation halo represents tumor cell infiltrate.
• Other primary tumors: Squamous cell carcinoma, Kaposi sarcoma.
• Metastases from hypervascular tumors: For example, angiosarcoma, choriocarcinoma, osteosarcoma, melanoma, gastrointestinal malignancies, pulmonary endometriosis (not a neoplasm). Ground-glass attenuation halo represents hemorrhage due to fragility of neovascular tissue.
• Bacterial infection: Mycobacterium (for example, Tuberculosis [ground-glass attenuation halo may represent granulomatous reaction without hemorrhage] and mycobacterium avium complex), Coxiella burnetii.
• Viral infection: Cytomegalovirus, herpes simplex virus, and myxovirus.
• Parasitic infection: Schistosoma.
• Vasculitides: Wegener granulomatosis, for example. Ground-glass attenuation halo represents hemorrhage.
• Septic emboli:
• Trauma: For example, biopsy, lung transplantation.
• Eosinophilic lung disease: Simple eosinophilic pneumonia, hypereosinophilic syndrome.
• Cryptogenic organizing pneumonia: Also see the reversed halo sign in this entity.
• Posttransplantation lymphoproliferative disorder: Ground-glass attenuation halo represents interstitial inflammation mixed with postobstructive endogenous lipoidosis.

References

• Lee YR, Choi YW, Lee KJ, Jeon SC, Park CK, Heo JN. CT halo sign: the spectrum of pulmonary diseases. Br J Radiol. 2005 Sep;78(933):862-5.
• Marten K, Rummeny EJ, Engelke C. The CT halo: a new sign in active pulmonary sarcoidosis. Br J Radiol. 2004 Dec;77(924):1042-5.
• Pinto PS. The CT Halo Sign. Radiology. 2004 Jan;230(1):109-10.