DOI: 10.1148/radiol.2301020649
(Radiology 2004;230:109-110.)
© RSNA, 2004
The CT Halo Sign1
Pedro S. Pinto, MD
1 From the Department of Radiology, University of California, San Diego. Received June 3, 2002; revision requested July 30; revision received September 25; accepted November 18. Address correspondence to the author, Department of Radiology, Faculdade de Medicina do Porto, Hospital São João, Al Prof Hernani Monteiro, Porto 4200, Portugal.
Index terms: Lung, CT, 60.1211 • Lung, ground-glass opacification, 60.1211 • Lung, nodule, 60.281 • Signs in Imaging
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APPEARANCE
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The computed tomographic (CT) halo sign, also known as the halo sign, refers to a zone of ground-glass attenuation surrounding a pulmonary nodule or mass on CT images (1) (Figure).
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Transverse CT scan obtained in a 50-year-old woman with invasive pulmonary aspergillosis treated with a high dosage of steroids to reduce cerebral edema from anaplastic oligodendroglioma. A large mass is seen in the right upper lobe, surrounded by a wide zone of ground-glass attenuation (arrow) demonstrating the halo sign. A smaller mass (arrowhead) is seen in the left lower lobe; it has indistinct margins but no well-defined halo sign.
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EXPLANATION
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The presence of a halo of ground-glass attenuation is usually associated with hemorrhagic nodules (2). This CT appearance was described by Kuhlman et al (1) in patients with invasive aspergillosis. In severely neutropenic patients, the CT halo sign is highly suggestive of infection by an angioinvasive fungus, most commonly Aspergillus. Vascular invasion by this fungus results in thrombosis of small- to medium-sized vessels, which causes ischemic necrosis (3). At pathologic examination, the nodules represent foci of infarction, and the halo of ground-glass attenuation results from alveolar hemorrhage (2,4). Although it is less common, the halo sign may also be observed in nonhemorrhagic nodules, in which case either tumor cells or inflammatory infiltrate account for the halo of ground-glass attenuation (2). Nonetheless, in the appropriate clinical setting, the halo sign is considered early evidence of pulmonary aspergillosis even before serologic tests become positive (5), and it warrants the administration of systemic antifungal therapy (6).
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DISCUSSION
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The halo sign has been described in patients with invasive aspergillosis and acute leukemia (1). Invasive pulmonary aspergillosis represents one end of the spectrum of conditions caused by Aspergillus organisms (7). In a severely immunocompromised patient with a fever that does not respond to antibiotics, pulmonary or systemic fungal infection must be considered in the differential diagnosis. Early recognition of this complication is critical because this disease is associated with a high mortality rate that ranges from 50% to 90% (8). The frequency of the halo sign in patients with invasive aspergillosis is relatively high in the early stages of the disease, but alters with time and becomes progressively less frequent. In a group of 25 patients with invasive pulmonary aspergillosis studied by using serial CT scans, the frequency of this sign ranged from 96% at day 0 to 19% at day 14 (9). The air crescent sign (10), which has also been associated with invasive aspergillosis, may be found later in the course of the disease, when the patient is recovering from neutropenia. Kami et al (5) showed that chest CT is more sensitive in the diagnosis of early invasive aspergillosis than is currently available serologic testing used to detect circulating Aspergillus antigens. The angioinvasive nature of Aspergillus is shared by other fungal species. In a series of eight patients with Mucor infection, the halo sign was seen on CT images in three patients (11). This sign has also been reported in patients with pulmonary candidiasis and coccidioidomycosis (2).
The halo sign has been demonstrated in a number of other conditions that are associated with pulmonary hemorrhagic nodules. Lung metastases from hypervascular tumors can manifest with a halo of ground-glass attenuation that most likely results from perilesional hemorrhage secondary to the fragility of neovascular tissue. This CT appearance has been found in patients with lung metastases from angiosarcoma, choriocarcinoma, and osteosarcoma (2,12,13). Pulmonary Kaposi sarcoma typically manifests on chest CT images as ill-defined (flame-shaped) nodules predominantly seen in a peribronchovascular distribution. The CT findings of this disease may occasionally include multiple foci of ground-glass attenuation or nodules with the halo sign (2). Wegener granulomatosis, a necrotizing vasculitis that involves the respiratory tract, may manifest with pulmonary hemorrhage and thus develop the halo sign (2). Patients who have undergone transbronchial biopsy can develop lung nodules that are in some cases associated with a halo sign. A review of the CT images in 40 patients with lung transplants demonstrated lung nodules in 12 patients that were unrelated to infection or rejection. The ground-glass component, which the authors related to hemorrhage secondary to the lung injury, was present on CT images in six of these 12 patients (14).
Although usually regarded as an indication of hemorrhagic nodules, the halo sign may also be present when tumor or inflammatory cells infiltrate the lung parenchyma. Two types of tumor growth in the lung have been described by Heitzman (15): hilic, which is characteristic of most masses that displace the lung while enlarging, and lepidic, in which tumor cells spread along the alveolar walls, thereby preserving the underlying architecture. This second type of growth can be found with bronchioloalveolar carcinoma, which explains the ground-glass attenuation component that is commonly found with this type of tumor. In a series of 22 patients with bronchioloalveolar carcinoma, the halo sign was identified on CT images in two patients (16). In another series, which comprised 65 patients with lung metastases from extrapulmonary adenocarcinoma, the halo sign was present in two patients. Histologic evaluation revealed tumor cell infiltrate consistent with lepidic growth (17).
The halo has been described in patients with eosinophilic pneumonia, bronchiolitis obliterans organizing pneumonia (18), and tuberculosis (19), and in patients infected by Mycobacterium avium complex (20), Coxiella burnetti (21), cytomegalovirus, herpes simplex virus (2), and myxovirus (18). Most of these cases were documented only in individual patients or in limited numbers of patients, and in most cases, no histologic-radiologic comparisons were obtained. However, in one patient with tuberculosis who had a solitary nodule with the halo sign at CT, histologic analysis demonstrated granulomatous reaction without alveolar hemorrhage in the peripheral zone of ground-glass attenuation. A review of the CT findings in 18 patients with intrathoracic lymphoproliferative disorders revealed the halo sign in three out of four patients with posttransplantation lymphoproliferative disorder (22). The ground-glass attenuation at CT consisted of nonspecific interstitial inflammation mixed with postobstructive endogenous lipoidosis that surrounded a central core of necrotic tumor.
In summary, the CT halo sign may be seen with a wide spectrum of pulmonary diseases; it is most commonly associated with hemorrhagic nodules and is more rarely associated with tumor cell or inflammatory infiltrate. Notwithstanding this wide spectrum of associated diseases, the CT halo sign is a useful diagnostic clue in the appropriate clinical setting and may be the first evidence of pulmonary fungal infection.
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ACKNOWLEDGMENTS
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I thank Michelle Wessely, BSc, for the final review of the manuscript.
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FOOTNOTES
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A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.
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REFERENCES
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