CT Halo Sign

CT halo sign is a CT Þnding of ground-glass opacity surrounding a nodule or mass. It was Þrst described as a sign of hemorrhage around foci of invasive pulmonary aspergillosis [1] (Fig. 6.1).

Diseases Causing the Sign

The halo sign is nonspeciÞc and may be caused by several pathologic processes: hemorrhagic pulmonary nodules [2], tumor cell inÞltration, and non-hemorrhagic inßammatory lesions. Hemorrhagic pulmonary nodules may occur in infectious diseases including angioinvasive pulmonary aspergillosis (Fig. 6.2), mucormycosis, and candidiasis and noninfectious diseases including antineutrophil cytoplasmic antibody (ANCA)-associated granulomatous vasculitis and primary and metastatic hemorrhagic tumors (Figs. 6.3 and 6.4). The hemorrhagic nodule can also be found in a patient with pulmonary endometriosis with catamenial hemorrhage

[3, 4] (Fig. 6.5). Tumor cell inÞltrations in lung adenocarcinomas, bronchus-associated lymphoid tissue (BALT) lymphoma, and pulmonary metastatic neoplasm may appear with the halo sign. Eosinophilic lung disease [5] (Fig. 6.1) and organizing pneumonia are representative of inßammatory lesions showing the halo sign (Table 6.1) [4].

Distribution

The nodule and halo sign usually show random distribution without speciÞc zonal, centralÐperipheral, or bronchovascular distribution. The nodule and halo sign may show subpleural distribution in angioinvasive pulmonary aspergillosis, metastatic hemorrhagic tumors, and non-hemorrhagic pulmonary metastatic neoplasms.

Angioinvasive pulmonary aspergillosis is seen in neutropenic subjects with immunosuppression (absolute neutrophil count, <500/L). ANCA-associated granulomatous vasculitis is usually associated with clinical presentation that included acute glomerulonephritis, chronic refractory sinusitis, or rhinorrhea. Serologic test including cytoplasmic ANCA is performed for the diagnosis of the disease. Catamenial hemorrhage occurs at the time of patient menstruation. Eosinophilic lung disease is usually seen in patients who have intake history of freshwater crabs [6] or raw cow liver [7].

Key Points for Differential Diagnosis

1.On CT scans, segmental areas of consolidation and surrounding ground-glass opacity or at least one nodule with the halo sign are seen in patients with angioinvasive pulmonary aspergillosis. However, the Þndings are nonspeciÞc and can be seen in neutropenic patients with mucormycosis, organizing pneumonia, or pulmonary hemorrhage [8].

2.Patients with ANCA-associated granulomatous vasculitis may have localized areas of hemorrhage related to pulmonary nodules or may develop diffuse alveolar hemorrhage [4].

3.A single persistent nodule and surrounding halo sign may suggest the presence of invasive adenocarcinoma, BALT lymphoma, or organizing pneumonia [4].

4.Nodule(s) and halo sign in eosinophilic lung disease are usually multiple, migratory, and transient [5].