From the Guidelines
The primary causes of the halo sign and reverse halo sign in radiology are distinct radiological findings with specific etiologies, including invasive pulmonary aspergillosis, metastatic tumors, and organizing pneumonia, among others, as identified in the most recent study 1. The halo sign appears as a ground-glass opacity surrounding a nodular lesion on CT imaging, and its primary causes include:
- Invasive pulmonary aspergillosis (especially in immunocompromised patients) 1
- Metastatic tumors (particularly hemorrhagic metastases like choriocarcinoma, melanoma, and renal cell carcinoma)
- Wegener's granulomatosis
- Occasionally tuberculosis The reverse halo sign (also called the atoll sign) presents as a central ground-glass opacity surrounded by a ring of consolidation, and its main causes include:
- Organizing pneumonia (formerly bronchiolitis obliterans organizing pneumonia) 1
- Pulmonary infarction
- Pulmonary tuberculosis
- Sarcoidosis
- Wegener's granulomatosis
- Invasive fungal infections like mucormycosis Both signs are important radiological findings that can help narrow differential diagnoses in pulmonary pathology, though neither is completely specific to any single disease entity and must be interpreted in the clinical context 1.
Key characteristics of these signs, as described in recent studies 1, include:
- Patchy consolidation with air bronchograms, typically in subpleural locations, for organizing pneumonia
- Ground glass change, reversed halo (atoll) sign, reticulation, or nodules for various conditions
- Histological features of organizing pneumonia, including fibroblasts and inflammatory cells embedded in extracellular matrix within the small airways and alveoli It is essential to consider these characteristics and the clinical context when interpreting the halo sign and reverse halo sign in radiology, as emphasized in the most recent study 1.
From the Research
Primary Causes of the Halo Sign and Reverse Halo Sign
The primary causes of the halo sign and reverse halo sign in radiology can be attributed to various infectious and noninfectious diseases.
- The halo sign is highly suggestive of early infection by an angioinvasive fungus, such as invasive pulmonary aspergillosis 2, 3.
- The reverse halo sign is more commonly associated with pulmonary mucormycosis, but can also be seen in other conditions such as organizing pneumonia, infectious diseases, and noninfectious diseases like Wegener's granulomatosis and lymphomatoid granulomatosis 4, 5, 2.
- In immunocompromised patients, the reverse halo sign is most commonly caused by infections, including fungal and bacterial pneumonia, with aspergillosis and mucormycosis being common causes 5.
- Noninfectious causes of the reverse halo sign, such as organizing pneumonia, are more likely in solid organ transplant patients, while infections are more common in patients with hematologic malignancy and stem cell transplant 5.
Disease-Specific Associations
Certain diseases are more commonly associated with the halo sign or reverse halo sign, including:
- Invasive pulmonary aspergillosis: halo sign 2, 3
- Pulmonary mucormycosis: reverse halo sign 2, 6
- Organizing pneumonia: reverse halo sign 4, 5
- Cryptogenic organizing pneumonia: reverse halo sign 4
- Wegener's granulomatosis: reverse halo sign 4
- Lymphomatoid granulomatosis: reverse halo sign 4
Clinical and Imaging Features
Clinical and imaging features can help differentiate between infectious and noninfectious causes of the halo sign and reverse halo sign.
- Neutropenia, rim thickness, central ground-glass attenuation, and pleural effusion can be used to differentiate between infectious and noninfectious causes of the reverse halo sign 5.
- A decision tree using these clinical and imaging features can help achieve a specific diagnosis with a high degree of accuracy 5.