From the Guidelines
The halo sign and reverse halo sign in radiology have distinct primary causes, with the halo sign most commonly associated with invasive pulmonary aspergillosis and the reverse halo sign classically seen in cryptogenic organizing pneumonia. The halo sign, characterized by a central opacity surrounded by a ground-glass ring, is often linked to invasive pulmonary aspergillosis, particularly in immunocompromised patients, as noted in studies such as 1. Other causes of the halo sign include tuberculosis, Wegener's granulomatosis, metastatic disease, and organizing pneumonia. On the other hand, the reverse halo sign, which presents as a central ground-glass opacity surrounded by a denser ring of consolidation, is typically associated with cryptogenic organizing pneumonia, as discussed in 1 and 1. However, it can also appear in pulmonary infarction, tuberculosis, sarcoidosis, Wegener's granulomatosis, and invasive fungal infections. While there is some overlap in the etiologies of these two signs, they generally represent different pathological processes. The halo sign typically indicates hemorrhage surrounding a nodule, whereas the reverse halo sign often represents inflammation with peripheral organization. These radiological findings are crucial for radiologists and clinicians to narrow down differential diagnoses when evaluating chest imaging, ultimately guiding appropriate treatment and management strategies to improve patient outcomes in terms of morbidity, mortality, and quality of life. Key points to consider include:
- The halo sign is most commonly associated with invasive pulmonary aspergillosis.
- The reverse halo sign is classically seen in cryptogenic organizing pneumonia.
- Both signs can have overlapping etiologies but generally represent different pathological processes.
- Accurate identification of these signs is critical for guiding appropriate treatment and management strategies.
From the Research
Primary Causes of the Halo Sign and Reverse Halo Sign
- The halo sign is a CT finding of ground-glass opacity surrounding a pulmonary nodule or mass, and is highly suggestive of early infection by an angioinvasive fungus, such as invasive pulmonary aspergillosis 2.
- The reverse halo sign is a focal rounded area of ground-glass opacity surrounded by a crescent or complete ring of consolidation, and is most commonly associated with pulmonary mucormycosis 2, as well as other conditions such as organizing pneumonia, invasive fungal infections, and noninfectious diseases 3, 4.
- In immunocompromised patients, the reverse halo sign is more likely to be caused by infection, including fungal and bacterial pneumonia, than by noninfectious processes 5.
Comparison of the Halo Sign and Reverse Halo Sign
- The halo sign and reverse halo sign are distinct radiological patterns with different underlying causes 2, 6.
- While the halo sign is more suggestive of angioinvasive pulmonary aspergillosis, the reverse halo sign is more suggestive of pulmonary mucormycosis in an appropriate clinical setting 6.
- Both signs can be useful for preemptive initiation of antifungal therapy in immunocompromised patients, but require correlation with clinical history and other radiological findings to narrow the differential diagnosis 3, 4, 2, 5.
Differential Diagnosis
- The differential diagnosis for the reverse halo sign includes a wide range of pulmonary diseases, such as organizing pneumonia, invasive fungal infections, paracoccidioidomycosis, pneumocystis pneumonia, tuberculosis, community-acquired pneumonia, lymphomatoid granulomatosis, Wegener granulomatosis, lipoid pneumonia, and sarcoidosis 3, 4.
- In immunocompromised patients, the presence of the reverse halo sign should prompt consideration of infection until further analysis proves otherwise, with aspergillosis and mucormycosis being common causes 5, 6.