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Can morphologic characteristics of the reversed halo sign narrow the differential diagnosis of pulmonary infections?

AJR 2014; 203:W557 0361–803X/14/2035–W557 © American Roentgen Ray Society Can Morphologic Characteristics of the Reversed Halo Sign Narrow the Differential Diagnosis of Pulmonary Infections? We read with great interest the wellwritten article by Walker et al. [1], who described common and uncommon imaging signs and patterns of pulmonary infections and discussed the underlying anatomic and pathophysiologic bases. They concluded that imaging plays an important role in the diagnosis and management of pulmonary infections. When imaging manifestations have a characteristic appearance, they may suggest a specific diagnosis or narrow the differential diagnosis. Walker et al. [1] reported that some characteristics of the reversed halo sign, such as associated irregular and intersecting areas of stranding or irregular lines within an area of ground-glass opacity, may be suggestive of invasive fungal infection (e.g., angioinvasive aspergillosis infection or mucormycosis). They found that imaging features favoring mucormycosis over angioinvasive aspergillosis in a patient with neutropenia are the reversed halo sign, multiple (> 10) pulmonary nodules, and evidence of the development of infection despite voriconazole prophylaxis. They also commented that the reversed halo sign is not specific for invasive fungal infection and may also be seen in a wide spectrum of diseases, including infectious and noninfectious conditions [2]. We would like to highlight additional morphologic CT features that can increase the specificity for the differential diagnosis of the reversed halo sign. In an examination of the morphologic characteristics of the reversed halo sign in 15 patients with invasive fungal infection and 25 patients with organizing pneumonia, reticulation inside the reversed halo sign was observed in 93% of patients with invasive fungal infection and in no patients with organizing pneumonia [3]. Maximum consolidation (± SD) rim thicknesses were 2.04 ± 0.85 cm for invasive fungal infection and 0.50 ± 0.22 cm for organizing pneumonia. Thus, reticulation inside the reversed halo sign and outer rim thickness greater than 1 cm suggest the diagnosis of invasive fungal infection rather than organizing pneumonia (Fig. 1). These features should be included among CT findings suggestive of invasive fungal infection [3]. Another reversed halo sign characteristic that is useful for differential diagnosis is the presence of small nodules in the wall or inside the halo, which usually indicates active granulomatous disease (granulomatous infection, particularly tuberculosis, or sarcoidosis) [2]. This nodular aspect of the reversed halo sign is related to the presence of granulomas. A study comparing reversed halo sign characteristics in 12 patients with tuberculosis and 10 patients with organizing pneumonia documented nodular walls in all patients with tuberculosis and small nodules inside the ground-glass component of the reversed halo sign in 83% of these patients (Fig. 2), whereas these characteristics were absent in all patients with organizing pneumonia [4]. Thus, careful analysis of the morphologic characteristics of the reversed halo sign may narrow the differential diagnosis when used in association with the clinical history. A nodular appearance of the reversed halo sign in patients with pulmonary infection indicates the presence of active granulomatous disease (especially tuberculosis), whereas the presence of reticulation inside the reversed halo sign and outer rim thick-