Background: The purpose of this study was to identify CT scan findings that differentiate the reversed halo sign (RHS) caused by invasive fungal infection (IFI) from the RHS caused by organizing pneumonia (OP). Methods: We retrospectively reviewed CT scans of patients with RHS caused by IFI or OP. The study included 15 patients with proven or probable IFI (eight men and seven women) and 25 patients with biopsy-proven OP (13 women and 12 men). The CT images were reviewed individually by two chest radiologists who were blinded to the final diagnosis. Results: Reticulation inside the RHS was observed in 14 of the 15 patients with IFI (93%) and in no patient with OP. The maximal thickness of the consolidation rim was 2.04 ± 0.85 cm for IFI and 0.50 ± 0.22 cm for OP. Pleural effusion was noted in 11 of the 15 patients with IFI (73%) and in no patient with OP. Other parenchymal abnormalities, such as consolidation and ground-glass and linear opacities, were observed in both groups. The number of lesions showing the RHS did not differentiate IFI and OP. Conclusion: The presence of reticulation inside the RHS, outer rim thickness > 1 cm, and associated pleural effusion strongly suggest the diagnosis of IFI rather than OP.
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