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Purpose: To re-examine the imaging characteristics of halo sign (HS) associated with solitary pulmonary nodules (SPN) for SPN differentiation with emphasis on the HS margins.Materials and methods: The halo sign in multi-detector spiral CT images (MDCT) of 150 SPNs with proven pathological results or clinical remission after anti-inflammatory treatment was retrospectively reviewed.The halo sign margin was categorized as well-defined and poorly-defined, and the underlying pathological findings were studied.Correlations between malignancy/benignancy of SPN grouped by size (<2 cm, 2~5cm, >5 cm) and margin categories as well as Halo/solid component ratio were analyzed for diagnostic sensitivity and specificity.In addition, imaging parameters that may affect HS visualization were investigated.Results: Of 150 SPNs with HS in MDCT examination, HS with well-defined margin (HSWM) was seen with 92 SPNs, while HS with poorly-defined margin (HSPM) with 58 nodules.Collectively,there existed weak correlation between HSWM and malignancy and between HSPM and benignancy (r=0.68 and 0.76, respectively).In terms of SPN size, strongest correlation was found in < 2 cm SPNs (HSWM-malignancy, r=0.94; HSPM-benignancy, r=0.91) and the diagnostic sensitivity and specificity of HSWM for malignant SPN were 91% and 87%, respectively.Halo/solid component ratio was heterogeneous and did not show consistent value for malignancy and benignancy differentiation.For better visualization of HS-associated HS and the corresponding margins, the optimal imaging parameters included less than 2 mm slice thickness, contrast-enhancement if no contraindication prohibited, multiple-planar reconstruction and high-resolution reconstruction of disease site.Conclusion: Visualization and characterization of SPN-associated HS is important for differentiation of solitary pulmonary nodules.For small SPNs (< 3cm), HS with well-defined margins tends to predict malignancy, while that with poorly-defined margins more frequently indicated benign diseases.The visualization of HS margin is subject to imaging acquisition and reconstruction parameters which deserve optimization and standardization studies.The predictive value of HS-margin should be examined in prospective studies.Nonetheless, other imaging features related to the SPNs should be taken into consideration for the final diagnostic decision.