Imaging Diagnosis of Isolated Lung Nodules Isolated lung nodules are defined as a single round or ovoid dense shadow in the lung parenchyma not associated with hilar or mediastinal lymph node enlargement, pneumonia, or pulmonary atelectasis, ≤50px in diameter, with a margin of some sharpness sufficient to measure its diameter. ＞Those >50px can be seen in a variety of diseases. The size of pulmonary nodules is further classified as micronodules (cornified nodules) if the diameter is less than 4 mm; small nodules (alveolar nodules) if the diameter is 4-10 mm; and nodules if the diameter is less than 20 mm if the diameter is less than 10 mm. Density of pulmonary nodules Pure ground-glass density nodules – the interior of the nodule is ground-glass-like. Mixed ground-glass density nodules – both ground-glass changes and solid components. Solid nodules-all of the nodule is realized and the bronchial and vascular texture is completely obscured. Ground-glass density nodules Ground-glass density shadows appear on CT as slightly hyperdense shadows in the lungs, with vascular and bronchial texture visible within them. If it is round or ovoid, it is called GGO nodule, and the degree of malignancy is reported to be as high as 71.4 in the literature, which is mainly adenocarcinoma and fine bronchioloalveolar carcinoma (slow-growing), followed by atypical adenomatous hyperplasia, focal inflammation and interstitial fibrosis. Mixed ground-glass density nodules are divided into two categories with and without halo manifestations. The former commonly include lung cancer, invasive pulmonary aspergillosis, cryptococcosis, focal mechanized pneumonia, pulmonary eosinophilic infiltration, Wegner’s granulomatosis and lung injury. The latter includes lung cancer and focal interstitial fibrosis. Solid nodules Solid nodular tumors and granulomatous lesions account for the majority of cases; malignant disease is most commonly peripheral lung cancer, with lung sarcoma and isolated lung metastases accounting for the minority. Among the benign lesions, nodular ball and inflammatory pseudotumor are the main ones, and a few are misshapen tumors and adenomas. Isolated nodules less than 5mm in diameter are mostly benign and less than 1% are malignant.