The ground-glass pattern is a common but nonspecific finding on CT. In certain clinical circumstances, it can suggest a specific diagnosis, indicate a potentially treatable disease, and guide a clinician to an appropriate area for biopsy. A pattern of centrilobular ground-glass nodules is fairly specific for the diagnosis of hypersensitivity pneumonitis with the appropriate clinical history. The tree-in-bud pattern indicates disease affecting the small airways. The differential diagnosis is lengthy; however, the most common process leading to this CT appearance is infection. Although commonly associated with M. tuberculosis, many infectious organisms can produce this pattern. When honeycombing is seen on HRCT, a confident diagnosis of lung fibrosis can be made. The most common causes of interlobular septal thickening on HRCT are pulmonary edema, pulmonary hemorrhage, and lymphangitic spread of cancer, and smooth thickening is characteristic of all three. Diffuse lung cysts in patients who are not immunocompromised generally signify Langerhans' cell histiocytosis, lymphangioleiomyomatosis, or centrilobular emphysema. Centrilobular emphysema can be diagnosed when the centrilobular artery is seen as a small nodular opacity in the center of the cyst. Langerhans' cell histiocytosis is often associated with parenchymal nodules, helping to distinguish it from lymphangioleiomyomatosis. When a nodular pattern is seen on HRCT, the differential diagnosis is very long, but can be narrowed by noting whether the nodules are random, centrilobular, or perilymphatic in distribution. A mosaic pattern of lung attenuation can represent an infiltrative, small airway, or vascular process. The distinction can often be made by noting the size of the pulmonary vessels in the abnormal areas of lung, and whether air trapping is present on expiratory scanning. Computed tomographic signs can be useful indicators of a specific disease process. For instance, the air bronchogram sign indicates that an opacity is intrapulmonary in location, and signals the possibility of two types of neoplasm: lymphoma and bronchioloalveolar cell carcinoma. An air crescent sign indicates recovery of the immune system in an immunocompromised patient with invasive pulmonary aspergillosis. The fallen lung sign is diagnostic of a bronchial transection in the correct clinical setting. The gloved finger sign is very suggestive of allergic bronchopulmonary aspergillosis. The halo sign is highly suggestive of early angioinvasive pulmonary aspergillosis in patients with acute leukemia. When a split pleura sign is seen, the diagnosis is often empyema, although other causes of pleuritis can lead to a similar CT appearance.