Helical CT depicts small pulmonary nodules not seen at conventional CT by elimination of respiratory misregistration and thorough analysis of a contiguous data volume. Pulmonary lesions can be analyzed in detail with use of retrospective reconstruction of the data volume. Multiplanar and three-dimensional images of lung masses and arteriovenous malformations are free of motion artifacts. The airways are clearly depicted by helical CT, a capability that enhances the radiologic evaluation of patients with hemoptysis, bronchial strictures, tumors, and recent lung transplantation. In comparison to conventional CT, helical CT provides a more consistent level of vascular enhancement because of the shortened acquisition time. Clinical applications include the detection of central pulmonary thromboemboli and the assessment of aortic aneurysms and dissections.