Pulmonary nodule simulated on the lateral chest radiograph by branches of the aortic arch

A prospective study of 500 lateral radiographs of the chest showed a nodular opacity superimposed on the trachea above the aortic arch in 15 patients (3%). Because no abnormalities were identified on the posteroanterior radiograph or on CT scans, we assume that this pseudonodule is produced by branches of the aorta. In all cases, CT showed that the left subclavian artery or the innominate artery-right subclavian complex protruded into the lung.